I've made sure it's connected one of the input faces, but it won't receive power! Anyone have a solution?
Cable losses? For solar panel you may use only up to 4 copper wires (or 39 tin wires).
No, your problem is too generic for a solution.
Check that it's daytime, that there is no power loss along the cable you're using to link the solar panel to the batbox (4 blocks max for insulated copper cable, 40 blocks max for glass fiber and tin), that the solar panel can see the sky and that it isn't raining (or cloudy, if you're in a desert biome).
Like Zjarek (who posted while I was typing this up), I suspect cable loss, as it's the least obvious potential problem.