Any given surface receives solar radiation energy, whether it is a planet, a terrestrial object inside the atmosphere of a planet, or any object exposed to solar rays outside of an atmosphere. Solar insolation measures this radiation energy as a snapshot of a location at a specific time. In most cases, (including the map below) the insolation is measured as a function of kilowatt-hours per square meter per day.

The following map shows the average solar insolation for the United States by color-code. Scroll over the map key for a zoomed-in view of the scale of kilowatt-hours.