Best Of

Interesting query. Does anything in the engineering toolbox help with that? Here's the link:

https://www.engineeringtoolbox.com/

Just type in Solar Radiation in the search bar.

What you need is the angle between the perpendicular to the panel and the sun. Given that angle — which, of course, changes with the time of day and the season — then the effective area of the panel is equal the the gross area time the cosine of that angle (not the cosine squared). Using that effective area you can calculate the gross solar radiation.

A useful reference for the math for the angle to the sun is here;

zenith-angle-calculation-Guoyuan-Li.pdf

Now.. That is not.. Repeat NOT related the the efficiency of a solar collector. The efficiency of the collector is, simply, the usable power output from the collecter divided by the net effective area of the panel and the instantaneous solar power — which, of course, varies with atmospheric condition. This efficiency varies with the design of the collector, and may — or may not — also be influenced by the angle between the perpendicular and the direction to the sun. This may be closely related to the cosine of that angle, in the case of collectors with flat glass surfaces or some (but not all) types of photovoltaic arrays. Or it may not…

Hi, Being more of a hands-on type, I'd be tempted to get or build a collector that fits what you're looking for and set it up. Data-log temperatures and you'll know with more certainty than any calculation can give you, just how well it performs. Feed it with warmer and warmer water to see how performance drops off. You'll be able to graph it, and get a clear idea of what sort of temps you can expect from a system using that collector. It certainly is some work to make happen, but you'll get results you can trust.

Yours, Larry