Terrestrial radiation is a term used to describe infrared radiation emitted from the atmosphere.
This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. All of which can be readily determined. However, the biggest factors affecting the available energy are cloud cover and other meteorological conditions which vary with location and time. Historically, solar measurements have been taken with horizontal instruments over the complete day. In the Northern US, this results in early summer values 4-6 times greater than early winter values.