Just to be clear, because I think it's easy to forget this: > If you are using a nuclear reactor, a small par of the mass of the atoms is transformed into ener

Just to be clear, because I think it's easy to forget this: > If you are using a nuclear reactor, a small par of the mass of the atoms is transformed into energy and you put that energy in the photons This applies equally to nuclear reactors and to AA batteries (like you mention earlier). All forms of energy have mass.