Solar electric modules will produce energy even on cloudy days. Although it might seem counter-intuitive, consider that solar panels on a rooftop in foggy San Francisco produce nearly the same as the ones in nearby sunny Sacramento. Consider too that Germany (with a climate not that different from Vancouver Canada) leads the world in residential solar right now, and it is generally an overcast climate.

While the amount of sunlight your panels receive is important, a more accurate representation of the amount of energy your panels can produce is referred to as peak sun-hours or equivalent full sun. It is important, first of all, to note that “peak sun-hours” are not the same as “hours of daylight.” Peak sun-hours refers specifically to how much solar energy is available in an area during a typical day. As a comparison Phoenix Arizona has equivalent full sun value of 6.5, in Western Montana we use the equivalent full sun value of 4.5.  The same equivalent sun hour value as a good part of the continental United States.