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

[mike miles]

The first thing you need to do is find out how many amps your pumps and heaters need to operate.then you need the power inverter as per amp draw from pump/heaters.then you need to find deep cycle batterys.
calculate reserve power and amps per hour.this will give you an estimated time your system will run and to be safe i would go .75%

Mike