What is Standard Test Conditions (STC)?
The laboratory conditions (1,000 W/m², 25°C) used to rate solar panel output, real-world output varies.
Quick Answer
Real-world output in the UK is typically 75–85% of the STC-rated output. UK irradiance levels average 100–400 W/m² (vs 1,000 W/m² at STC), but panel temperatures are usually below 25°C, which actually boosts efficiency slightly. PVGIS provides accurate estimates accounting for UK conditions.
Fact-checked by John Rooney, Solar Energy Editor. Editorial policy
Standard Test Conditions (STC) Explained
Standard Test Conditions (STC) are the internationally agreed laboratory conditions used to test and rate solar panel output. STC specifies: irradiance of 1,000 W/m², cell temperature of 25°C, and air mass of 1.5. These conditions allow fair comparison between panels from different manufacturers. In practice, real-world conditions differ from STC, irradiance is usually lower, and panel temperatures are often higher, resulting in output below the rated wattage.
How Does Standard Test Conditions (STC) Work in the UK?
Real-world output in the UK is typically 75–85% of the STC-rated output. UK irradiance levels average 100–400 W/m² (vs 1,000 W/m² at STC), but panel temperatures are usually below 25°C, which actually boosts efficiency slightly. PVGIS provides accurate estimates accounting for UK conditions.
Frequently Asked Questions
Will my panels produce their rated output?
Rarely in the UK, as STC assumes 1,000 W/m² of sunlight (roughly noon in June). Actual output varies with weather and season. Annual output estimates from PVGIS are more useful than peak ratings.
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John Rooney is the founder of Solar Info and has been covering the UK solar energy market since 2023. He researches every battery and inverter brand against manufacturer datasheets, MCS and Ofgem data, and feedback from the MCS-certified installers in our directory before publishing.