I Am New to Solar. Where Do I Start?

Q: What will a solar photovoltaic (PV) system do for me?

A: It will make Kilowatt-Hours (KWH) of electricity. A KWH is 1000 Watts (one Kilowatt) used for one hour. This is the unit of electricity used to determine how much you pay your electric company. Watts are instantaneous, but Kilowatt-Hours are Watts times hours, so they take place over a period of time. If you use 6000 Watts of electricity for one minute, you have used 100 Watt-Hours or 0.1 Kilowatt-Hours (KWH). If you use 100 Watts of electricity for 24 hours, you have used 2400 Watt-Hours or 2.4 Kilowatt-Hours (KWH). In a solar PV system, solar panels and inverters are rated in Watts, while batteries are rated in KWH.

Q: How can I “zero” (eliminate) or reduce my electric bill?

A: The practice of utility customers selling back (“exporting”) their excess solar electric production to their utility company was started in the 1970s by government-regulated electric utilities trying to encourage solar investment. This practice made sense for utility customers since the lead-acid batteries used at that time with a 4 year life and 25% available capacity made self-storage of electric energy too expensive. Many utilities offered net metering, crediting their customer’s accounts the value of 1 Kilowatt-Hour (KWH) for each KWH that the customer sent to them during the daytime. However, this arrangement was not economically beneficial to utilities since daytime solar production does not reduce the size of the generating plant that the utility must purchase to meet peak demand between 4 p.m. and 10 p.m. For this reason, state regulators and the electric companies they regulate have been moving away from net metering and decreasing the “export” rate they pay customers for solar electricity. Today many grid companies offer wholesale buyback, meaning that the rate they pay for each KWH you produce is a fraction of the rate they charge for those you consume, and the export rate structure may change over time.

Grid-tied inverters have no electric storage capacity of their own, and they are designed to shut down when the grid goes down. Since solar production effectively ends by about 4 p.m., grid-tied solar customers depend completely on the grid for the electricity they use in the evening and at night. For example, a grid-tied customer with no battery storage capacity who charges their electric vehicle at night has simply exchanged a hydrocarbon they buy at the pump (gasoline or diesel fuel) for one used to power an electric generating plant, typically coal or natural gas.

Today, technology has made BOTH a reduced electric bill AND increased energy self-reliance much more affordable, and they can be achieved with the same system. Our 48V 5.12 KWH battery has Lithium Iron Phosphate (LiFePO4) cells that are UL rated for 7000 cycles or 20 years of off-grid use, with an 80% depth of discharge (DOD) vs. 25% for lead-acid. Our best-selling LiFePO4 battery contains the usable storage capacity of 13 traditional 12 Volt lead-acid batteries and lasts 5 times as long. The low per-usable-KWH price of these batteries, along with that of our “stackable” 5 KW off-grid inverters with 7.5 KW of solar panel input and grid backup during cloudy days, makes it possible for anyone to gradually increase their energy self-reliance and reduce their electric bill. Many customers do this by installing an off-grid breaker panel to operate critical appliances independently from the grid. As they purchase more panels and batteries, they can move more of their power needs to the off-grid breaker panel, thus eliminating a larger portion of their power bill.

Q: How does the grid pay me back for the solar power I produce?

A: The bulk of solar electric production happens between 10 a.m. and 3 p.m. The only power you can sell back to the grid is excess power you produce above what you are using at that instant. At those moments, the smart meter measures the excess power you are selling back to the grid. The rate you are paid, as well as your ability to sell power back to the grid, depend on your location and your electric provider. This export rate has been decreasing over time as utility regulators recognize the underlying economics of residents selling power back to the grid. During daytime periods when your production is lower than your use, as well as at night, you are buying from the grid as you always have been.

Q: How can I go solar and still benefit from the electric utility grid?

A: A solar electric system can work with the grid or supplement the grid, depending on your equipment. In particular, the type of inverter you have determines how you interact with the grid. Grid-tied inverters only work when they detect power coming from another source, usually your electric utility. If the grid goes down, your solar electric system will not operate even if your solar panels are functional. Off-grid inverters store electric power in batteries, so they are not dependent on the grid, but they can accept power from the grid to power your appliances or to charge their batteries during cloudy days. Our best selling off-grid inverters are stackable (expandable), meaning you can start with a small system and expand your capacity later. This allows you to keep some of your critical appliances operating when the grid goes down. We can help you select the best equipment for your situation.

Q: How can I have an off-grid inverter and still be connected to the grid?

A: An off-grid inverter is designed to produce power from either solar panels or batteries, so it can run independently 24 hours a day. The off-grid inverters we sell can accept power from the grid when the Voltage in their batteries drops below a certain pre-set level because of high demand and/or low solar production, such as on cloudy days. When the percentage of battery charge gets low, our inverters will automatically draw power from the grid to run your appliances. They can also charge your batteries at the same time if you choose.

Q: I would like to keep certain appliances running if the grid goes down. How many appliances can I run this way?

A: Any battery-based system that can operate independently of the grid operates on a Kilowatt-Hour (KWH) budget. If the number of Kilowatt-Hours charging the batteries does not equal or exceed the number of Kilowatt-Hours being used, you will not be able to keep your batteries charged. A goal of solar electric system design is to include enough solar panels to keep the batteries charged almost all of the time, with minimal help from the grid or from a generator. Before you connect any appliance to a battery-based system, you need to know how many Kilowatt-Hours it uses each day. Then, you need to add up the daily Kilowatt-Hours used by all of the appliances you plan to connect to the system. Next, compare this with the number of Kilowatt-Hours produced each day by your solar panel array. The Solar Panel section has more information about how solar production is calculated.

