I'm thinking about powering my Raspberry Pi web server (which needs to run 24/7) from solar power. Let's assume that it's a model A, which consumes about 0.65 W in average, if I'm not mistaken.
I live in an area where the average sun hours worst case occurs in December, about 60 hours of sunlight for the whole month, or 2 hours of sunlight per day. We need to calculate based on the worst case if we don't wan't to run out of power for the Pi during the winter (the average in the summer is about 9 hours of sunlight per day).
So the Pi consumes 0.65 W for 24 hours. That's a total of roughly 16 W a day. With 2 hours of sunshine in December, that means that we need an 8 W solar panel. Make it 10 W just to be sure.
The battery needs to store energy for at least 5 days in advance (there might not be any sunshine for 5 days), that means that it must be able to store 80 W. But the deep cycle batteries recommended for solar applications should not be drained below 50% of their capacity, so we need to double that to 160 W in order to ensure that if there will be no sunlight for 5 days, the Pi will still not drain a fully charged battery below 50%. Assuming that it's a 12V battery, which are most common, 160 / 12 = 13.3, so we will need a battery rated at 13.33 Ah. 12 Ah might do the job, or I could go for 20 Ah, which is the next available size.
Obviously, the solar panel and the battery would have to have triple capacity for a Raspberry Pi model B. Also, these are calculations based on the worst case which is still realistic. In the summer there will definitely be plenty of solar power for the Pi. In the winter, if the sun does not shine for a week, the Pi is going to get thirsty
What do you think, are these calculations realistic?