Aside from whether it's worth it, how do you balance this? I think, to be worthwhile for its primary (draining) mechanic, this unit needs to have higher energy(stolen)/cost than the energy sources it steals from have energy/cost. Otherwise:
1) If you are using it to sabotage the enemy, It's cheaper to send in a glaive or a scythe and permanently destroy however many energy sources you can get to. This causes their energy to be drained permanently.
2) If you are using it to power your own economy, it's cheaper to build the usual energy sources which are cheaper.
Of course, maybe you are using it to create a difference between your economies. Then it only needs to be half as good because "stealing" a solar is removing one enemy solar and adding your solar, for two solars worth of difference.
However, then comes the problem of counterplay. If i have a solar which because of the energy thief is powering my enemy, and i cannot kill the thief, i can self-destruct the solar, halving this difference - a blown up solar is no longer powering me, but it's also no longer powering the enemy. This is perverse.
(Alternatively, as the attacker, i can instead send in a Dominatrix and steal an unlimited number of energy sources, while preventing their self-destruction. Perhaps this energy thief is just a different capture unit?).
If i
can reliably kill the thief, then its high killability needs to be factored into its price and again it's better to just honestly build/raid energy?
That all aside, it could of course be just an energy-draining siderm on an otherwise useful unit. Like the AA gun on gremlin scouts. This seems like to be the case because it's suggested it have a leveler sidearm, which would let it permanently sabotage an unlimited number of energy sources.