Solar insolation is how solar radiation - or energy from the sun - is measured. It's also called "solar irradiation", and either measured in hourly or daily outputs.
Usually, professionals in the solar industry use the measurement unit watt-hours per square meter to express the energy. Therefore, the solar insolation - or irradiance - is usually expressed as so many Watts, per square meter, per hour.
Therefore, normally when you see a solar insolation chart (such as below), the numbers indicate the average measured watts per square meter per hour.
This insolation chart shows the estimated average solar production for the United States.
You can download a solar insolation chart for a specific month, angle, or output using the resources found at the National Renewable Energy Laboratory's (NREL) website.
Our technical support team assists our dealers in solar system design, sizing, and output calculations using charts such as these. If you would like assistance in solar system design or output projections, a service we provide to our network of dealers, please contact us.