Solar Panels Needed Calculator
Sizing a solar array is a division problem. Work out how much energy one panel produces where you live, then divide your daily consumption by that number.
Panels needed
20
- System size
- 8
- Daily output per panel
- 1.548
- Annual output
- 11,300.4
- Share of usage covered
- 103.20%
- Roof area needed
- 430
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
The formula this calculator uses
panels = ceil(daily kWh / (panel kW x peak sun hours x (1 - losses)))
- daily kWh
- Your average electricity use per day
- peak sun hours
- The equivalent hours of full strength sun per day
- panel kW
- The rated output of one panel, watts divided by 1000
- losses
- Everything lost to wiring, heat, dirt and the inverter
How to check the result by hand
- 1
Find your daily electricity use
Read the annual total off your bill and divide by 365. A home using 10950 kWh a year uses 30 kWh a day, which is the number the array has to cover.
- 2
Work out what one panel produces
A 400 watt panel is 0.4 kilowatts. At 4.5 peak sun hours with 14 percent system losses it delivers 0.4 times 4.5 times 0.86, which is 1.548 kWh a day.
- 3
Divide and round up
Thirty divided by 1.548 is 19.38 panels, so 20 go on the roof. Rounding up matters because a short array leaves you buying grid power at the worst tariff.
- 4
Check the roof and the coverage
Twenty panels at 21.5 square feet each need about 430 square feet, and the array produces 11300.4 kWh a year, which is 103.2 percent of the 10950 kWh the home uses.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Solar Panels Needed page.