About this tool
Solar Panel System Size Calculator is a free, in-browser tool that turns your daily electricity use into the array size you need: the system rating in kilowatts, the number of panels, and a rough roof-area estimate. Everything is computed locally as you type — nothing is uploaded.
The array size is dailyKWh ÷ (peakSunHours × derate), where derate (0.75–0.85, default 0.8) accounts for real-world losses from inverter efficiency, wiring, dust, heat and panel tolerance. The panel count is ceil(kW × 1000 ÷ panelWatts), rounded up so the array meets or exceeds the target, and roof area is panels × the area per panel.
Use it to scope a rooftop or off-grid project: enter your average daily kWh (from a utility bill divided by 30), your location's peak sun hours, and the wattage of the panels you are considering. Try 400 W versus 500 W panels to see how module choice changes the count and footprint.
Frequently asked questions
What are peak sun hours?
Peak sun hours are the equivalent number of hours per day at full 1000 W/m² sunlight — a way to fold a whole day's variable irradiance into one figure. Typical annual averages run from about 3 hours in cloudy northern climates to 6+ in desert regions.
Why include a derate factor?
A panel almost never delivers its nameplate wattage. Inverter losses, wiring resistance, heat, soiling and manufacturing tolerance shave off 15–25%, so a derate of 0.75–0.85 (0.8 by default) gives a realistic size rather than an optimistic one.
Why is the panel count rounded up?
Rounding up with a ceiling ensures the installed array meets or slightly exceeds the target kW. Rounding down would leave the system undersized for your stated daily use.
Is this the same as the size an installer will quote?
It is a solid first estimate. A full design also weighs roof orientation, shading, string sizing and local code, but this tool gives you the ballpark kW and panel count to sanity-check any quote. All math runs in your browser.
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