Q: With an off-grid solar electric system, do I even need the grid at all?

A: You may not, if your solar electric system is properly sized. More solar panels mean the ability to produce more power when the sun is shining. A larger battery bank means greater ability to store power at night and during cloudy days. Statistically, the more solar panels and battery storage you have, the less likely it is that you will need to supplement your power from the grid or from a generator. This is particularly true if you live in an area that does not have long periods of cloud cover.

Solar Electricity

Q: How does a solar PV system make Kilowatt-Hours?

A: As stated earlier, you pay for electricity based on the number of Kilowatt-Hours (KWH) you use. The solar panels make direct current (DC) from sunlight. A charge controller (also called an MPPT) regulates the DC charge coming from the panels, which is then sent to an inverter which converts DC to the alternating current (AC) use in your appliances. If the inverter is grid-tied, this DC charge is converted to AC for immediate use. If the inverter is off-grid, this DC charge can be used immediately OR it can be stored in batteries.

Q: What’s the difference between DC and AC electricity?

A: When drawn out on paper, direct current (DC) looks like a straight line. Alternating current (AC) is a sine wave with crests and troughs, like a wave in the ocean. In order for two sources of AC to be connected to the same wire or device, such as a breaker panel, the two AC sine waves must be synchronized!

This means two things:

(1) they must have the same frequency and Voltage (amplitude), and…

(2) the crests and troughs must occur at the same time.

Typically, grid-tied inverters synchronize with the grid, while off-grid inverters do not. Off-grid inverters that are stackable can synchronize with each other, but not with the grid. On the other hand, DC electricity from different sources can be combined if the sources are nominally the same Voltage. Since DC looks like a straight line, DC sources do not need to be synchronized.

Q: What is a Volt?

A: A Volt (V) is a unit of electric driving force. Think of it as pressure pushing water in a pipe. In North America, anything that plugs into a standard outlet is using 120 Volt AC. Certain larger appliances such as a dryer, oven, hot water heater or air conditioner/heater unit use 240 Volt AC.

Q: What is an Amp?

A: An Amp (A) is a unit of electric current flow rate. Think of it as a flow rate of water. Most appliances will have a placard that tells how many Amps they use, so if you know the number of Amps and Volts an appliance is using, you can calculate the maximum number of Watts it uses.

Q: What is a Watt?

A: A Watt (W) is a unit of power, similar to horsepower. Watts take place at a given instant in time. Solar panels and inverters are rated in Watts. 1 Amp x 1 Volt = 1 Watt (electricians call this the power rule),so if you know the number of Amps and Volts for an appliance, you can calculate how many Watts it uses at maximum output. By the same token, if you know the Wattage and Voltage, you can determine Amperage. For example, a 5000 Watt (5KW) inverter producing 240 Volt AC outputs 20.83 Amps at full production. If its surge output is 10,000 Watts (10 KW), its surge Amperage is 41.67 Amps.

Q: What is a Kilowatt (KW) of solar panels?

A: A Kilowatt of solar panels is 1000 Watts of panels based on the laboratory rating, which is how panels are quoted in the solar industry. For example, two 500 Watt solar panels, four 250 Watt solar panels and ten 100 Watt solar panels are each a Kilowatt of solar panels. Do not confuse a Kilowatt of solar panels with a Kilowatt-Hour of electric use.

Q: How does AC electricity work in North America (and parts of the Caribbean and South America)?

A: We have a split-phase electric system. The electric grid sends out 240 Volt AC made by two lines of 120 Volt AC that are 180 degrees out of phase, meaning they are opposites. Visually, this looks like a series of hourglass shapes going down the page. When one AC sine wave has a “bump” (crest) to the right, the other has a bump to the left. The horizontal distance between the bumps represents 240 Volts. However, most appliances use 120 Volt AC, so this is produced by the “neutral bus bar” on your breaker panel. This bus bar takes a Voltage between the two 120 Volt AC lines and sends it back to the electric grid as the “return” line completing the circuit. Visually, this is like drawing a straight vertical line in the middle of the hourglass shapes, and it is called a center tap. In this way, the Neutral Bus Bar splits the 240 Volt AC into two 120 Volt “legs”. On a breaker panel without the breakers, there are two channels going through the breaker panel in a serpentine pattern, so that every other breaker is on a different channel. These two channels are the two “legs” of 120 Volt AC, and this is done so that electricians can balance the circuits between the two legs as they attach them to the breaker panel. A single pole breaker carries 120 Volt AC because it connects to only one of these legs. A double pole breaker carries 240 Volt AC because it connects to both 120 Volt AC legs, so it is automatically balanced.

Q: What is the basis of a solar electric system design?

A: A solar electric system design is based on the number of Kilowatt-Hours (KWH)per day that you want the system to produce for you, either as an annual average or during a particular month of the year. Keep in mind that solar panels produce different amounts of power at different times of the year, and this amount also varies based on geography. For example, you may want to ensure that you are producing enough KWH/day in December, the minimum solar production month.

