Irradiance

Irradiance is the instantaneous power of solar radiation hitting a surface, measured in watts per square meter (W/m²). While insolation measures total solar energy over time, irradiance measures the intensity of sunlight at any given moment.

Solar panels are rated under Standard Test Conditions (STC) at an irradiance of 1,000 W/m², which represents bright, direct sunlight at noon on a clear day. In practice, irradiance changes constantly throughout the day — it’s zero at night, increases through the morning, peaks around solar noon, and decreases through the afternoon. Cloud cover, haze, and atmospheric conditions also affect irradiance moment by moment. A panel rated at 400W will produce 400 watts only when irradiance is exactly 1,000 W/m² and the cell temperature is 25°C. At 500 W/m² (overcast sky), the same panel produces roughly 200W. Understanding irradiance helps explain why solar production varies throughout the day and why your system’s real-time output shown on the monitoring app fluctuates. Solar design software uses historical irradiance data to model expected production for your specific location.