Theoretically, if your computation is energy efficient, you won't need any electricity at all since the real computation costs zero energy.

That's not correct. For ordinary computers there is Landauer's principle, which gives a theoretical lower limit for the energy needed for computation [0].

I say "ordinary computers" because other comments mentioned "reversible computers" for which this limit doesn't apply.

According to the linked wikipedia page, this theoretical limit is around a billion times smaller than current computers use for an operation, so you may call me pedantic.

[0]: https://en.wikipedia.org/wiki/Landauer%27s_principle