Sun angle effect on heat output of solar panel

[dustygroundhog]

Can anyone point me to a simple chart of the effect that varying the angle at which the sun strikes a solar panel has on the actual heat output of fafco type solar pool water heating panels? I want to determine whether cost of mounting panels at optimum angle for winter heating (which I know) is justified by difference in heat gain between simply affixing panels to flat roof and mounting them at 50 degree angle best for mid-winter heat gain in San Diego county desert. Can't find a thing on this and obviously can't determine value of angle mounting without knowing how much it increases heat output (btu). Any help would be greatly appreciated.

[chem geek]

This link explains a little about the optimum tilt/angle based on latitude. As for what happens if your panels are not angled pointing towards the sun, the reduction in energy (so rate of heating and associated temperature rise) is just the cosine of the angle relative to perpendicular to the panel so:

cos(0º) = 100%
cos(10º) = 98%
cos(20º) = 94%
cos(30º) = 87%
cos(40º) = 77%
cos(50º) = 64%
cos(60º) = 50%

The reason for having the angle be 15 degrees more or less than the latitude during winter and summer, respectively, is to compensate for the 23 degree tilt of the Earth. One doesn't use 23 because that's the maximum at summer and winter solstace.

You can see that the angle makes some difference, but is not terrible if you aren't perfectly lined up with the perpendicular of the panel pointing to the sun (at solar noon). Generally, most angled roofs facing south will do a decent job. It is more important for you to angle the panel (compared to flat) if you are in a very northern latitude. For example, in Canada at 50 degree latitude (near the U.S. border), the tilt should be around 35 degrees. If you do not do this, then you lose around 20% efficiency near noon, but even more the rest of the day.