Overall Design Considerations

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: What are the components of a solar PV system?

A: Strings of solar panels connected in series produce high Voltage direct current (DC) from sunlight. The charge controller (also called an MPPT) converts this to a lower Voltage and either sends it to an inverter for immediate use or stores it in a battery bank. The inverter converts DC from the battery bank to the alternating current (AC) used in your appliances. Today, many inverters called all-in-one perform both the charge controller and inverter functions, which used to be done in separate boxes. Solar panel arrays and inverters are rated in Kilowatts (KW), while battery banks are rated in Kilowatt-Hours (KWH).

Q: How can I reduce or eliminate my electric bill AND increase my energy self-reliance?

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 lead-acid batteries with a 4 year life and 25% depth of discharge made self-storage of electrical 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 needed to meet their 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 this export rate structure may decrease over time.

Grid-tied inverters have no electric storage capacity of their own, and are designed to shut down when the grid goes down. Since solar production effectively ends by about 4 p.m. or earlier, 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 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. Each 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 appliances to the off-grid breaker panel, thus eliminating a gradually increasing portion of their power bill.

Q: What is the basis of a solar PV 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. This may be only certain appliances connected to a “critical loads” breaker panel, such as a fridge, chest freezer, lights, and maybe a mini-split air conditioner/heat pump. Our stackable inverter and battery systems make it possible for you to start small and add to the system at your own pace, up to the point where you could power your entire house off-grid and still have grid backup for periods of low solar production.

If you are operating any type of battery-connected system, design your system to produce at least as many KWH/day, on average, as the connected appliances are consuming. Solar production varies greatly during the year in northern areas, so you can base your design on your annual average demand OR on you can base it on your monthly average demand during a particular month. For example, in the northern tier of states in the U.S., December solar production can be as low as 35% of annual average production, so customers in those areas may want to ensure that they can meet heating demands when solar production is minimal. In the southern tier of states, December production is roughly 70% of the annual average production, and summer cooling is a much greater concern. Customers in those areas may want to base their system design on annual average production, or they may want to base it on monthly average production during the summer months.

You can estimate the current KWH/day use for your entire home 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: What are the major design choices in a solar PV system design?

A: There are three major parts of a solar PV system design: (1) solar panels, (2) charge controllers and inverters, and (3) batteries. Each of these design choices will be described in a later section.

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: How should I use solar panel ratings to design a solar electric system?

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 usually about 800 Watts/square meter, cell temperature is closer to 115 degrees F, and panels are connected in strings. Lower irradiance decreases Amperage and Wattage, and the last 2 factors increase electrical resistance.

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 maximum power under load. For most of the U.S., the 80% irradiance factor can be applied to the Maximum Power Point Current (Impp) shown on the spec sheet or panel label to determine actual Amperage, which directly effects Wattage. For example, if your Maximum Power Point Current (also called Operating Current) is 10.86 Amps, actual current at 80% irradiance will be 10.86 Amps x 0.8 = 8.69 Amps.

Solar panels typically exceed 80% of their rated laboratory Wattage for only about 5% of the day, at solar noon. Since laboratory Wattage is higher than actual Wattage, solar designers often use a DC to AC ratio of (1/0.8) = 1.25 or greater to capture more Watts during the morning and evening. Even though some clipping (Amperage and Wattage loss) occurs at solar noon, the overall Wattage production of the system is increased by this design. Solar production models such as pvwatts.nrel.gov already account for this “de-rating” factor. If clipping occurs, some Amps (and thus Watts) will remain unused on the solar panels, but no harm will be done to the inverter.

Voltage is different from Amperage because an inverter or charge controller’s maximum Voltage input limit cannot be exceeded for very long without damage to the device. Connecting solar panels in series (positive to negative) adds the Voltage of each panel together to form a string Voltage which should never exceed the inverter or charge controller’s maximum input Voltage. Voltage increases as temperature drops, so those in northern climates should use the Voltage Open Circuit (Voc) rating as stated on the spec sheet. Those in southern climates can use actual Voltage (maybe 80-90% of Voc) for design purposes if they stay above 32 F. However, a cold snap (close to 20 F or lower) could cause actual panel Voltage to approach or equal Voc. Exceeding the inverter or charge controller’s maximum input Voltage will cause a fault code and will damage the equipment if not corrected.

