How to estimate solar system size from your electric bill

Turn monthly electricity spending into a planning range, then refine it with usage, sunlight, and roof constraints.

Sources reviewed: 2026-09-10

Start with electricity use, not dollars

Collect twelve consecutive months of electricity usage in kilowatt-hours. Spending alone is a weak sizing input because bills can include fixed customer charges, demand charges, taxes, credits, and changing rates. The U.S. Energy Information Administration publishes electricity data for context, but your own bills are the correct starting point.

Add the monthly kWh figures to find annual consumption. Then adjust for known changes such as an electric vehicle, heat pump, pool, home addition, efficiency upgrade, or occupants moving in or out. Separate a confirmed change from a vague future possibility.

If only the energy charge and energy rate are available, estimated usage equals the energy charge divided by dollars per kWh. A $180 energy charge at $0.18 per kWh suggests 1,000 kWh for that billing period. Do not divide the full bill by the rate if the bill includes fixed charges.

Worked example: annual usage to system size

Assume annual use is 10,800 kWh and the goal is to produce 90%, or 9,720 kWh. If a preliminary PVWatts run estimates 1,350 kWh per year for each installed kW, the planning size is 9,720 ÷ 1,350 = 7.2 kW DC.

If the chosen panels are rated at 400 watts, 7.2 kW requires 18 panels. Panel count is a layout result, not the first sizing step. A designer still needs to confirm usable roof area, fire setbacks, structural conditions, shade, inverter limits, and utility interconnection rules.

Production per installed kilowatt

Production varies with solar resource, roof direction and tilt, shade, temperature, inverter selection, clipping, wiring, soiling, and downtime. The PVWatts model uses location and system inputs to estimate energy production, but NREL describes it as an estimate rather than a prediction of a particular system's performance.

Use the Arizona solar guide and Florida solar guide to see how local sunlight, prices, and policy context can differ, then browse the state guide index for your location. A state average cannot replace a roof-specific shade and layout assessment.

Choose an offset target deliberately

A 100% annual offset is not automatically optimal. Low export compensation can favor a smaller array with more on-site consumption. Conversely, planned electrification may justify additional capacity if roof space, budget, and utility sizing rules allow it.

Compare at least three scenarios in the Solar Cost Atlas calculator, such as current usage at 80%, 100%, and a future electrification case. For each, record annual production, expected self-consumption, exports, gross cost, and estimated savings.

Validate the design before signing

Ask each installer for DC system size, AC inverter capacity, panel count, first-year kWh, shade assumptions, loss assumptions, and monthly production. Large differences between proposals often reflect different roof layouts or modeling inputs, not simply better equipment.

Finally, confirm the utility's maximum eligible size, export tariff, application process, and whether historical consumption limits the design. The estimate from a bill is a planning range until the installer and utility approve a site-specific system.