I have run fan overnight like that on several occasions with no problem. In the scenario you are describing, it would usually only be a relatively short time of lights, water pump use, etc.. before lights out...just to stop and sleep, so the batter if in good shape would be charged form the engine's alternator furring the drive and will have more than enough power to get you through...
Also what i have done is to run the genny for a few hours in the evening while watching TV and such. get the house really chilly with the AC, then shut everything down for bed. The residual cold might last till it gets cool enough outside...might crack the window open in the early hours if I wake up...so there's often no need in running the genny and Ac all night
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