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Solar System Size Calculator

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

How to use

  1. Enter your values in the fields above.
  2. Press Calculate to see your result instantly.
  3. Use the Share button to copy a link to your result.

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 × 30.4 × 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.

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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