Estimate only. Your installer will confirm the size with your roof, shading and utility rules.
How to find your yearly usage
Add up the kWh of your last 12 electric bills, or look for the yearly total that many utilities show on the bill or in your online account. A full year captures both summer cooling and winter heating.
How the size is calculated
System size (kW) = yearly kWh × share you want to cover ÷ yearly production of 1 kW of panels in your state.
The production per kW comes from PVWatts by the National Laboratory of the Rockies (formerly NREL), for a standard roof system facing south at a 20° tilt with 14% losses, at your state's 2020 Census center of population. Sunny states need fewer kW for the same usage: 1 kW produces about 1,750 kWh a year in Arizona and about 1,060 kWh in Washington.
Without the federal tax credit
The federal Residential Clean Energy Credit does not apply to systems placed in service after December 31, 2025 (IRS). If you are deciding on size by cost, covering less than 100% of your usage can make sense when your utility pays little for exported energy.
Example: 10,800 kWh a year
A home that uses 10,800 kWh a year (900 kWh a month) needs a different system in each state, because the same panel produces more in sunny places:
| State | Production of 1 kW | System for 100% |
|---|---|---|
| Arizona | 1,750 kWh | 6.17 kW |
| California | 1,679 kWh | 6.43 kW |
| Texas | 1,507 kWh | 7.16 kW |
| New York | 1,253 kWh | 8.62 kW |
| Washington | 1,060 kWh | 10.19 kW |
The calculator does the same division with your own usage, your state and the share you want to cover.
How to read your result
- The size is in kW of panels (DC). It is the number installers use in quotes. The yearly production is what those panels should generate in an average year.
- Production changes from year to year with the weather, and panels slowly lose output over time. Plan around the average, not a promise for every year.
- The number of panels depends on their wattage: higher-wattage panels need fewer of them for the same kW, which matters when roof space is tight.
Choosing how much to cover
Covering 100% of your usage makes sense when your utility credits exported energy at or near the retail rate. When exports earn much less, as under California's net billing, a smaller system that matches your daytime use can pay back faster. Compare both with the solar payback calculator or, in California, the NEM 3.0 calculator.
Common mistakes
- Using one month of usage. Add up 12 months so both cooling and heating seasons count.
- Ignoring future loads. An electric car or a heat pump can add thousands of kWh a year.
- Comparing quotes by panel count. Compare the system size in kW and the price per watt instead.