Solar Panel Calculator
Size a rooftop solar system from your electricity use and sunshine, and estimate cost, savings and payback.
About the Solar Panel Calculator
Each kilowatt of panels produces roughly peak sun hours × 365 × (1 − losses) kWh a year. Divide your yearly use by that to get system size, then by the panel rating for the number of panels. Savings depend on how much power you use yourself versus export, and on export tariffs. In the US the 30% residential clean energy credit (25D) ended for systems installed after 31 December 2025.
The formula
kW = yearly kWh ÷ (sun hours × 365 × (1 − losses))
Worked example
Example: System size = 7.99 kW (17 panels)
- Inputs: Electricity use per month: 990 kWh; Peak sun hours per day: 5 hours; Share of use to cover: 100%; Panel rating: 470 W; System losses: 15%; Installed cost per watt: $3; Electricity price per kWh: $0.
- Formula: kW = yearly kWh ÷ (sun hours × 365 × (1 − losses))
- Result: System size = 7.99 kW (17 panels)
- Installed cost: $23,970
- Yearly saving: $0
- Payback: ∞ years
Things to keep in mind
- Shading, roof direction and tilt change output; use a site tool such as NREL PVWatts or PVGIS for a precise estimate.
Questions people ask
How much roof space per panel?
About 2 m² (20 ft²) for a modern 400–450 W panel.
Do I need a battery?
Not to save money in most places; batteries help with backup and time-of-use tariffs.