Agree, window light sucks. With the window, (pristine clean), you lose at least 10%, i'd say more like 35-45% for having a dirty window. These are just generic figures, I do not own a light meter, but one day I'll definitely test this to get accurate figures. Anything obstructing the light will cause it more so, I've never had any real problems with direct sunlight in a window unless the temps are getting too hot, 25*C+.
Go el-natural if you want great results
. (year round is near impossible for many though)