[jillesvangurp]

I was in the Aosta valley (on the Italian side of the Swiss border, surrounded by the highest mountains in Europe) a few weeks ago in the middle of the winter.

In short: yes there are only about 9-10 hours of daylight but it can get very bright when the sun comes out over the mountains for 3-4 hours. They also grow wine on the south facing parts of the mountains (which requires plenty of sun). And you have to consider that mountains often have clouds/rainy weather. Another thing to consider is the peaks of the mountains are often above the cloud layers. You can get a sun burn in the middle of the winter in Switzerland quite easily. Just because it's north of most of the US, doesn't mean solar is uneconomical.

So yes, it's not optimal but there's plenty of light. And people do put solar on their roofs there and it apparently makes economical sense to them.