What is solar irradiance and how is it measured?

Solar irradiance is the power of solar radiation per unit area, measured in W/m^2. Key terms: Global Horizontal Irradiance (GHI) - total on horizontal surface, Direct Normal Irradiance (DNI) - direct beam perpendicular to sun, and Diffuse Horizontal Irradiance (DHI) - scattered light. Standard Test Conditions use 1000 W/m^2 to rate panels. Measurement uses pyranometers (GHI, DHI) and pyrheliometers (DNI). Solar resource data from satellites or ground stations enables yield predictions. Peak sun hours is irradiance over 1000 W/m^2 threshold per day.