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.
I know people say not to put the heating pad under the tank. But I have a small one under the side of the tank with the log hide. That is where she spends almost all of her time except for I guess at night when I can't see her. I'm going to try unplugging it and see how far it drops without a heating source. I also went ahead and bought a 50 watt IR bulb. If I end up not using it for the scorp. I haveother animals I could use it for.