Remus had thought sun at first. Surely, the simplest. But they're in Scotland, sun isn't going to happen every day, not even every week.

He points out to the pointed stick under the contraption. It's covered in runes. He studied them while the others were busy making themselves animagi. "Stick it into the ground, it'll tap into the warmth of the ground itself and power up. The radius is five meters to every direction. Could probably power up a solarium."