And as an aside, the 747-8 uses GenX engines, same as the 787. These engines and their accessory gear drive have the ability to drive two 250KVA generators EACH. (That is not what is installed on the 747-8, but could be.) With four engines and eight generators, that means 2 gigawatts of installed power, NOT including the APUs (yes, two APUs are required, each also with BIG generators.). I'll leave it up to the reader to conjecture on what all this power is for, but, yes, all that power is needed to power some of the stuff that will be installed on the new Air Force Ones.
KenV, the GEnx engine on the 747-8 is not the same as the GEnx on the 787. The GEnx-2B on the 747 has a ~7" smaller fan, one less LP stage on each end, an aircraft bleed system, and naturally a different gearbox without two generators, relative to the GEnx-1B on the 787. So it would be highly impractical to install the 787 engine on the new AF1. Now, it may be possible to install the -1B gearbox on the -2B, but that would be a major engineering effort (oil cooling would be a big problem, and I'm sure it would drive major engine nacelle changes).
I think what may be more likely if they need lots of electricity would be to install 120 KVA IDGs (the 747-8 currently has 96 KVA IDGs which appears to have been a penny wise/pound foolish decision by management very early in the 747-8 program).