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

A: When solar panels are connected in series with the positive of one panel connected to the negative of the next, their Voltage is added together but their Amperage does not increase, so the Amperage of the string is the same as the Amperage of a single panel. The Voltage resulting from connecting panels in series is called the string Voltage. Your inverter or charge controller’s spec sheet should show an input Voltage range. Exceeding the maximum Voltage in this range can cause error codes and/or damage the inverter or charge controller.

Voc, or Voltage Open Circuit, is shown on the back of the solar panel and on the spec sheet. Voc is a theoretical lab Voltage, andyour panels will typically operate at about 80% of Voc for most of the year. However, Voltage increases as temperature drops. If your ambient temperature drops to around 20 degrees F or less your panel Voltage could reach or exceed Voc, so you must use Voc to calculate the number of panels in the string. Dividing the upper limit of the inverter/charge controller’s Voltage range by the Voc of the panel tells you how many solar panels you can place in a string, but you should keep string Voltage at least 20 points below the maximum input Voltage to avoid problems. For example, if your inverter’s Voltage limit is 250 Volts and your solar panel’s Voc is 50 Volts, you cannot put 5 panels in a string because you are at the maximum input Voltage with no safety factor. If your ambient temperature never goes much below freezing (32 F), you may be able to use the lower “actual” Voltage (80-90% of Voc) to do this calculation.

Q: How many solar panel strings can I bring together in parallel?

A: The spec sheet for your inverter or charge controller should show a PV Amperage input limit. Unlike Voltage, exceeding this limit will not damage your unit, but will cause “clipping”, meaning that some of the Watts collected by your solar panels will not be used. Solar designers often use a DC:AC ratio up to about 1.3 to capture more Watts during the morning and evening. Even though some clipping occurs at solar noon, the overall KWH production of the system is increased.

Your panel’s spec sheet will show the panel’s maximum power current, Impp. In most of the U.S., your panels will typically make about 80% of this lab rating. Connecting panels in series (a “string”) increases Voltage but does not increase Amperage, so the Amperage of the string is the same as the Amperage of one panel. First, determine your panel’s actual Amperage by considering the irradiance factor. Most of the U.S. can use an irradiance of 800 Watts/square meter or a factor of 0.8 (800/1000). Since string Amperage is the same as the Amperage of one solar panel, you can multiply this factor by your panel’s maximum power current (Impp) to get the actual Amperage of the string. Next, divide the inverter or charge controller’s maximum PV Input Amperage by this actual Amperage to determine how many strings can be combined in parallel. Exceeding this limit may cause clipping but will not damage your equipment. The actual Amperage of a string of solar panels 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 have an upper limit on the total Wattage of solar panels that it can accept, and this may limit the number of strings you can connect in parallel. This input Wattage is actually limited by the inverter or charge controller’sDC Amperage used to charge the batteries, typically at 58 Volts for a nominal 48 Volt battery bank. For example, if an inverter’s Maximum PV Charging Current shows 100 Amps, the maximum Wattage available to charge the batteries is 58 Volts x 100 Amps = 5800 Watts. If a solar panel is operating at 80% of its laboratory Wattage, the inverter can accept (1/0.8) x 5800 Watts = 7250 laboratory Watts of solar panel input.

Q: What if my solar panels send more Amperage to an inverter or charge controller than it is designed to accept?

A: Unlike the Voltage limit, this will not damage your inverter or charge controller. Instead, you will experience “clipping” meaning that the excess Amperage is not processed, so you do not get the benefit of those Watts.

Q: Should I install my solar panels on the roof of my house?

A: If you have the room, we strongly recommend installing panels on a ground mount rack. The benefits are (1) reduced chance of falling, (2) no holes in your roof, (3) much easier access to wiring connections under the panels for maintenance, (4) better panel cooling, and (5) the opportunity to use the ground mount rack as a storage structure. Also, the back side of a bifacial panel (cells on both sides) mounted on a ground mount rack will typically produce 15% of the front side rating. If your area gets snow, back side production will help to melt the snow or ice on the front side. If a ground mount is not possible, second choice is the gently-sloped roof of an outbuilding, such as a barn or shed.

Q: What is the optimal direction and tilt of my solar panel array?

A: For countries in the northern hemisphere, the optimal direction is facing due south (azimuth 180). In most of the U.S., maximum annual production results from a tilt that is about 5-6 degrees less than your location’s latitude, but in extreme northern areas optimal tilt may be the same as or greater than your location’s latitude. A lower tilt favors summer production and a steeper tilt favors winter production.

