Solar System Size Calculator
Work out how many solar panels and what system size you need, and how much roof they take.
Your result
Enter your numbers and press the button. Your result appears here.
System size in kW equals your yearly use divided by how many kWh each kW of panels makes in your area.
How it works
Size (kW) = yearly kWh to cover ÷ yearly kWh per kW. Panels = size in watts ÷ panel watts, rounded up.
Example: 825 kWh a month needs 7.24 kW, or 19 panels of 400 W covering about 399 ft².
At a glance
Same settings as the calculator, changing only the monthly bill.
| Monthly bill | System | Panels | Roof area |
|---|---|---|---|
| $100 | 4.83 kW | 13 | 273 ft² |
| $150 | 7.24 kW | 19 | 399 ft² |
| $200 | 9.65 kW | 25 | 525 ft² |
| $250 | 12.06 kW | 31 | 651 ft² |
| $300 | 14.48 kW | 37 | 777 ft² |
Common questions
How many solar panels does an average house need?
The average U.S. home used 865 kWh a month in 2024. At the national average output of 1,367 kWh per kW, that takes about 7.6 kW, or 19 panels of 400 W.
How much roof space do solar panels need?
A 400 W panel is about 21 ft², so 20 panels cover about 420 ft² before walkways and setbacks.
Should I cover 100% of my use?
Under full net metering, yes. Where exports earn less, a smaller system that matches your daytime use may pay back faster.
Sources
- U.S. EIA, Today in Energy: average residential use 865 kWh a month in 2024: https://www.eia.gov/todayinenergy/detail.php?id=65244
- NREL PVWatts v8 (20° tilt, south-facing, 14% losses), state results compiled by TheGreenWatt: https://thegreenwatt.com/average-peak-sun-hours-by-state/
- EnergySage, Solar panel cost by state (updated November 6, 2026): https://www.energysage.com/local-data/solar-panel-cost/