You can estimate your KWH/day use in several ways. (1) Divide the number of KWH shown on your highest monthly electric bill by 30. (2) Find the dollar cost of your monthly electric bill and subtract fees that are not related to your KWH use. Then, divide the remaining cost of your bill by your cost of electricity (current average is $0.113/KWH in the U.S.) to get KWH/month. Next, divide this number by 30 to get daily use in KWH. (3) Use a spreadsheet to calculate the daily KWH use from all of the appliances in your home. Remember that Amps x Volts = Watts and account for the fact that the compressors in your fridge, freezer or AC/Heating unit are running for only part of the hour, so multiply the unit’s running Watts by this fraction to get KWH/Hour. For example, multiply running Watts by 0.33 if running 20 minutes/hour, 0.5 if running 30 minutes/hour, etc. You can purchase a plug-in meter to determine an appliance’s KWH use during a 24-hour period. A typical 2000 square foot American home uses about 50 KWH/day, larger homes (3000-4000+ SF) use about 75 KWH/day or more.

Q: How can I calculate the number of Kilowatt-Hours (KWH) each appliances uses?

A: If the appliance doesn’t have a Wattage rating, you can multiply Amps x Volts to get Watts, as mentioned earlier. Appliances will use close to this number of Watts whenever they are running at full power, but they are not operating at full power all of the time. Your goal is to figure how many minutes per hour that they are using their full power rating. You can guess this factor from experience. Some appliances run continuously, such as lights, TVs, and computers. Anything that uses resistance heating, such as electric space or water heaters, an oven, a dryer, or kitchen electrics, will use close to the maximum Wattage when they are on, typically for more than half of the time they are operating. Appliances that use compressors, such as a refrigerator, freezer, or air conditioner/heat pump, typically have their compressor on for one-fourth to one-half of the hour (15 to 30 minutes), so their maximum Wattage should be multiplied by a factor between 0.25 and 0.5 when calculating Kilowatt-Hours. You can also purchase a plug-in meter to measure an appliance’s KWH use during a 24 hour period.

Q: Should I design my solar PV system based on my current Kilowatt-Hour (KWH) per day use?

A: Not necessarily. Before you “go solar”, analyze the Kilowatt-Hour (KWH) per day electric use of each appliance in your home, remembering that the Wattage used by each appliance will be multiplied by the number of hours it runs. 240 Volt appliances are typically the largest users. In normal operation, a central air conditioner/heater uses about 1000 Watts per “ton” (12,000 BTU) of heating or cooling. By comparison, a mini-split air conditioner/heat pump uses 600 Watts to cool or heat a 600 SF area rather than the entire house. An electric resistance-based hot water heater uses about 4000 Watts, but a heat pump water heater can use one-half to one-third this amount. Dryers and ranges typically use over 3000 Watts each. By comparison, most kitchen electrics (120 Volt AC) use 1500 Watts or less. Also, remember that solar production varies with the seasons, particularly in northern areas, so you may want to use other sources of energy strategically. For example, a wood stove can heat your home, cook your food, dry your clothes and improve the operation of a heat pump water heater.

Solar Panels

Q: What is a Kilowatt (KW) of solar panels?

A: A Kilowatt of solar panels is 1000 Watts of panels based on the laboratory rating, which is how panels are quoted in the solar industry. For example, two 500 Watt solar panels, four 250 Watt solar panels and ten 100 Watt solar panels are each a Kilowatt of solar panels. Do not confuse a Kilowatt of solar panels with a Kilowatt-Hour of electric use.

Q: What are bifacial and split-cell panels?

A: A bifacial panel has solar cells on both sides. They are considered premium panels since there is no plastic back sheet on the back of the panel. On some lower-end panels, the back sheet can peel away from the panel over time. Bifacials are particularly valuable in northern areas where the back side can still produce electricity if the front side is covered with snow. The resistance of the electricity moving through the back side of the panel produces heat, which helps to melt the snow.

Note: The laboratory Wattage shown for a bifacial panel refers to front side production only. To get total laboratory Wattage for both sides of the panel, multiply the front side Wattage by 1.05 if the panel is placed on a roof OR by 1.15 if it is placed on a ground mount rack.

Split-cell panels have a bus bar in the middle of the panel. They are also considered premium panels since the electric current travels a shorter distance than it does in a regular panel, thus increasing the panel’s longevity. Shading on one half of the panel does not affect the other half of the panel.

Q: Do solar panels actually produce the number of Watts shown on their rating?

A: Solar panels are rated in perfect laboratory conditions: Irradiance of 1000 Watts/square meter, cell temperature 77 degrees F, and not connected to any other panels. In reality, irradiance is about 800 Watts/square meter in most of the U.S., cell temperature is closer to 115 degrees F, and panels are connected in strings. Lower irradiance means less Amperage and Wattage, and the last 2 factors increase electrical resistance. Typically, panels exceed 80% of their laboratory rated Wattage for only about 5% of the day, at solar noon. Solar production models such as pvwatts.nrel.gov already account for this “de-rating” factor.

Q: How many solar panels do I need?

A: There are 4 steps in this process:

(1) Decide how many Kilowatt-Hours (KWH) per day you want the system to produce. This is the basis of your solar electric system design. You are deciding this when you decide what part of your appliances will be operated independently from the grid. This can be based on your current use, or it can be based on a lower expected use once more electrically efficient appliances, such as mini-split air conditioners/ heat pumps or heat pump water heaters, are installed.

(2) Determine the number of Kilowatt-Hours (KWH) produced per day, on average, for every Kilowatt (KW) of panels in your area, depending on the direction and tilt of the array.

Note: You can use the annual average number for KWH/day/KW of panels as the basis of your design, OR you can use the monthly average from a certain month that is of particular interest to you. For example, December solar production may be as low as 35% of annual average production in the northern tier of states, where winter electric demand is high. December production is typically about 70% of annual average production in the southern tier of states.

