When deciding on collector orientation, if the only criteria is sun angle, of course, perfectly south is best. But, in our real world, in addition to sun angle, there are several other considerations that will go into your decision on how best to orient your panel.
- Odds are, your most southerly wall or roof is not perfectly south, but the very worst case is that your wall is within 45 degrees of south and it's likely much closer to south than that. Using a house wall (or building the collector just in front of your wall) offers several advantages, including nice aesthetics, insulation from the house itself and a really short run to your house.
- You might have shade from a nearby tree or structure in the morning or afternoon, so it makes sense to orient your collector to favor the morning or evening sun as appropriate. That was my case here.
So, how narrow a target do we have before taking a significant performance hit?
The great news is that we have 30 degree window (15 degrees east or west of south) in which we are within 98% or better of optimum performance! In fact, expand that window to 60 degrees and we are still within 90% or better of optimum!
When I was analyzing collector placement for my 24' X 8' pex collector I used Gary's calculator to determine the results due south (12:00 PM solar noon), 15 degrees off (11:00 AM / 1:00 PM solar) and 30 degrees off (10:00 AM / 2:00 PM solar). This assumes a 60 degree tilt angle to maximize the winter sun.
The target is wide. Don't worry if your collector isn't perfectly south. Any aim capable of hitting the broad side of a barn will serve you well!