Solar Panel Array Sizing Calculator
Size a solar array from your daily energy use, local peak sun hours and panel wattage — the kW needed and the number of panels.
How it works
Array W = daily kWh × 1000 ÷ (sun hours × efficiency) · Panels = ⌈array W ÷ panel W⌉
Peak sun hours convert a whole day of sunlight into an equivalent number of hours at full 1000 W/m² intensity, so an array’s daily output is its wattage times sun hours. Working backwards from your daily kWh gives the wattage required, inflated by a system derate factor (~75–85%) that covers inverter loss, wiring, heat, dust and panel tolerance. Dividing by panel wattage and rounding up gives the panel count.
Worked example
For 20 kWh/day at 5 peak sun hours and 80% efficiency: 20 × 1000 ÷ (5 × 0.80) = 20,000 ÷ 4 = 5000 W (5 kW). With 400 W panels that is ⌈5000 ÷ 400⌉ = 13 panels, an installed 5.2 kW.
Frequently asked questions
What are peak sun hours?
Peak sun hours are the day’s total solar energy expressed as hours of full-strength (1000 W/m²) sunlight. A location averaging 5 kWh/m²/day has 5 peak sun hours, even though the sun is up longer. Look up your region’s figure from a solar irradiance map.
Why include a system efficiency factor?
Panels never deliver their nameplate wattage in the field — inverter conversion, wiring resistance, heat, soiling and manufacturer tolerance all take a cut. A derate of 75–85% turns the ideal figure into a realistic one, so the array is sized large enough.
Does this size the battery bank too?
No — this covers only generation to match daily use. Off-grid systems also need battery storage sized for the nights and cloudy days you want to ride through, plus extra panel capacity to recharge them.
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This calculator is for educational purposes. Double-check important results before acting on them.