Solar Offset 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.
Offset is the share of your yearly use that solar covers. Many installers aim for 80% to 100%; under net billing a lower offset can pay back faster.
How it works
Size (kW) = yearly kWh to cover ÷ yearly kWh per kW. Panels = size in watts ÷ panel watts, rounded up.
Example: 865 kWh a month needs 6.07 kW, or 16 panels of 400 W covering about 336 ft².
At a glance
Same settings as the calculator, changing only the covered.
| Covered | System | Panels | Roof area |
|---|---|---|---|
| 50% | 3.80 kW | 10 | 210 ft² |
| 70% | 5.32 kW | 14 | 294 ft² |
| 80% | 6.07 kW | 16 | 336 ft² |
| 90% | 6.83 kW | 18 | 378 ft² |
| 100% | 7.59 kW | 19 | 399 ft² |
| 110% | 8.35 kW | 21 | 441 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/