How many solar panels fit on my roof?

Divide usable roof area by one panel's footprint and round down. Setbacks, fire access, obstructions, roof shape and structural limits can reduce the real count.

How many hours will a home battery run essential loads?

Divide usable delivered battery energy by the average essential load in kW. Real backup also depends on inverter power, surge loads, temperature, battery reserve and whether solar can recharge during the outage.

Does a battery save the full retail value of stored solar?

No. Exported solar already has an export value. The incremental value is delivered battery energy times the retail price minus all solar energy used for charging times the export credit. This accounts for lost export revenue on charging losses.

Solar & Battery Calculator: Output, Backup and Payback

Estimate rooftop solar panels, output, self-use, exports, battery size, backup hours, shifted energy, payback and 20-year cash flow.

Size rooftop solar, storage and backup

Turn roof area into panel count and annual energy, then compare solar-only economics with usable battery capacity, shifted exports, essential-load backup and a transparent 20-year scenario.

Roof area (sq ft)Usable roof share (%) Panel area (sq ft)Panel rating (W) Specific yield (kWh/kW/year)Total system losses (%) Home electricity use (kWh/year)Solar self-consumption (%) Utility price ($/kWh)Export credit ($/kWh) Installed cost ($)Annual operating cost ($) Annual degradation (%)Utility price growth (%)

Optional home battery scenario

Battery nameplate capacity (kWh)Usable depth of discharge (%) Round-trip efficiency (%)Full-equivalent cycles/year Installed battery cost ($)Essential backup load (kW) Annual battery capacity fade (%)

Battery shifting is limited by PV surplus, annual cycling capacity and the household demand left after direct solar use. Charging uses more energy than the battery delivers: export revenue is reduced by all energy sent to charge, including losses. This annual envelope assumes feasible self-consumption, not optimized hourly dispatch; it can have a negative bill value when export rates are higher. Backup duration is a simplified conservative energy estimate using the entered efficiency, not a power or outage guarantee. NREL SAM: battery efficiency and energy flows.

How the solar and storage estimate works

Panel count is usable roof area divided by panel footprint, rounded down. Annual output is DC kW × location-specific yield × output after losses. Self-used energy cannot exceed generation or home use.

Usable battery AC energy = nameplate kWh × usable depth × round-trip efficiency. Charging is capped by available solar exports and usable DC capacity × cycles. Delivered energy is also capped by remaining household demand. Extra value = delivered kWh × retail price − charging kWh × export credit; charging losses also forgo export revenue.

Use location and hourly data before buying

NREL PVWatts refines solar output using weather, tilt, azimuth, array type and losses. NREL SAM models behind-the-meter storage with hourly dispatch; this annual scenario cannot reproduce seasonal state of charge, time-of-use rates, demand charges or outages.

NREL PVWatts · NREL SAM storage documentation · DOE residential renewable energy guide

This tool is not a roof check, electrical or protected-load design, tax analysis, battery warranty model or installer quote.