Welcome to the boards.
Well, that depends on a few things, where you live, the ambient temp of the room you house them in, the size of the enclosure, what species you are housing etc. the good thing with lamps is that you can move them closer or further from the setup for small adjustments. A thermometer is obviously needed so you can achieve perfect conditions. A 50w would probably be suitable for a 10gal pandinus / heteromtrus setup.