Lets compare our radiation to that of Honolulu (see below). In the summer there is not much difference, since our northern latitude is balanced out by our longer days. During the winter..... well...you don't need a meteorologists to tell you about that.
Here is the actual radiation reaching the surface that last few days, based on the measurement at the UW:
A number of folks are experimenting with solar collectors in our area as a way of acquiring more green energy for their homes. There are significant number of solar contractors and non-profits ready to do installations around here. Sounds crazy for cloudy Seattle? It turns out that we get more solar radiation than Germany and they have a very large installed base of photovoltaic and other collectors (see map).
Clearly, the SW U.S. is just perfect for solar energy: LOTS of sun and a huge population that wants air conditioning while the sun is blazing. Our country needs to do more....a LOT more...to take advantage of this huge resource.