The amount of electromagnetic energy (solar radiation) incident on the surface of the earth. Basically that means how much sunlight is shining down on us.

Why is knowing the irradiation level useful?

By knowing the irradiation levels of a particular region we can determine the size of solar collector that is required. An area with poor irradiation levels will need a larger collector than an area with high irradiation levels. Once you know your region's irradiation level you can more accurately calculate collector size and energy output.

What units are used to express irradiation levels?

The values are generally expressed in kWh/m²/day. This is the amount of solar energy that strikes a square metre of the earth's surface in a single day. Of course this value is averaged to account for differences in the day's length. There are several units that are used throughout the world.

The conversions based on surface area as follows:

1 kWh/m²/day = 317.1 btu/ft²/day = 3.6MJ/m²/day

The raw energy conversions are:

1kWh = 3412 Btu = 3.6MJ = 859.8kcal

Is my region's irradiation level low, moderate or high?

The following scale is a basic guide for irradiation levels. Although a value of 5 is not considered very high during the summer months, as an average annual value this is very high. You will see that in central Australia, which is a hot, sunny place, the annual average irradiation is 5.89.

You may compare you location to the following two extreme locations.
Average annual irradiation levels:

Central Australia = 5.89 kWh/m²/day - Very High
Helsinki, Finland = 2.41 kWh/m²/day - Very Low

What is the irradiation level in my area?

If you would like to find the irradiation levels for your area, please visit the NASA website below. You just need to know your latitude and longitude.
Free registration at the site is required for access.
Nasa Surface meteorology and Solar Energy Data Set

Now I know my irradiation level, how do I calculate my collector's size?

Click here to visit the collector sizing guide