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Solar Irradiance Explained: Why Some Roofs Make More Power
Irradiance is the sunlight hitting a surface — the single biggest driver of solar output. How to read it, why it varies, and what it means for you.
WaterFuelAndSolarEnergy.com Editorial Team Updated December 2024 6 min read
Solar irradiance is the amount of sunlight energy hitting a square metre of surface per second, usually measured in W/m². It’s the direct driver of how much electricity your panels produce — panels convert a share of irradiance into power.
Rough irradiance on a clear day
| Condition | Irradiance | Notes |
|---|---|---|
| Midday, full sun | ~1000 W/m² | The rated “peak sun” condition |
| Bright overcast | ~200–400 W/m² | Output drops sharply |
| Heavy cloud/rain | ~50–150 W/m² | Minimal output |
| Early morning/late day | ~100–300 W/m² | Low angle sunlight |
Does irradiance change with the seasons?
Yes — sun angle and day length shift irradiance a lot through the year, which is why winter output is far lower in higher latitudes even on clear days.