As a rough approximation, a south-facing solar panel array of 20 degree tilt in the eastern half of the U.S. will make an annual average of 4 Kilowatt-Hours (KWH)/day/KW of panels. The actual number is more like 3.7 KWH/day/KW in the northern tier of states and 4.3 KWH/day/KW in the southern tier of states, including Florida and the Caribbean. It approaches or exceeds 5 KWH/day/KW as you move toward the dry areas of the west, particularly Arizona and southern California. It is about 3.2 in the Pacific Northwest due to the rainy climate.

The PV System Design FAQ has more information about how to calculate annual average and monthly average production for your area.

(3) Divide your KWH/day use by KWH/day/KW of panels.

The result of your calculation is the number of Kilowatts (KW) of panels you need to make your KWH/day goal. This result refers to laboratory Watts, not actual. We usually add a few KW to this number as a safety factor, depending on how many solar panels the inverter can accept.

Note: This is not the number of solar panels needed for you system; it is the number of Kilowatts (1000s of Watts) of panels needed for your system, so go to Step 4.

(4) Multiply KW of panels by 1000, then divide by the number of Watts per panel to determine the number of panels needed for your system.

Note: The laboratory Wattage shown for a bifacial panel refers to front side production only. To get total laboratory Wattage for both sides of the panel, multiply the front side Wattage by 1.05 if the panel is placed on a roof OR by 1.15 if it is placed on a ground mount rack.

Q: How should I use the information from a solar panel’s label or spec sheet?

A: On a solar panel spec sheet, the open circuit or short circuit ratings refer to one panel by itself, and the maximum power ratings refer to panels connected in a string. Typically, the 80% factor can be applied to the Maximum Power Current shown on the spec sheet or panel label to determine actual Amperage. This directly effects Wattage, so your actual Wattage will typically be about 80% of laboratory Wattage or less. Voltage increases as temperature drops, so those in northern climates should use the Voltage Open Circuit (Voc) rating as stated. Those in southern climates such as Florida can use about 90% of Voc for design purposes if they are certain that their ambient temperature will never get below 20 degrees F. If you exceed the inverter’s upper input Voltage limit the inverter will typically throw a fault code. Long term over-Voltage fault codes can damage the inverter or charge controller.

Q: Are my solar panels 24 Volt?

A: In the past, solar panels were nominally referred to as “24 Volt”. Today, the Voc of solar panels in the upper 300 to 400+ Watt range is close to 50 Volts. Use the panel’s actual Voc, and not a nominal Voltage amount, to determine if panels will work in a string. Today, “24 Volt” or “48 Volt” systems refer to the Voltage of a battery bank connected to the inverter, not to any particular type of solar panel.

Q: What is meant by “series” and “parallel”?

A: Every solar panel is a source of DC Voltage, just like a battery. Each panel has a positive and negative lead of PV wire (typically 10 gauge wire with extra insulation) attached to the back of the panel. The wires have male (+) and female (-) snap-together MC4 connectors. A “string” is a group of panels connected in series, meaning the positive of the first panel is connected to the negative of the second panel, etc. on up the string. When connected in series, the Voltage of each panel is added but the Amperage does not change. Several strings of panels are then connected in parallel, meaning the positive ends of each of the strings are connected together and the negative ends of each of the strings are connected together. Strings of panels connected in parallel add the Amperage of each string together, but do not change the Voltage of each string. Connecting strings of panels together in parallel is often called “series-parallel”, since each string is a series, and the strings are connected in parallel.

Q: How are solar panels connected?

A: Every solar panel is a source of DC Voltage, just like a battery. Each panel has a positive and negative leads of PV wire (typically 10 gauge wire with extra insulation) attached to the back of the panel. The wires have snap-together MC4 connectors, typically male is positive and female is negative. A “string” is a group of panels connected in series, meaning the positive of the first panel is connected to the negative of the second panel, etc. on up the string. When a set of solar panels is connected in series, positive to negative, the Voltage of each panel is added but the Amperage does not change. Several strings of panels are then connected in parallel, meaning the positive ends of each of the strings are connected together and the negative ends of each of the strings are connected together. Strings of panels connected in parallel add the Amperage of each string together, but do not change the Voltage of each string. Strings connected in parallel to the same charge controller (MPPT) must be at the same Voltage, but strings connected to different charge controllers may be at different Voltages.

Q: What determines how many solar panels I can put in a string?

A: Your inverter or charge controller spec sheet should show an input Voltage range. Your solar panel spec sheet should show a number for Voc or Voltage Open Circuit. Voc is also shown on a sticker on the back of the solar panel. Dividing the inverter/charge controller’s maximum input Voltage (the highest number in the range) by Voc tells you how many solar panels you can place in a string, but you should keep string Voltage at least 10-20 Volts below the upper limit to avoid problems. Voltage increases as temperature drops, so if your ambient temperature drops much below 32 degrees F you must use Voc to calculate the number of panels in the string. If your ambient temperature does not get this low, you may be able to use the lower “actual” Voltage (80-90% of Voc) to do this calculation.

Q: Can I connect different types of solar panels in the same string?

A: Generally not. Panels connected in the same string need to be within about 1 Volt of each other, based on their Voltage Open Circuit (Voc) rating. Strings connected in parallel to the same charge controller (MPPT) must be at the same Voltage. Many inverters contain several different charge controllers. Strings connected to one charge controller may operate at a different Voltage from those connected to a different charge controller.

