Solar Panel Output Calculator

Estimate how many kilowatt-hours your solar panels make a day, a month and a year.

Formula explained below

Hours of full sunshine a day, averaged over the year, for your location. Find yours from a solar map or tool.

Wiring, inverter, dirt, heat and shade. 14% is a common planning figure, not a measured value; change it.

Energy per day

17.20 kilowatt-hours

Per month
523 kWh
Per year
6,278 kWh
Array size (kW)
4

The array makes about 17.2 kWh a day on average.

Calculation breakdown

StepFormulaResult
Energy per day400 W × 10 panels × 5 h × (1 − 14%) ÷ 1,00017.200 kWh
Energy per year17.2 kWh × 3656,278 kWh

How it works

Enter the panel rating, the number of panels, your peak sun hours and the system losses.

kWh per day = Watts × Panels × Peak sun hours × (1 − Losses) ÷ 1,000kWh per month = kWh per day × 365 ÷ 12 kWh per year = kWh per day × 365

Worked example

Ten 400 W panels, 5 peak sun hours, 14% losses:

  1. Per day: 400 × 10 × 5 × 0.86 ÷ 1,000 = 17.2 kWh
  2. Per month: 17.2 × 30.42 = about 523 kWh
  3. Per year: 17.2 × 365 = 6,278 kWh

Sources

Frequently asked questions

How do you calculate solar panel output?

Multiply the panel watts by the number of panels, the peak sun hours and one minus the losses, then divide by 1,000. Ten 400 W panels at 5 peak sun hours and 14% losses make 17.2 kWh a day.

What are peak sun hours?

They are the hours in a day at which sunlight is the strength of 1,000 watts per square meter, which would give the same energy as the real day. It is not the same as hours of daylight, and it varies by place and season. Enter an average for your location.

What are system losses?

Energy lost to wiring, the inverter, dirt, heat, shading and panel mismatch. The 14% in the calculator is a common planning figure, not a measured value for your system, and you can change it.

How do I get kWh per month and per year?

The calculator multiplies the daily figure by 365 for a year and by 365 divided by 12 for a month. Summer months make more than winter months, so the monthly figure is an average.

Will my real output match?

Rarely exactly. Weather, panel angle, shading, temperature and panel age all change it. Use this as an estimate, and use a tool that models your roof, such as NREL's PVWatts, for a detailed forecast.

Solar System SizeArray size in kW and panels for your usage.Solar Panel AngleBest tilt and direction for fixed solar panels.Battery RuntimeHow long a battery bank runs a load.Electricity CostCost of running an appliance from watts, hours and rate

An estimate for information only. Peak sun hours and system losses are your own inputs, and NREL's PVWatts documentation could not be opened to cite its default.