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Solar System Size Calculator

Enter your monthly usage, peak sun hours, and derate to size your solar system.

Result

8.33 kW
21 panels
30.00 kWh/day

How it is calculated

The array size is found by dividing the energy you need per day by what one kilowatt of panels produces per day. kW = monthly kWh / (30 × peak sun hours × derate). Peak sun hours are the hours per day of full-strength sunlight at your site, and the derate is a fraction (0.8 means 20 percent lost to inverter, wiring, heat and dust). The month is taken as a flat 30 days. The tool then counts panels as ceil(kW × 1000 / 400), assuming 400 W panels and always rounding up, and shows the daily output kW × peak sun hours × derate in kWh per day. All three inputs must be above zero. Roof space, shading, tilt, seasonal variation and local grid rules are not considered.

Worked example

A home uses 900 kWh a month, with 4.5 peak sun hours a day and a derate of 0.8. Energy per kW per day: 30 × 4.5 × 0.8 = 108 kWh per month for each kW of panels. System size: 900 / 108 = 8.33 kW, shown as 8.33 kW. With 400 W panels that is 8,333 / 400 = 20.8, rounded up to 21 panels. Daily output is 8.33 × 4.5 × 0.8 = 30.00 kWh, which matches 900 divided by 30 days.

Try this example →

Common mistakes

  • Entering the annual kWh or the daily kWh in the monthly field. The field is for a monthly figure, so use a typical month from your bill, or the yearly total divided by 12.
  • Using the sunshine hours from a weather report instead of peak sun hours. Peak sun hours are the equivalent hours at full intensity, normally much fewer than hours of daylight.
  • Entering the derate as a percentage such as 80. The field expects a fraction like 0.8; entering 80 would shrink the system to about 1 percent of the right size.
  • Sizing to a single monthly average. Summer and winter output differ, so a system sized on the yearly average will produce a surplus in summer and a deficit in winter.
  • Treating the panel count as final. It assumes 400 W panels, so if your panels are 300 W or 550 W, divide the kW figure by your own panel rating.

About this calculator

Sizing a home solar array starts with how much electricity you actually use. The standard formula is System size (kW) = Monthly kWh usage ÷ (peak sun hours × 30 × derate factor), where peak sun hours measure the daily equivalent of full-intensity sunlight for your location and the derate factor (typically 0.75–0.85) accounts for real-world losses from inverter inefficiency, wiring, dust, and panel temperature. The panel count shown assumes 400 W panels, rounded up.

Solar installers and homeowners use this calculation during the design phase to decide how many panels to quote, and utilities reference similar math when evaluating net-metering applications. Because peak sun hours vary by region and season, the same monthly usage can call for a noticeably larger system in a cloudier climate than in a sunny one.

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  • Editorial team

    Panel count assumes 400 W modules and rounds up. If you have a different wattage in mind, divide the system size by it yourself: the default 7.5 kW needs 19 panels at 400 W but 17 at 450 W. Peak sun hours are not daylight hours, and a size based on your average month may undershoot in winter. What panel rating are you considering?

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