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Solar Panel Output Calculator

Estimate the daily, monthly, and yearly energy production of your solar panel system.

How it is calculated

Daily energy is kWh/day = capacity × sun hours × efficiency / 100. Capacity is the array's rated size in kW, sun hours are the peak sun hours per day at your location (hours of full-strength sunlight, not hours between sunrise and sunset), and efficiency is the system derate in percent that covers inverter, wiring, dust and heat losses. If you leave efficiency empty the tool uses 80%. Monthly output is the daily figure times 30 and yearly output is the daily figure times 365, so it is an average and ignores seasons and panel ageing.

Worked example

A 6 kW system at a site with 5 peak sun hours and an 80% derate. Ideal output: 6 × 5 = 30 kWh per day. After losses: 30 × 80 / 100 = 24.00 kWh per day. Monthly: 24 × 30 = 720 kWh. Yearly: 24 × 365 = 8,760 kWh.

Try this example →

Common mistakes

  • Entering daylight hours (say 12) as sun hours. Peak sun hours are much fewer, since morning and evening light is weaker.
  • Typing the capacity in watts. The field is in kW, so a 6,000 W array is 6.
  • Entering efficiency as 0.8. It is a percentage, so type 80.
  • Treating the yearly figure as guaranteed. Winter months produce far less than summer ones, and the 365-day number is only an average.

About this calculator

This calculator estimates a solar system's energy output from its size in kW, the local peak sun hours per day (a measure of usable sunlight, not daylight hours), and a system derate factor that accounts for inverter losses, wiring, dust, and temperature effects, giving daily, monthly, and yearly kWh production.

Homeowners considering solar use it to sanity-check an installer's quote against their own electricity bill, and installers use the same math to size a system for a customer's expected usage. Peak sun hours vary by location and season, so the estimate is only as good as the input.

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