Insolation

Insolation (incoming solar radiation) is the total amount of solar energy received on a given surface area over a specific period of time. It is typically measured in kilowatt-hours per square meter per day (kWh/m²/day) and is a key factor in estimating how much electricity a solar system will produce at a given location.

Insolation values vary dramatically across the United States. The Southwest (Arizona, Nevada, New Mexico) receives the highest insolation at 6–7+ kWh/m²/day, while the Pacific Northwest and upper Northeast receive 3–4 kWh/m²/day. These values are often expressed as “peak sun hours” — the equivalent number of hours per day at full 1,000 W/m² sunlight. A location with 5.0 kWh/m²/day of insolation gets 5 peak sun hours. Solar installers use insolation data from sources like NREL’s NSRDB (National Solar Radiation Database) and tools like PVWatts to estimate annual production for a specific site. Higher insolation means more solar production, but even areas with moderate insolation can generate excellent returns on solar investment due to higher electricity prices in those regions.