Solar Size Calculator

Find the system size and panel count needed to offset your electricity usage.

This sizes a system to your electricity usage. Want to know what your roof can physically fit instead? See the Solar Roof Calculator.

This tool provides a planning estimate based on the assumptions shown below the results — it is not a quote, and actual costs and savings will vary by installer, local rates, and your specific home.

Your state

Sets peak sun hours below to your state's average (NREL/PVWatts) — edit it if you know your actual site conditions.

Annual electricity usage

kWh/yr

Check your annual usage total on your utility account — average U.S. households use roughly 10,000–11,000 kWh/yr.

Peak sun hours per day (your region)

hrs

Roughly 3.5–4 in the Northeast/Midwest, 4.5–5.5 in the Southwest — check NREL's PVWatts map for your ZIP code.

Target offset of your usage

100% targets fully offsetting current usage. Go above 100% if you're planning to add an EV or heat pump.

Panel wattage (for panel count estimate)

Enter your usage and region to see a recommended system size.

How this calculator works

System size = (target annual production) ÷ (peak sun hours × 365 × a real-world derate factor of 0.8, accounting for inverter losses, wiring, soiling, and temperature effects vs. nameplate rating). Panel count is system size divided by your chosen panel wattage, rounded up.

This estimates a starting point for a conversation with an installer, not a substitute for a site-specific production estimate that accounts for your actual roof orientation and shading.

Source: NREL PVWatts Calculator methodology; DOE Solar Photovoltaic Technology Basics, last updated 2026-07-01.