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Solar Panel Count Calculator

Estimate the number of solar panels and the system size needed to cover your daily energy use.

How it is calculated

First the tool works out what one panel delivers per day: Wh per panel = panel watts × peak sun hours × efficiency ÷ 100. Then it divides your daily need by that and rounds up to a whole panel: panels = ceil(daily kWh × 1000 ÷ Wh per panel). System size is panels × watts ÷ 1000 in kW, and the expected output is panels × Wh per panel ÷ 1000 in kWh per day. Consumption, panel watts and peak sun hours must be above zero. Efficiency is a percentage between 1 and 100; if you leave it empty, or enter 0 or more than 100, the tool silently uses 80%. Peak sun hours are not clock hours of daylight: they are the equivalent hours of full-intensity sunlight, which varies with location and season. The tool gives a single average day and does not size batteries or account for roof shading or orientation.

Worked example

A household uses 12 kWh a day, panels are rated 400 W, there are 4.5 peak sun hours and efficiency is 80%. One panel yields 400 × 4.5 × 0.80 = 1,440 Wh a day. The need is 12,000 Wh, and 12,000 ÷ 1,440 ≈ 8.33, rounded up to 9 panels. System size = 9 × 400 ÷ 1000 = 3.60 kW, and daily output = 9 × 1,440 ÷ 1000 = 12.96, shown as 13.0 kWh, slightly above the need because of the rounding up.

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Common mistakes

  • Entering the monthly or yearly consumption in the daily field. Divide a monthly bill figure by about 30 first.
  • Using the hours between sunrise and sunset as peak sun hours. The correct figure is much lower, since it is the equivalent of full-intensity sun.
  • Using the same sun-hours figure for winter and summer. Sizing for the average day leaves a shortfall in the darkest months.
  • Entering efficiency as 0.8. It is a percentage, so type 80. A value below 1 is read as a very low efficiency, and a value above 100 or 0 is replaced by 80 without warning.
  • Reading the panel count as a final design. Shading, roof angle, available area and local rules also decide what can be installed.

About this calculator

This calculator works backward from your daily energy consumption in kWh to the number of solar panels needed, dividing that demand by the output a single panel realistically produces in a day — panel wattage × peak sun hours (the local average of full-intensity sunlight equivalent hours) × system efficiency, which discounts for inverter losses, wiring, temperature derating and panel soiling, typically bringing real-world output to 75-85% of the panel's rated capacity.

Solar installers use this sizing method during initial consultations to give homeowners a rough panel count and system size (in kW) before a formal site survey, and homeowners planning an off-grid cabin or RV system use it to make sure their proposed array can actually cover daily usage through cloudier, lower-sun-hour seasons.

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Comments (1)

  • Editorial team

    Peak sun hours are numerically the same as the daily solar irradiation for your location in kWh per square metre, so a local solar map or installer quote gives a usable figure. Run the tool twice, once with an average-day value and once with a winter value, to see how many more panels a December-proof system needs. Where is the system going, and which sun-hours figure did you use?

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