In northern areas, tilt is more critical because designers are trying to overcome a large variation between summer and winter production. In southern areas, the variation between summer and winter production is much less, so tilt is not as helpful. If you want to find the optimal tilt for your solar panel array, you can use the on-line computer model pvwatts.nrel.gov described later. When designing a solar panel array, you should also consider maintenance and ease of access, as well as your seasonal power requirements.

Q: What if the sides of my roof face east and west?

A: That’s OK. You can run this on the computer model pvwatts.nrel.gov to find out how much you will make. Use azimuth 90 for east and 270 for west. You will still make about 85% of the south-facing production.

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, either as an annual average or a monthly average. This is the basis of your solar electric system design. This can be based on your current use, or it can be based on a lower expected use in the future 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.

A south-facing solar panel array of 20 degree tilt in the eastern half of the U.S. will make an annual average of roughly 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 monthly average KWH/day produced by one KW of panels will vary greatly throughout the year in northern areas, and not as much in southern areas.

To find the actual number for your area, go to the website pvwatts.nrel.gov, type in your city and state or zip code, and hit “Go”. This will take you to a map of your location. Then, use the large orange arrow to the right to move forward to the “System Info” screen. Type in 1 KW for the system size, choose premium panels (19% efficiency), choose array type (open or roof mount), type in system loss of 6% for a grid-tied system or 9% for an off-grid system (based on our experience), enter your tilt in degrees and your azimuth, which will be 180 if facing due south. Then click the right arrow again to get your results. If something doesn’t work, type in your city/state or zip code again and start over.

The second column of the pvwatts “RESULTS” page is titled “AC Energy”. Since you specified system size as 1 KW, this chart shows the number of KWH produced by 1 Kilowatt (KW) of solar panels in your location at the tilt and azimuth(direction) you specified during each month of the year, as well as the annual total. Divide the annual total number of KWH by 365 to get the annual average KWH production per day for every KW of solar panels. Also, you can take the amount shown for each month and divide it by the number of days in that month to get monthly average production. Remember, these results are for 1 Kilowatt (KW) of solar panels, not for one panel.

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

KWH/day/KW of solar panels is the production ratio for your area. The second column of the Results page, titled “AC Energy”, shows how many Kilowatt-Hours (KWH) will be produced in your area by 1 Kilowatt (KW) of solar panels at the tilt and azimuth (direction) you specified. You can use the annual average number for KWH/day/KW of panels OR you can use the monthly average from a certain month that is of particular interest to you. For example, December production is particularly critical in northern states, where winter electric demand is high and solar production is much lower than the annual average.

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 this result by the number of laboratory Watts per panel to determine the number of panels needed for your system.

If you are using bifacial panels, the laboratory Wattage pertains to the front side only. Before you do your division problem, multiply this number by 1.05 if the panels will be placed on a roof or by 1.15 if the panels will be placed on a ground mount rack.

The result of this division problem is the number of panels needed to produce your KWH/day design goal in your area. This number will probably not be an integer, so choose a number that makes sense. Best practice is to have the same number of panels in each string and to load the inverters equally to preserve system longevity.

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: Should I buy Monocrystalline (“Mono”) or Polycrystalline (“Poly”) solar panels?

A: Statistically, new solar panels fail at the rate of 4 in 10,000 or 0.04% over their 25 to 30 year lifespan. ”Monocrystalline” means the solar cells are made from a single silicon ingot, while “polycrystalline” means the cells are taken from different ingots. In our experience, both will produce Watts over a long period of time.

Q: Are your 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 Watt to 400+ Watt range is close to 50 Volts. Today, “24 Volt” or “48 Volt” systems refer to the Voltage of battery bank connected to the inverter, not to any particular type of solar panel.

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. The 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 upper limit of the inverter/charge controller’s Voltage range by the Voc of the panel tells you how many solar panels you can place in a string, but you should keep string Voltage at least 10-20 points below the upper limit to avoid problems. Voltage increases as temperature drops, so if your ambient temperature drops to about 10 degrees F or lower 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: 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 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.

Charge Controllers and Inverters

Q: What does a charge controller do for me?

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 several solar panel strings in parallel, adding the Amperage together. This is necessary when the charge controller limits string Voltage to a low number. 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). Since string Voltage is high you need fewer strings added in parallel, so a combiner box is not necessary. Solar panel strings generally carry less than 9 Amps, so you can combine up to 3 of them in parallel using 10 gauge (30 Amp) Y-shaped “branch pairs” (“splitters”), which are much less expensive than combiner boxes.

Q: What does an inverter do for me?

