Of the sunlight that hits the earth:
19% is absorbed by the atmosphere
8% is reflected by the atmposphere back into space

On the equator, you're losing 27% of that energy to atmospheric losses. If you're at, say, Edmonton at 51 degreees north, the angle of inclination means that light has to shine through 1.6x as much atmosphere, so you end up losing not 27%, but somewhere around 43%. And that's not even taking clouds into account yet; if it's a cloudy day, your solar panel output goes to practically nothing. Plus, there's thermal derating for the cold which takes a few percent, too. Semiconductors simply work way better when they're hot (don't confuse this with the properties of the metal interconnects which dominate for microchips and work better cold) There are very very few cloudy days in the desert. There are quite a few cloudy days in Alberta, though, and that has a HUGE impact, way beyond simple atmospheric losses. A panel placed properly in the california or arizona deserts will produce over twice the annual power as one placed in Edmonton and even more than that when compared to New England and Seattle. Poor Seattle, one of the "greenest" places on earth, but also one of the worst places in the world to put in a solar panel...

But hey, at least Edmonton is better off than Toronto!

[Linked Image from solar4power.com]

Another map:
http://1.bp.blogspot.com/_cwrSE63jF...SuXMFZmM/s1600-h/us_solar_energy_map.jpg