It is why fixed panels work so well, but also an advantage here. 8 degrees off only drops 1%, so for an hour, you still get 99%.

18 degrees, (about 2 1/2 hours) you are still at 95%. So some slip or error or sag will still give me most of the energy. It doesn't have to be precise.

But at 60 degrees off - you only get 50%, so if you have clear mornings, but cloudy mid-days you won't get most of the energy.

Also the tilt is part of the error. If you are pointing at your latitude, or below (you would want more in winter) this adds to the error.

http://www.grc.nasa.gov/WWW/K-12/airplane/tablcos.html