A: An inverter is the heart of a solar electric system. The inverter converts the direct current (DC) that is coming from solar panels or from a battery into the alternating current (AC) that is used by the appliances in your home.

Q: What are the different types of solar inverters, and how do they effect system design?

A:Grid-tied inverters are designed to synchronize with an AC sine wave coming from another source. Typically, this source is the utility grid. Grid-tied inverters must meet the requirements of UL 1741 anti-islanding, meaning that they will shut down immediately if they do not see an AC sine wave coming from another source. They were designed this way so that they will not send power back up the utility lines when the power is down, which could hurt line workers. This means that a Grid-tied inverter will shut down when the utility grid goes down, unless you can safely supply it with an AC sine wave from a different source.

Off-grid inverters make their own AC sine wave by converting DC Voltage in a battery bank. As long as there is enough Voltage in the battery bank, the off-grid inverter will run. They do not even attempt to synchronize their AC sine wave with the grid. (Stackable off-grid units synchronize their outputs with each other, but not with the grid). As a result, 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 insure 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: Can I operate an off-grid inverter without a battery?

A: Although this may work for a short time, we don’t recommend it. Off-grid inverters are designed to work with batteries.

Q: Can I attach an off-grid inverter directly into my existing breaker panel?

A: No. The off-grid inverter’s output does not synchronize with the grid’s output, so grid output and the output from an off-grid inverter cannot enter the same breaker panel at the same time. The off-grid inverter’s output must be connected to a breaker panel that is not powered by the grid. However, an off-grid inverter can be powered by the grid when solar production is low and battery Voltage drops below a pre-determined level.

Q: Can I have off-grid backup of certain appliances without going completely off-grid?

A: Yes you can. Our system allows you to choose the amount of off-grid backup you want, and you can change the amount of your off-grid backup over time. Critical circuits that you want to operate continuously, such as a fridge, chest freezer, lights, or even a mini-split AC/heat pump can be placed on an off-grid breaker panel that is not connected to the grid. As solar equipment is added, more circuits can be added to the critical loads panel.

Q: How are inverters rated?

A: Inverters are rated by how many Kilowatts (KW) of AC power they can provide continuously at any given time. They also have the ability to surge (increase) their AC output for a few seconds to start motors. An inverter’s surge capacity is typically 2-3 times its continuous capacity.

Inverters that are stackable are able to combine their output to the same breaker panel, so you can start with one inverter and add capacity later. Older non-stackable inverters are not able to do this.

When looking at an inverter, also consider its MPPTinput Voltage range. This is referring to the charge controller inside the inverter. The maximum number of solar panels in a string is determined by dividing the inverter’s maximum input Voltage by the Voc of the panels, so the higher the input Voltage, the more panels you can connect in a string. Connecting more panels in a string could make your panel wiring easier and less costly by eliminating the need for a combiner box and breakers for each string.

Q: How much inverter capacity do I need?

A: The number of solar panels and batteries used in a solar electric system is based on an estimate of Kilowatt-Hours (KWH) used per day. Inverter capacity, on the other hand, depends on the maximum number of Kilowatts (KW) you are using instantaneously, at any given time. Your maximum Kilowatt use is the total of the maximum Kilowatt use from all of your appliances (called “loads”). Also, when purchasing an inverter make sure the surge capacity is enough to handle motor startup loads in addition to your continuous loads.

Q: How many Kilowatts (KW) of solar panels can I connect to an inverter?

A: The inverter’s spec sheet or owner’s manual should tell you the maximum number of Watts of solar panels that the inverter can accept. Keep in mind that rated solar panel Watts and actual Watts are not the same thing. The hard limit is the number of DC Watts that can be charged to the batteries. For example, our stackable SPF 5000 ES inverter can accept 100 Amps charge at 58 Volts, or 5800 actual Watts. Since panels may produce up to 80% of rated power during the peak of the day, this inverter can accept 5800/0.8 or 7250 Watts of solar panel input. If the number of Watts of panels connected exceeds this amount, a small amount of Amperage (and Wattage) will be lost to clipping during the peak of the day, but the inverter will not be damaged. By contrast, the inverter’s maximum input Voltage is a hard limit that cannot be exceeded.

Q: How can I use my grid-tied inverters when the grid goes down?

