How to calculate how long a car battery will last as backup

[r_wellwood]

5 amps, seems like a lot of equipment could run on that. Wouldn't the idea behind the backup be to maintain temperature, water flow (PH or two, return pump from sump if heater is there), and areation? Maybe some minor lighting could be powered, but why bother with keeping MH, VHO, all PowerHeads, etc going?

Strange things happen randomly in nature, and a power outage for a Fish/Reef tank for even a day should be tolerable for the system (with temperature, basic flow, and aereation maintained).

I could see something heavy-duty being necessary on a larger/complex system if say a chiller was also involved (closed loop pump and compressor).

RW