I couldn’t find the answer with a Straight Dope search, so I thought I’d launch a new thread.
The reason why I’m wondering is that it seems to me that the Sun’s light is actually stronger in the winter than it is in the summer. It could be my imagination, it could be because of the higher reflection/lower absorption of sunlight because of snow and there being little vegetation. It could be because there’s much less humidity. Or the Earth could be closer to the Sun.
Related question: Does the Earth’s variance in distance from the Sun have a measurable affect on ambient light, weather, tides, what have you?