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.