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Solar System Size Calculator

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

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.

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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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