Q: What determines how many solar panel strings I can bring together in parallel?

A: The Amperage of the charge controller you are using. The actual Amperage of each string is typically less than 9 Amps, so if your charge controller has a limit of 18 Amps, you can bring 2 strings together in parallel. Note that the inverter or charge controller you are using may also have an upper limit on the total Wattage of solar panels that it can accept, so this may limit the number of strings you can connect in parallel.

Batteries

Q: How should I evaluate batteries?

A: Look at Kilowatt-Hours (KWH) stored and battery life. For example, our 48 Volt (nominal) 5.12 KWH battery has Lithium Iron Phosphate (LiFePO4) cells that are UL rated for 7000 cycles or 20 years of off-grid use, with an 80% depth of discharge (DOD) vs. 25% for lead-acid. Our best-selling LiFePO4 battery contains the usable storage capacity of 13 traditional 12 Volt lead-acid batteries and lasts 5 times as long.

Q: Should I compare batteries based on Amp-Hours (AH)?

A: No. What you need from a battery is Kilowatt-Hours (KWH). These are determined by multiplying the Voltage of the battery by the number of Amp-Hours, then dividing by 1000. In our battery for example, 51.2 Volts x 100 Amp-Hours = 5120 Watt-Hours or 5.12 Kilowatt-Hours (KWH).

Q: How many batteries do I need?

A: For solar purposes, nighttime is roughly defined as the period between 4 p.m. and 8 a.m. (16 hours) or possibly longer depending on your location. You need batteries with enough Kilowatt-Hours (KWH) of storage to at least keep your inverter(s) running during this time period. To get the correct answer, you should calculate your nighttime KWH use for each appliance and add these amounts. As a rule of thumb, if you are willing to move most of your large electric loads to daytime use, multiply your daily use (KWH/day) by a factor of 1/2 (0.5). This is fairly accurate for most people. However, this can vary greatly depending on your energy usage habits at night.

If your goal is to be able to continue operating during longer periods of cloudy days, you will need more batteries to give you more days of autonomy. With our system, you can start with a few batteries and add them as you need them.

Q: How much charge can I put into my batteries at one time?

A: Each battery can accept up to 50 Amps of regular charge, so the total Amperage output of all of your chargers should not exceed this amount per battery.

Q: What does ‘State of Charge’ (SoC) mean?

A: State of charge (SoC) is the level of charge of a battery relative to its capacity. The units of SoC are percentage points (0% = empty; 100% = full). An alternative form of the same measure is the depth of discharge (DoD), the inverse of SoC (100% = empty; 0% = full). All types of batteries have one thing in common: the Voltage at their terminals decreases or increases depending on their charge level. The Voltage will be highest when the battery is fully charged and lowest when it is empty. This relationship between voltage and SoC depends directly on the battery technology used.

Q: How do I size the batteries in my solar energy storage system?

A: Generally, it is better to have more battery capacity than you need in order to meet the home’s power requirements. The faster you discharge batteries, the faster the batteries will wear out. Our lithium iron phosphate (LiFePO4) batteries are designed to last for 7000 cycles (20 years) at 80% depth of discharge (DOD) every night.

Inverters

Q: What are the different types of inverters?

A:Grid-tied inverters are designed to synchronize with an AC sine wave coming from another source (typically the grid). Grid-tied inverters must meet UL 1741 anti-islanding, so they will shut down immediately without an AC sine wave coming from some other source. This prevents them from sending powering to utility lines that are down, because this could hurt line workers. A grid-tied inverter will shut down when the grid goes down unless you can safely supply it with an AC sine wave from a different source. Operating a grid-tied inverter with a sine wave from an off-grid inverter is called AC coupling.

Off-grid inverters make their own AC sine wave by converting DC Voltage in a battery bank to AC. These inverters will run whenever there is enough Voltage in the battery bank. They do not synchronize their AC sine wave with the grid, so you cannot have the utility grid and an off-grid inverter powering the same breaker panel at the same time. If you want both the grid and an off-grid inverter connected to the same breaker panel, you must have some type of transfer switch (manual or automatic) to make sure that only ONE of these sources is connected at any given time.

Hybrid inverters are just these 2 types of inverters placed together in the same box. You can accomplish the same thing using manual or automatic transfer switches.

Q: How are inverters rated?

A: Inverters are rated by the number of Kilowatts (1000s of Watts) that they can produce at any moment in time during normal operation. Must inverters can surge (increase) their production for a short period of time, usually a few seconds. This allows them to start electric motors, such as compressors, that typically require 2-2.5 times the running Watts at startup.

Q: What is meant by “stackable” inverters?

A:Stackable means that the outputs from several inverters can be combined because their AC output sine waves are synchronized with each other. They are NOT synchronized with the grid.

Q: Should I buy an inverter that makes 120 Volt AC or 240 Volt AC?

A: This depends on what appliances you are trying to operate. Most appliances you plug into the wall use 120 Volt AC, while some of the larger appliances such as an oven, dryer, water pump, water heater or central air conditioning unit use 240 Volt AC. Most homes use 240 Volt AC in some way, even if it is to power the outdoor unit of a mini-split air conditioner. If you use a 120 Volt AC, you are only powering one of the two AC legs on the breaker panel. Today’s stackable inverters allow you to connect two 120 Volt AC inverters to make 240 Volt AC. However, it is often easier and less expensive to purchase stackable 240 Volt inverters from the beginning, even if you only use 120 Volt AC.

