How to size a solar system
Solar is sized by energy, not by roof or house size. Add up a year of kWh from your bills, decide how much of it you want solar to cover, and divide by how many kWh one kilowatt of panels makes where you live:
kW needed = annual kWh × offset ÷ kWh per kW per year, then panels = kW × 1,000 ÷ panel watts, rounded up.
Worked example: 1,000 kWh a month is 12,000 kWh a year. A typical U.S. site makes 1,340 kWh per kW, so 12,000 ÷ 1,340 = 8.96 kW. With 440-W panels that is 8,955 ÷ 440 = 20.4 → 21 panels (9.24 kW), producing about 12,382 kWh a year.
Find your real usage first
Your bill lists kWh used each month; many utilities also show a 12-month history online. Add up all twelve months rather than doubling a summer or winter bill. If you only know your dollar total, divide it by your all-in price per kWh — the calculator does this in “Monthly bill” mode using your state's 2026 average. For scale: the U.S. average home used 874 kWh a month in 2025, from 494 in California to 1,225 in Louisiana.
Where the kWh-per-kW numbers come from
Each state uses a representative city modeled for a fixed roof at 20° tilt facing south with 14% system losses (wiring, soiling, shading, heat and inverter) — the defaults of PVWatts v8 from the National Laboratory of the Rockies (formerly NREL). Ten states come straight from PVWatts; the rest use the European PVGIS model, which landed within about 7% of PVWatts where both were run. Results range from about 1,077 kWh per kW in Seattle to 1,780 in Albuquerque.
Peak sun hours are the same idea in different units: the day's sunlight on the panels expressed as hours at 1,000 W/m². Phoenix averages 6.54, Boston 4.7. Sun-hour tables based on 1961–1990 weather data still circulate; the numbers here use the NSRDB typical year behind PVWatts v8, or ERA5 data for the PVGIS states.
Panels needed by monthly use
| Monthly use | System (U.S. typical) | 440-W panels | Arizona | Washington |
|---|---|---|---|---|
| 500 kWh | 4.8 kW | 11 | 8 | 13 |
| 1,000 kWh | 9.2 kW | 21 | 16 | 26 |
| 1,500 kWh | 13.6 kW | 31 | 24 | 38 |
| 2,000 kWh | 18 kW | 41 | 32 | 51 |
Panel wattage and roof space
Residential panels in 2026 are mostly 430–460 W and 22–23% efficient; 250–400 W figures in older guides overstate the panel count. A 440-W panel covers about 21.5 sq ft. Add roughly 25% for fire-code access paths, ridge setbacks and gaps — the calculator divides the panel area by 0.8. Berkeley Lab found arrays cover a median 22% of a one-story roof and 31% of a multi-story roof, so south- and west-facing planes without shade are what count, not total roof area.
How much should solar cover?
- 1:1 net metering: size to about 100% of use. Exports bank credits at the full retail price.
- Net billing (California, Arizona, Utah, Georgia, Idaho and others): exports earn a fraction of retail, so about 70% often pays back faster.
- Plan ahead: an EV or heat pump can add thousands of kWh a year. Adding panels later costs more per watt than sizing for it now.
Common mistakes
- Sizing from one summer bill. Use a 12-month total; usage swings a lot by season.
- Using square footage. Two 2,000 sq ft homes can differ by 2× in kWh depending on heating, cooling and appliances.
- Oversizing past 100%. Most utilities pay little for surplus at year-end, and many cap system size at your annual use.
- Forgetting shade and orientation. West-facing arrays produce less in total but more in the evening, which helps under time-of-use rates.
Next: see the cost, 2026 tax-credit status and break-even year in the solar payback calculator, or jump to California, Texas, Florida or Arizona. Check your rate on electricity rates by state.