Solar irradiance is the measurement of solar radiation power received on a surface, expressed in watts per square meter (W/m²). It is the fundamental metric that determines how much electricity a solar panel system can produce at any given location and is a key factor in designing and sizing residential solar installations.
Solar irradiance varies based on several factors. Geographic latitude affects the angle at which sunlight strikes the Earth's surface — locations closer to the equator receive more direct sunlight. Time of year matters significantly, as the sun's path across the sky changes with the seasons. In the Northeast, summer irradiance can be 2–3 times higher than winter levels. Daily weather patterns, cloud cover, and atmospheric conditions also cause real-time fluctuations.
The solar industry uses Standard Test Conditions (STC) of 1,000 W/m² as the benchmark for rating panel output. When a panel is rated at 400 watts, that means it produces 400 watts under 1,000 W/m² irradiance. Real-world conditions rarely match STC perfectly, which is why actual production is typically 75–85% of the rated maximum.
Solar installers use historical irradiance data from sources like the National Renewable Energy Laboratory (NREL) to model expected system production. Tools like PVWatts use irradiance maps combined with your specific roof orientation, tilt angle, and shading factors to predict annual kWh output with impressive accuracy.
Understanding solar irradiance helps homeowners set realistic expectations. Even in the cloudier Northeast, annual irradiance levels support excellent solar production — especially when combined with the region's high electricity rates, which amplify the financial value of every kilowatt-hour produced.