Q: How does an inverter send its AC output to my electric system?

A: In a breaker box powered by the grid, the power comes in from the top of the breaker panel. The breakers on the panel only distribute power to house loads; they never bring power into the panel. In a solar electric system, the “AC output” of an inverter is wired to a breaker on a breaker panel. This breaker “energizes” (brings power into) one or both of the panel’s 120 Volt AC legs, thus powering all of the other breakers that are connected to that leg. 120 Volt AC output from an inverter will be sent to a single-pole breaker, which powers one 120 Volt leg. 240 Volt AC output will be sent to a double-pole breaker which powers both legs of 120 Volt AC, thus providing either 120 Volt or 240 Volt AC.

Q: How many Amps (A) does an inverter send to a breaker panel?

A: This is easily calculated by dividing the inverter’s output in Watts by the Voltage it is delivering to the breaker panel. For example, if a 5000 Watt (5KW) inverter is sending 240 Volt AC to the panel, it is sending 5000/240 or 20.83 Amps. If it has a surge capacity of 10,000 Watts (10 KW), it is sending 10,000/240 = 41.67 Amps during its surge output.

Q: How should I wire Grid Backup for an off-grid inverter?

A:Grid Backup means that the grid will supply power to your house loads when the Voltage in the off-grid inverter(s) battery bank drops below a predetermined level. Customers configure this Voltage level in the inverter’s settings. Grid backup requires two breaker panels, one powered by the grid and one powered by off-grid inverter(s). An electrician can move your main grid wires to a different breaker panel. How you distribute your appliances between these two panels is up to you. Some customers leave most of their appliances on the grid-connected panel and place a few appliances on an off-grid “critical loads” panel which can accept more appliances later as solar panels, inverters and batteries are added. Other customers choose to move their grid electric service to a separate breaker panel and power all their existing appliances from the off-grid inverter(s), with grid backup. Plan for how you will allocate your appliances in the future.

The off-grid inverter’s AC input will draw power from the grid-connected breaker panel and pass it through, unmodified, to meet your house loads. It gets this power from wires connected to a breaker located on the grid-connected panel. The grid-tied breaker panel sees this as an additional breaker either single-pole (120 Volt) or double-pole (240 Volt). The breaker is usually of the same size and type as that used by the off-grid inverter’s AC output.

Q: Where should I locate my inverter(s)?

A: Inverters and other solar electric equipment typically do not have to be climate-controlled, but they should be located out of direct sun and rain. Look at your inverter’s spec sheet for temperature requirements. If you are grid-tied, they can be located at the array or near your breaker panel. If you are off-grid, you should locate inverters, charge controllers and batteries close to your breaker panel.

Q: How far away from my inverter can I place my solar panel array?

A: Tables are available in books and on the internet to help you calculate the Voltage drop over a run of wire. This depends on the distance of the circuit (both directions), the gauge of the wire used, and the Amperage being carried. Your goal is to keep Voltage drop well below 5% of the total string Voltage, which is typically several hundred Volts. If you need to use a junction box to switch from solar wire (10 gauge) to a thicker gauge of wire at the array, run a “trunk line” in conduit, and then switch back to a smaller gauge when you get close to your breaker panel, you can do so.

Q: How does an off-grid inverter with 240 Volt AC output make 120 Volt AC?

A: Most off-grid inverters produce 240 Volt split-phase AC, just as the grid does. This power is sent to a double pole breaker on a breaker panel. They use a center tap transformer to take a Voltage in the middle of the 240 Volt AC hourglass shapes. This transformer is not changing Voltages; it is simply splitting the hourglass shapes the same way the grid does. The transformer may be located inside the inverter, or it may be a separate device. If it is a separate device, it is connected to the breaker panel by a double pole breaker (black and red wires) and by a white wire which connects to the breaker panel’s neutral bus bar. In an off-grid system, the neutral bus bar is connected to a ground rod.

Q: What size breaker should I use to connect the inverter’s AC output to the breaker panel?

A: This is shown in the inverter’s user manual. Remember that Amps x Volts = Watts and remember to use a breaker large enough to accommodate the inverter’s surge Amperage. If you know the Wattage and Voltage, you can determine Amperage. For example, a 5000 Watt (5KW) inverter producing 240 Volt AC outputs 20.83 Amps at full production. If its surge output is 10,000 Watts (10 KW), its surge Amperage is 41.67 Amps.

Q: Can I connect a grid-tied inverter to my existing grid-connected breaker panel?

A: Yes, if you have the open slots on your breaker panel to accept it. Grid-tied inverters have a local oscillator which allows them to synchronize their AC output with that of the grid.

Q: Can I connect an off-grid inverter to my existing grid-connected breaker panel?

A:ONLY if you can ensure that the breaker panel is NOT energized by the grid AND by an off-grid inverter AT THE SAME TIME. If you use an off-grid inverter to energize your breaker panel, the panel must be electrically disconnected from the grid, either by turning off the main breaker or by using a manual or automatic transfer switch.

Q: How should I connect the output of my inverter to the breaker panel?

A: 120 Volt AC output should connect one “hot” wire (typically black) to a single-pole breaker. 240 Volt AC output should connect 2 “hot” wires (typically black and red) to a double-pole breaker.

Q: What if my 240 Volt inverter’s output says: “Line 1”, “Neutral” and “Ground”?

A: This is written for use in other countries that do not have split phase AC. If you are using this in a split-phase country such as the U.S., interpret “Neutral” as Line 2.