A:Grid-tied inverters, whether string-based or microinverters, need both (1) incoming sunlight and (2) an AC sine wave coming from somewhere else (typically the grid) in order to operate. As installed, they will only operate during daylight hours when the grid is working. They do not work when the grid is down unless you can use an AC sine wave from a non-grid source to activate them. This can be done by a method called AC coupling, which requires an off-grid (battery-based) inverter, a Voltage-sensing switch, and a double-pole relay(s) to control the output from the grid-tied inverter(s). We do not provide technical support for AC coupling, but information is available on the internet.

Q: What does a transformer do for me?

A: This may be necessary if you live in the United States or any other country with a split-phase electrical system. If your inverter makes only 240 Volt AC, you need a separate transformer to counteract the imbalance between the load on each 120 Volt leg of your split-phase breaker panel. For example, if you have a 5000 Watt (5KW) inverter but you are pulling 3750 Watts on one 120 Volt leg and 1250 Watts on the other 120 Volt leg, your imbalance is 3750 – 1250 = 2500 Watts. One Growatt transformer is designed to handle this imbalance.

Q: How many transformers do I need?

A: This depends on the maximum amount of imbalance you have in your split-phase breaker panel at any given time. If you have more than 2500 Watts of imbalance you will need a second transformer.

Batteries

Q: Why are batteries necessary?

A: Most solar production takes place between 9 a.m. and 3 p.m., but your electric demand takes place throughout the 24-hour day. For a solar PV system, night refers to the 16 to 18 hour period when you are making little or no solar power. If you are grid-tied, you are relying on the grid to power all of your appliances during this period. If you have any type of battery backup, the batteries store the excess power made during the day so that it can be used at night. Additional batteries are necessary if you want more days of autonomy to carry you through periods of low solar production.

Q: How many batteries do I need?

A: This depends on your estimate of how many Kilowatt-Hours per Day (KWH/day) your system will use. This is the basis of your system’s design. It also depends on how many Kilowatt-Hours (KWH) you use at night, as defined above. Most people use about half of their total daily use at night. For example, a typical 2000 square foot home using 50 KWH/day should have about 25 KWH of battery capacity to go through the night if they are totally off-grid. Since each of our 5.12 KWH Lithium Iron Phosphate batteries provides 4.1 useable Kilowatt-Hours (KWH), 6 batteries would be sufficient to get this customer through the night if they followed the average use pattern. Customers who use a higher proportion of their daily power demand at night, and those who want more days of autonomy, will need more batteries.

Q: Why do I need a battery rack?

A: The bus bars in a battery rack produce more even distribution of charge between the batteries. This is important because batteries that are charged and discharged unevenly will degrade much faster. There are wiring schemes on the internet to equalize charging and discharging rates within a set of batteries, but they are somewhat complicated so many customers prefer to store their batteries in a rack to get the benefit of the bus bars.

Q: Why didn’t more solar PV systems have batteries in the past?

A: In the past, lead-acid batteries with a 4 year life and 25% depth of discharge (DOD) made self-storage of electrical energy too expensive. They typically had to be placed in series to make the inverter input Voltage, and they required more maintenance since they were liquid-based. Today, our 48V 5.12 KWH battery has Lithium Iron Phosphate cells that are UL rated for 7000 cycles or 20 years of off-grid use, with an 80% depth of discharge vs. 25% for lead-acid. Each battery contains the usable storage capacity of 13 traditional 12 Volt lead-acid batteries and lasts 5 times as long, so battery storage is now much more affordable.

Q: Do I have to put batteries in series?

A: Only if the Voltage of the battery does not match the input Voltage of the inverter. Our 48 Volt and 24 Volt batteries can be connected directly to our inverters without being placed in series.

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

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: What Voltage of battery should I purchase for a solar PV system?

A: 48 Volt input is the standard for solar PV system batteries.

Q: How can I add battery capacity?

A: Attach multiple batteries in parallel to the same bus bars. All positive terminals should be connected together and all negative terminals should be connected together. In this way, the number of Amp-Hours of each battery and the total Amperage delivered by each battery are added together.

Q: What if I need more than the 6 batteries that fit in a rack?

A: You can buy multiple racks. A set of positive and negative wires should run from each battery rack to each inverter.

Q: Do I have to use a hygrometer to check the density of the acid in the battery cells?

A: No. Lithium Iron Phosphate is a solid. There are no liquid components.

Q: What is included with the EG4LL batteries?

A: Each battery comes with red and black 100 Amp cables that screw into the bus bar of a flat-stack rack. It also has a green RS485 cable so the battery management system (BMS) of each battery can communicate with the other batteries in the rack so they can charge and discharge themselves evenly.