Solar array and battery sizing for home backup

Size the panels and battery that keep your critical loads running for several days, using the worst month of sun rather than the yearly average. A system sized on the average runs short in December. Get your daily kWh from the backup power calculator or your own list of loads.

Your numbers

Critical loads (kWh per day) Days of autonomy (battery alone) Worst-month peak sun hours at your site

Look up your site in NREL's free PVWatts and take the lowest month of "solar radiation" (kWh/m²/day) for your panel tilt. 3 is a placeholder, not data for your area.

System loss (%, typical 10 to 20) Coupling Usable battery depth of discharge (%, typical: LFP 80 to 100, lead-acid about 50) Battery inverter efficiency (%, typical) Panel size (W) Roof area per 200 W of panel (sq ft, typical about 11)

Reverse mode (optional)

Enter a roof area and/or a budget to see what it yields. We store no prices: type the installed cost per watt from your own quote.

Roof area for panels (sq ft) Budget for the array ($) Your cost per watt ($/W) Battery size you have in mind (kWh nominal, optional)

How this is worked out

Array kW = daily kWh / (worst-month sun hours x (1 - loss) x coupling efficiency). It sizes the array to cover the whole daily load on a worst-month day; on cloudier days the battery makes up the gap.
Battery nominal kWh = daily kWh x days / (inverter efficiency x depth of discharge).
Roof area uses a typical 200 W per about 11 sq ft; real panels vary, and roofs lose area to setbacks, vents and shading. Check your panel's spec sheet.
Not covered: array voltage and string limits (see solar string sizing), battery and inverter power limits, shading, tilt and orientation, and recharge time after a long cloudy run. Efficiencies and loss are typical values, not measurements of any product.
Solar wiring, rapid shutdown and battery installation fall under NEC Article 690 and 706 (2023 NEC); local amendments apply.