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Solar Array Sizing Calculator

Enter your daily energy need, panel wattage, sun hours, and derate to size your array.

How it is calculated

Inputs are daily energy need in watt-hours (energy), panel rating in watts (Panel wattage), peak sun hours per day (sun) and a derate factor (derate) for losses. Blank sun hours default to 5 and a blank derate to 0.8. One panel yields per panel = panel W × sun hours × derate Wh per day. The panel count is panels = ceil(energy ÷ per panel), always rounded up. Array size is panels × panel W ÷ 1000 in kW and daily generation is panels × per panel Wh. All inputs must be above zero. Derate is a fraction, not a percentage, and the tool rejects a value above 1. It sizes for an average day and ignores batteries, shading, panel tilt, seasonal variation and the inverter size.

Worked example

A home needs 10,000 Wh per day. Panels are 400 W, the site gets 5 peak sun hours and the derate is 0.8. One panel yields 400 × 5 × 0.8 = 1,600 Wh a day. Panels: 10,000 ÷ 1,600 = 6.25, rounded up to 7 panels. Array size: 7 × 400 ÷ 1000 = 2.80 kW. Daily generation: 7 × 1,600 = 11,200 Wh, about 12% more than the need because of the rounding up.

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Common mistakes

  • Entering energy in kWh (10) instead of Wh (10,000); the tool then returns a single panel.
  • Entering the derate as a percentage (80) rather than a fraction (0.8). The tool rejects any derate above 1 and shows no result, so enter 0.8, not 80.
  • Using hours of daylight (12) rather than peak sun hours; peak sun hours are much fewer, so the array would come out far too small.
  • Using the best summer sun-hour value, so the array is too small in winter. Use a figure for the worst month you must cover.
  • Treating the panel count as the full system: battery storage, inverter size and available roof area are not part of this calculation.

Sources

About this calculator

Sizing a solar array starts from your target daily energy demand in watt-hours (Wh; multiply kWh by 1,000) and works backward through two real-world loss factors: peak sun hours (the number of equivalent hours per day the sun delivers a full 1,000 W/m² of irradiance — a location-dependent value, typically 3–6 hours, that's usually much less than actual daylight hours) and a system derate factor, entered as a decimal fraction (accounting for inverter losses, wiring resistance, panel soiling, temperature effects and mismatch, commonly around 0.75–0.80 per NREL's PVWatts methodology).

The calculator divides daily energy need by (panel wattage × peak sun hours × derate) and rounds up to get the number of panels required, then multiplies panel count by wattage to report total array size in kW, along with the daily generation that array would deliver. This is the standard method solar installers and DIY off-grid planners use to size a system so it reliably meets — not just theoretically covers — actual daily energy demand under real irradiance and equipment losses.

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Comments (1)

  • Editorial team

    Most people have their usage on a monthly bill, while this tool wants watt-hours per day. Divide the monthly kWh by 30, then multiply by 1,000: 300 kWh a month is 10,000 Wh a day. Better, take the bill from your highest-use month if you want the array to cover it, since an average month can hide a summer cooling or winter heating peak. Which month on your bill is the heaviest, and roughly what does it show?

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