Thanks for the ping, @Tigerle!
My C17 is subject to bouts of seasonal depression, and my C23 has a pretty severe case of it. And we live in Alaska, so it was definitely at the forefront of our college search process.
C17 ended up going to the mid-Atlantic (Muhlenberg, specifically), which was fine for her. That wouldn’t have worked for C23, though, so in our spreadsheet we had a column for solar energy. The US government has a site designed for people who want to put in a solar system, and if you put in a college’s zip code and click through on the next screens (just accept the default settings) you get a list of solar radiation in kWh/m²/day for each month and overall for the year.
We collected the annual value and the lowest wintertime value (usually December, but some places—like most of Colorado—it’s January). So, for example, if you plug in 20742 (which is the University of Maryland College Park, where I went for undergrad), you get an annual average of 4.79 kWh/m²/day and a monthly minimum (in December) of 2.98. If you put in 99775 (the University of Alaska Fairbanks, the northernmost university in the country) you get 3.04 and 0.05(!!), and for 76203 (the University of North Texas, where C23 is attending and very happy with the level of sunlight) you get 5.50 and 3.85, with the lowest monthly value in January.
We set an arbitrary minimum of 2.90 for the monthly minimum and 4.50 for the annual average (though we learned the former was more restrictive than the latter). Those were based mainly on vibes, though, based on our experiences traveling in different geographies over the years. It took only a couple hours of plugging in values to get them for a hundred or so colleges, and gave us something more objective than simply eyeballing the Mason-Dixon line or such. (Especially since you have phenomena like parts of the northern Intermountain West—e.g., the Front Range in Colorado—getting more sunlight than you’d expect due to the rain shadow effect resulting in fewer clouds, while parts of the southern Intermountain West have less sunlight than areas further north despite having more daylight hours in the winter.)
And now I’ve gone on too long and in probably too much detail, but maybe it’s a useful resource for you.