Ours was off for just on 24 hours.
The bigger tank coped fine, but my small 30L didn't fare so well. I only lost two fish, but .. as fate would have it they were my two favourites. A beautiful Blue Ram .. his colouring was perfection and he was such a friendly little guy, and a white/ red female fighter. I was gutted.
Had to do the floating bottle trick , but it was while I was sleeping that those two died. Came out in the morning to the surviving fish doing headstands on the bottom of the tank.
I'm thinking generator myself.