Q: How should I ground my inverter?

A: Ground connections should be made using bare 6-gauge copper wire.

(1) If your breaker panel IS NOT connected to the grid, connect the inverter’s ground to the breaker panel’s neutral bus bar. This neutral bus bar should then be connected to a ground rod close to it.

(2) If your breaker panel IS connected to the grid, connect the inverter’s ground DIRECTLY to a ground rod close to it. DO NOT connect it to the neutral bus bar.

Q: Can the off-grid inverter send power back to the grid?

A: No. (1) It has diodes that only allow one-way electricity flow into the AC input, and (2) it does not have a “local oscillator” necessary to synchronize its AC output with that of the grid. The off-grid inverters we sell send their AC output to a breaker panel that is not connected to the grid.

Wiring

Q: When I buy wire at the hardware store, what do the wire colors mean?

A: When you purchase wire from the hardware store or a home center store, it is designed for use with AC appliances, electronics, etc. Typically, Line 1 (the hot line) is black, Line 2 (a second hot line if used with 240 Volt AC) is red, Neutral is white and Ground is green or bare.

Q: What is the wiring color standards for DC devices, such as solar panel wires and/or batteries?

A: Generally, red is positive and black is negative for any DC Voltage source. Most solar panel wires are black, with male and female MC4 connectors to denote positive and negative, respectively.

Q: What do the wire ratings / numbers mean?

The first number in the rating is the gauge of the wire. The second number is the number of separate wires it has, not including the ground. For example, 12/3 is 12-gauge wire with black (Line 1), red (Line 2) and white (Neutral) wires, and a green (Ground) wire. This can carry 240 Volt AC since it has two hot wires (black and red). Another example would be 12/2. This means it is a 12-gauge wire with black and white wires and a ground. It can only carry 120 Volt AC since it only has one hot line (black). This is used for most of the circuits in your home.

Important Note: The smaller the gauge number of wire, the larger its size. For example; 10-gauge wire is bigger than 12-gauge wire.

Q: How many Amps does each gauge of wire carry?

A: This depends on the type of wire being used. Hardware stores and the internet have tables for this. Typically, 12 gauge carries 20 Amps, 10 gauge carries 30 Amps, 8 gauge carries 55 Amps, 6 gauge carries 75 Amps, etc. The National Electrical Code (NEC) recommends you use 80% of these values.

Q: Does the distance of the wire affect the wire size?

A: Yes. Voltage drop is proportional to current (in Amps), wire length and the size of the wire. The formula is Vdrop = I (current in Amps) x Rc (resistance of the wire in Ohms/1000 feet) x L (Length of the wire in 1000s of feet). If you are bringing DC current from a solar panel array to an inverter or charge controller, your design goal is to keep the Voltage drop below 5% of the total Voltage of the string. For example, if you have 8 panels in a string and each panel has a Voltage Open Circuit (Voc) of 50 Volts, your string Voltage is 400 Volts so you would want to keep your Voltage drop under 20 Volts.

Q: What is the Difference Between Ground and Neutral Wires?

A: Truth be told, the Earth is a gigantic electrostatic generator. Charges travel around vertically, making it more prone to electrical currents. To provide a solution to this problem, the ground wire is added.

The neutral wire forms a part of the live circuit along with the hot wire. In contrast, the ground wire is connected to any metal parts in an appliance, such as a microwave oven or coffee pot. This is a safety feature, in case the hot or neutral wires somehow come in contact with metal parts. Connecting the metal parts to earth ground eliminates the shock hazard in the event of lightning or a short circuit.

While it may appear that ground and neutral wires are similar, especially considering that they are generally connected at the service panel, the neutral wire and ground wire serve very different purposes. At the device, the neutral wire is the path that is used for returning current. Remember, electricity always wants to go home (complete a circle).

All of the current that comes from the hot leg returns through the neutral wire. The ground wire should only carry current if a fault condition has occurred. Typically, the ground wire is physically connected to a rod that penetrates the Earth near the breaker box. Meanwhile, the neutral wire goes back to the source, often to a transformer or inverter. While both wires deal with excess electrical current, they operate under somewhat different principles and are both very important for protecting your devices (as well as yourself).

Important Note: Grounding can be used as a Neutral, but Neutral CANNOT be used as a ground. For this reason, do NOT ground the neutral wire twice!

Q: When do I need to use solar wire in my PV system?

A: The solar wire we sell, like the wire on a solar panel, is stranded 10-gauge wire with snap-together “MC4” connectors. This wire is convenient to use because you can snap it together and it has extra insulation to protect it from Ultraviolet (UV) radiation. At any point in the system, you can choose to run regular wire with wire nuts, junction boxes, etc. from the hardware store, but you will need to put the wire in conduit to protect it from UV. If you place solar wire in conduit, we recommend 2-inch conduit so that so you can easily move MC4 connectors through the conduit.

Q: How should I ground my solar panel array?

A: Connect the panel frames to a bare 6-gauge copper wire that is connected to a ground rod. Optimally, the ground rod should be a few feet away from the array. If your array is on your roof, you can connect directly to your house ground rod.

Important Note: Do not connect the ground wire to the neutral bus bar of your grid-connected breaker panel.

Transformers

Q: If I have a balancing (“Center-Tap”) transformer, how many Watts can I use on each 120 Volt AC leg?

