You should know from your work.
Well, that's been the general trend for the different kinds I've tested at least.
Another thing I've noticed is that the measured voltage drops by ~1-2V when through a window and a ~50% drop under medium cloud cover.
Of course this isn't exactly definitive science here, but that should give you a rough estimate of how powerful of a panel you'll need based on your climate and power usage.
@angelscar: Also, in the future please use more specific topics such as "Powering My Robot with Alternative Energy Sources". It just helps to get a sense of the problem.