You're right. And three of them would use 21 energy an hour, or 504 energy per day, which equates to about 14 hours, not seven. Though it's a moot point, as both of use chose to use the generators.

Hhmm...it might have been better to use one generator and one set of energy cells instead of two generators. The 18 energy a day would cover the daily operation of all of the other cargo except the Multipurpose Prefab (which sits empty anyway), and the cells would cover spikes in needed energy.