Enter your monthly kWh, local peak sun hours and a panel wattage to estimate the system size and panel count that offsets your bill.
What sizes a solar system
Three numbers size every rooftop system: how much electricity you use, how much usable sun your location gets, and what each panel produces. Enter your monthly kWh from the utility bill, your area’s peak sun hours, and the panel wattage you’re considering — the calculator divides through, applies a standard 20% allowance for real-world losses, and returns the system size and panel count.
How to use it
- Read the monthly kWh off your electricity bill (average a full year if you can).
- Look up peak sun hours for your region — typically 3–6 depending on climate.
- Enter the panel wattage; 400 W is a common modern residential panel.
The formula
A worked example
900 kWh a month is 30 kWh a day; at 5 peak sun hours with losses that needs a 7.5 kW system — 19 panels at 400 W. The same usage in a 3.5-sun-hour climate needs 10.7 kW and 27 panels: location moves the answer more than any equipment choice, which is why the sun-hours input matters most.
An honest boundary
This is the sizing arithmetic, not a quote: roof orientation, shading, local rules and net-metering terms all move the real design. Use it to sanity-check installer proposals — a quote wildly off this math deserves questions.