Also, the answer to "how much energy": with stacking levels of Battery for a combined mecha, and a current maximum reactor of III for Supers: 440/5. 88. So, you could merge Getter Emperor for a single turn (more, if EVERY pilot had Spiral Power or whatever it's called and burned all their spirit to do that).
EDIT: Wait, I tell a lie. 132 a turn (88 is for Reactor II). Therefore, for Getter Emperor to sustain itself independently, it needs Reactor VI.