Solar panel calculator
Work out what an array will produce, or how many panels it takes to cover your bill, using published sun hour data and the standard loss assumptions.
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Where the losses come from
The PVWatts default loss assumptions, which the derate factor above is built from. The individual figures add up to slightly more than the total because they compound rather than simply sum: each one acts on what the previous one left.
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How it works
Array size is the DC nameplate: panel wattage times the number of panels. Peak sun hours come from NREL's annual daily averages by state, listed on the solar hub. The derate factor converts the nameplate into what actually reaches your meter.
Why the derate factor matters more than people expect
A 400 W panel does not make 400 W in normal use. The default here, 0.83, comes from the PVWatts assumptions: 14% total system losses across soiling, shading, mismatch, wiring, connections, degradation and availability, then a 96% inverter efficiency on top. Multiply those and about 17% of the nameplate disappears before the energy is usable.
Use a lower figure if you know the array will be partly shaded, if the roof faces away from the equator, or if the panels are old. Use a higher one only with good reason: 0.83 already assumes a well-sited, well-maintained system.
Peak sun hours are not daylight hours
A peak sun hour is one hour of sunlight at 1,000 watts per square metre, the condition panels are rated at. Phoenix gets about 6.5 of them a day and Seattle about 3.8, though both see far more hours of daylight than that. The difference is why the same array produces nearly twice as much in Arizona as in Washington.
Questions
How many solar panels do I need for a 2,000 square foot house?
Floor area does not decide it; your electricity use does. A home using 1,000 kWh a month in Texas needs roughly 7.5 kW, which is about 19 panels at 400 W. The same home in New York needs around 25, because there are fewer peak sun hours.
How much power does a 400 watt solar panel produce?
About 1.7 kWh a day at 5 peak sun hours once losses are taken out, so roughly 600 kWh a year. In Arizona it is closer to 2.2 kWh a day, in Washington nearer 1.3.
What is a peak sun hour?
One hour of sunlight at 1,000 watts per square metre, which is the condition panels are rated under. A location with 5 peak sun hours receives the equivalent of five hours at full strength, spread across the whole day.
Why is my array producing less than its rating?
The nameplate is measured in laboratory conditions. Real systems lose output to soiling, shading, wiring, panel mismatch and inverter conversion, which together take roughly 17%. Output also falls as panels get hotter, which is why a cool bright day often beats a hot one.
Should I size for 100% of my bill?
Not always. Many utilities pay less for exported energy than they charge for imported, so covering every kilowatt hour can mean selling a surplus cheaply. Check how your utility handles net metering before sizing past about 100%.