Factors that contribute to low power production

Yesterday was the best day for power production that we've had in December and January (31.5 Kwh). The days are getting longer, and the sun is higher in the sky, but the days are still overcast, snowy, and rainy. I haven't been keeping track of the times the sun rises and sets, but I did look at the time of the beginning of power generation to get a rough idea of how the days are lengthening. On December 22, 2015, the first day after Winter solstice, the first power was generated at 8:15 am (2 WH). This morning (January 16, 2015), the first power was generated at 7:45 am (4 WH). That is approximately 30 minutes of additional power generation. Since the sun now sets later in the evening, I'm assuming symmetry between sun-rise and sun-set, and there is approximately 60 more minutes of power generation now compared to the day after Winter solstice. I thought I'd list, based on my brief experience with solar power, my opinion about the factors that affect power generation in northern Utah from solar cells. These factors are ordered according to importance.

1. Length of day and height of sun.
2. Snow on the solar cells.
3. Clouds in the sky.

I'm interested in hearing the experience of others. You may have a different amount of sun and a different slope and direction of your cells.