Bill-based sizing is an early planning estimate. It converts the bill's estimated energy portion into kWh using the selected state's residential average electricity rate, then divides the target annual solar production by a state planning yield. Your actual utility rate, fixed charges, taxes, time-of-use pricing, export rules, shading and roof geometry can materially change the result. If your bill shows annual kWh, use that value in the Solar Panel Calculator for a better starting point.
How the bill-based estimate works
- Subtract the fixed monthly charges you enter from the average bill.
- Convert the remaining annual energy spend to estimated kWh using the selected state's residential average electricity rate.
- Apply your target solar offset percentage.
- Divide target annual solar production by the state's planning solar yield to estimate DC system size.
- Round up to a whole number of panels using the panel wattage you entered.
Why an electric bill is only a shortcut
A $200 bill does not mean the same electricity use in every state or utility territory. Bills can include fixed customer charges, taxes, fuel riders, demand charges and time-of-use pricing. Solar export credits can also be lower than the retail import rate.
If you have the last 12 months of actual kWh from your utility account, use the Solar Panel Calculator instead. If you know your ZIP code, use the Home Solar Calculator for ZIP-specific PVWatts production.
System size is not the same as roof fit
- Shading, orientation and roof pitch change real production.
- Setbacks, vents, chimneys, skylights and fire-access paths reduce usable roof area.
- Panel dimensions differ by model even when wattage is similar.
- Electrical-service upgrades or interconnection limits can affect the final design.
Use the Solar Roof Analyzer for address-level roof insights and the Roof Solar Calculator for usable-area planning.
Data used by this planning tool
Residential electricity-rate inputs come from U.S. EIA Electric Power Monthly, Table 5.6.B, Year-to-date through June 2026. State solar yield is a planning estimate derived from NREL residential PV resource-class ranges, not a site-specific PVWatts result.
These are screening assumptions, not utility quotes or site-specific production forecasts. Review our methodology and sources for more detail.