Ok so this has always confused me and I still can't seem to find an answer anywhere! A general black hole has parameters M, Q, and J which are given the meaning of mass, charge, and angular momentum. My question is what what is the contribution to M by Q and J?? Presumably if you start with a Schwarzchild black hole and drop "massless" charge (not possible in reality but electrons at rest come pretty close) into it, it will not only gain charge but M will be increased due to the compression of like-signed charges. The situation is similar for angular momentum where a rotating black hole partially repels objects with angular momentum in the same direction. The extra energy required to get the object into the black hole should add to M. So how much of M is due to charge and angular momentum, and how would you go about calculating it? Naively I would guess that the mass gain is equal to the work required to bring the charge up to the event horizon (since no energy can escape whatever happens within the horizon can't increase the mass).