[MrMikeB]

IMO, you should worry about flow and oxygenation first, which go hand in hand. You don't want to have still/potentially toxic water sit around your reef.

I have seen some creativity where folks have put an air pump on a battery backup and when the power failed, the pump would activate and provide both oxygenation and flow (with the bubbles moving to the surface it creates flow).

Most outages are not too terribly long where heat becomes a huge issue, and given the slow decline in temp over time, there is a degree of flexibility in this area.