What is a good formula to estimate the solar load on an electronics enclosure?

I realize simulation is probably the way to go on this but I wanted to get an understanding of it. I’m going to put some electronics in a plastic box and it has to sit outside in the full sun. The electronics are directly attached with thermal paste to an aluminum heat-sink on the outside of the box. The box is 100mm x 100mm x 50mm. The electronics run at 7Watts.

After searching for a while I found a formula that says it tells you how many Watts the sun will add to your cooling needs. It is W = (CF * SA) / 4 where W is the added watts from solar, CF is a constant number based on the color of your device and SA is the surface area in square feet. For CF they give 0 for shade or 28 for light gray. So for my example:

my enclosure is 0.43 square feet so for light gray W = 28*0.43/4 = 3Watts.

That number sounds reasonable but I don’t know the source of this formula or what is based on. Is there a better approach I should be using?