Solar System Size Calculator — How Many Panels Do You Need?
Calculate the right solar panel system size for your home based on your electricity usage and location. Find out how many panels and kW capacity you need.
· Free · No signup required
How to use this calculator
- Enter your average monthly electricity usage in kWh (from your utility bill).
- Use the state selector to load your location's daily sun hours automatically.
- Set what percentage of your electricity you want solar to cover (80-100% is typical).
- Select your panel wattage (400W is current standard).
- See your recommended system size, number of panels and roof space needed.
Understanding your results
How system size is calculated: Required kW = (monthly kWh × 12 × offset %) ÷ (sun hours/day × 365 × efficiency factor). For a home using 875 kWh/month targeting 100% offset at 5 sun hours with 80% efficiency: (875 × 12 × 1.0) ÷ (5 × 365 × 0.80) = 10,500 ÷ 1,460 = 7.2 kW. Round up to the nearest 0.4 kW (one panel increment) for the recommended system size.
The 80% efficiency factor explained: Real solar systems don’t achieve their nameplate output every hour of every day. Losses come from: inverter conversion (~2–3%), wiring resistance (~1–2%), panel temperature (hot panels produce less: ~3–5% loss), soiling (~1–3%), and shading or panel mismatch (~1–5%). NREL’s standard derate factor of 0.86 is used for design; 0.80 is a conservative practical estimate appropriate for sizing calculations.
Roof space planning: A standard 400W panel measures approximately 68" × 44" = 20.8 sq ft. The calculator uses 21 sq ft per panel as a practical estimate including minimal spacing. For a 7kW system using 18 panels: 18 × 21 = 378 sq ft. This assumes unobstructed, south-facing roof space. A roof with multiple skylights, vent pipes or chimneys may reduce usable area significantly — have an installer assess before finalising system size.
Oversizing considerations: Sizing for 120% of current usage makes sense if you plan to add an EV or heat pump in the next 3–5 years. The incremental cost of 2–3 extra panels at time of installation is minimal compared to adding them later (which requires a new permit, new wiring run and additional labour). However, many utilities cap net metering at 100–120% of historical usage, so verify your utility’s policy before oversizing significantly.