A: The transformer limits the discrepancy between the number of Watts being used on each leg. For example, if I have a stackable 10 KW system and am using 7500 Watts on one leg and 2500 Watts on the other leg, I have a discrepancy of 5000 Watts. This would require 2 transformers since each can balance 2500 Watts. If I had used a 12 KW 240 Volt inverter with an internal transformer, I would have had a hard limit of 6000 Watts on each leg, and this would not have been possible.

Q: What type of breaker should I use with the 5000-Watt transformer?

A: We recommend a 30 Amp double-pole breaker.

Charge Controllers

Q: Will I need a charge controller?

A: Charge controllers convert solar panel string DC Voltage to a useable level and prevent current back flow in battery-connected systems. A charge controller optimizes the combination of Amps (I) and Volts (V) shown on an “I-V curve” on the panel’s spec sheet to maximize the power produced by the string. This function is called “Maximum Power Point Tracking” or MPP or MPPT, and the charge controller itself is often called an MPPT.

Q: Do I need to buy a separate charge controller?

A: Today, virtually all grid-tied inverters and many off-grid inverters include charge controllers. Our stackable off-grid inverters are designed so that you will not need a separate charge controller, even if your system is large. If you are off-grid and need more charging capacity than the inverter(s) provide, a separate charge controller can provide additional Amperage to charge the batteries. The output from the charge controller would be connected directly to the battery bank, not to a breaker panel.

Q: Do I need a combiner box?

A: Combiner boxes connect strings in parallel, adding the Amperage from many strings together. This is necessary when the charge controller limits string Voltage to a low number. A Combiner Box has a breaker + lightning arrester (EMR Pole). Many off-grid charge controllers in the past could only accept a 150 Volt input, which means 3 of today’s panels in a string. Grid-tied inverters usually have very high string Voltages (5-600 Volts), and some of today’s off-grid inverters, such as our stackable 5 KW, have a high Voltage input limit (450 Volts).

When string Voltage is high, you need fewer strings added in parallel, therefore a combiner box is not necessary. Solar panel strings generally carry less than 9 Amps, so you can combine up to three of them in parallel using 10-gauge (30 Amp) Y-shaped “branch pairs” (“splitters”), which are much less expensive than combiner boxes.

Generators

Q: What is meant by “clean” and “dirty” power?

A: “Clean” power means an AC sine wave has less than 5% Total Harmonic Distortion (THD). The output from most generators has between 15% and 30% THD. Typically, only very expensive generators have filters that are adequately sized to reduce THD significantly. This means the output from a generator can damage electronics, like the power boards on your appliances or your solar inverter. By contrast, a pure sine wave off-grid inverter like the SPF 5000 ES stackable inverter makes the cleanest power available (less than 3% THD), even cleaner than that produced by the grid. During cloudy days, instead of trying to run all of your house loads from a generator which makes dirty power, your goal should be to run all of your loads from your clean-power off-grid inverter(s). This is accomplished by keeping the batteries charged, the #1 goal of off-grid operation. Batteries store Kilowatt-Hours (KWH) which are Watts times hours, so best practice is to “trickle charge” your batteries using a smaller, quieter, more fuel-efficient generator over a longer period of time. This requires checking the weather to see when periods of clouds are coming.

Q: Should I connect the output of my generator to the inverter’s AC input?

A: This will cause your inverter’s power board to blow, or at a minimum, to degrade over time. Always send your generator output directly to a battery charger.

Q: How large of a generator should I buy to power my house entire house during cloudy days?

A: Virtually all of the appliances in your home, and your solar inverter, contain printed circuit boards which are easily damaged by the Total Harmonic Distortion (THD) of an AC sine wave. Why would you want to power your appliances with an AC sine wave that has 30% THD when you could power them with the output from our 5KW off-grid inverter with less than 3% THD? Why would you want to spend $10-20K on a large generator requiring storage of large amounts of hydrocarbons when you could purchase batteries and/or solar panels that will greatly decrease the probability that you will go without power during cloudy days. Instead of powering your house with a generator, you should use a small generator to charge your batteries so that you can run your house from a pure sine wave inverter, which makes the cleanest power available. To charge batteries, we recommend a generator size of 3500 to 4000 Watts. For example, if your generator has two 120 Volt AC outlets, you can plug two of our 48V 25A chargers directly into the generator and make 2400 Watts of DC charge for a 48 Volt battery bank (58V x 25A = 1450W = 1.45 KW x 2 chargers = 2.9 KW). The red and black clamps from each charger connect to the positive and negative wires running between your battery bank and the inverter(s). These chargers are making DC, so there is no problem connecting multiple units in parallel (same polarity to the same wire). This way there is no possibility of damaging your inverter or appliances. If you run two of the 48 Volt 25 Amp chargers for 8 hours, you have added 23.2 KWH to your battery bank, meaning you have fully charged one of our 6-battery racks. If you plan ahead and use electricity carefully, this should be enough to get you through the night.

Q: What if my generator has a filter?

A: Generator filters often do not have enough Ampacity to completely filter the amount of current they are producing for a long period of time, because their Field Effect Transistors (FETs) will burn out with frequent use. Even if the generator company offers to replace your inverter or your appliances in a few weeks, do you really want to go that long without their operation? It is much better for a battery bank to receive a trickle charge, a small amount of charge over a long period of time,rather than a large amount of current at one time.

Q: Can chargers accept 240 Volt AC output from a generator?

A: Yes, they can, but the two wires on the charger that are normally called “hot” (black) and “neutral” (white) must be connected to the two “hot” wires (typically black and red) from the generator’s output.