Outdoor Reef Tank?

[TwoHedWlf]

AND a chiller would need battery backups. We usually have at least one short outage for repair purposes, every several months, as they add on to the system.

If it's short, then no you won't need a backup. More than 5-6 hours maybe, but less than that should be fine. Any lights you have on the tank would obviously be off, you could throw a blanket over it to shade and insulate it a bit more. I'd be more worried about airation than cooling during a power outage.