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Solar Irradiance vs Solar Panel Output

A panel is a converter: sunlight in, electricity out, at an efficiency of about 20 percent. Irradiance sets the input. Cell temperature, dirt, shading, angle and wiring decide how much of the theoretical output survives.

By the Akermin Editorial Team · 2 min read

Solar panel output rises almost in proportion to irradiance: halve the sunlight and the power roughly halves. The panel’s rated power is its output at 1,000 W/m² and 25 °C cell temperature. In the field the irradiance is usually lower, the cells are usually hotter (each degree above 25 °C costs about 0.3 to 0.4 percent), and soiling, shading, angle and wiring take a few percent more. Expected output is therefore rating × (irradiance ÷ 1,000) × temperature factor × system losses, and a healthy panel lands within about 10 percent of that.

The conversion

A panel converts a fraction of the irradiance on its area into electricity; that fraction is its efficiency, 19 to 23 percent for current crystalline modules. A 2 m² panel at 20 percent efficiency under 1,000 W/m² produces 400 W, which is its rating. See what solar irradiance is.

How output tracks irradiance

  • Current is nearly exactly proportional to irradiance. Half the light, half the amps.
  • Voltage is almost independent of irradiance above a few hundred W/m²; it depends mainly on temperature.
  • Power is therefore close to proportional, with a slight efficiency loss below about 200 W/m².

What reduces it

Factor Typical effect
Cell temperature above 25 °C −0.3 to −0.4% per °C; −10 to −15% on a hot roof
Soiling (dust, pollen, bird droppings) −2 to −5% typical; far more in dusty or dry regions
Shading, even partial −10 to −50%; one shaded cell can drop a whole string
Angle of incidence and reflection −2 to −5% away from noon; more at low sun
Mismatch between panels in a string −1 to −3%
Wiring and connector resistance −1 to −3%
Inverter conversion −2 to −4%
Age (degradation) −0.3 to −0.7% per year

Worked example

A 400 W panel, irradiance measured at 780 W/m² in the plane of the array, panel back temperature 48 °C (cell about 51 °C), temperature coefficient −0.35 %/°C.

  1. Irradiance scaling: 400 × 0.78 = 312 W.
  2. Temperature: 26 °C above 25 °C × 0.35% = 9.1% loss → 312 × 0.909 = 284 W.
  3. Allow 3 to 5% for soiling and wiring → about 270 to 275 W expected.

A tester reading 265 W is healthy; 220 W suggests soiling, shading or a fault worth investigating. See how to measure solar panel output.

Over a day and a year

Because output tracks irradiance, daily energy tracks insolation. Multiply system size by peak sun hours and a loss factor (commonly 0.75 to 0.85) for daily kWh. The solar output estimator does this. Why the curve rises and falls through the day is covered in why solar panel output changes during the day. Instruments for both sides of the comparison are in best solar panel testers.

Fluke IRR1 Solar Irradiance Meter

Fluke IRR1 Solar Irradiance Meter

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ZIBOO FT-1000W Solar Panel Tester MPPT Meter - 1000W Max Power, 80V/35A PV Module Tester for Voc/Isc, Open Circuit Voltage & Short Circuit Current, with Backlight & Data Hold

ZIBOO FT-1000W Solar Panel Tester MPPT Meter - 1000W Max

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Is panel output proportional to irradiance?

Current is nearly proportional; voltage changes little. So power is close to proportional, with a small efficiency drop at very low light and a temperature penalty when the panel is hot.

Why does my 400 W panel only produce 300 W in full sun?

At 1,000 W/m² a panel on a summer roof runs at 50 to 65 °C, losing 10 to 15 percent to temperature. Add a few percent for soiling and wiring and 300 to 340 W is normal.

What efficiency do panels have?

Modern crystalline silicon modules are 19 to 23 percent efficient at standard test conditions. A 2 m² panel at 20 percent captures 400 W from 1,000 W/m².

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.