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

[Chin_Lee]

I've been looking for a system to use as a backup power supply for my tank in the event of a power out. I was considering a UPS but most of them won't last long enough (and expensive) or provide enough power for heaters if needed. I've been looking at the Motomaster Eliminator PowerBoxes that come in 300A, 500A, 600A and 1200A.
http://www.canadiantire.ca/assortmen...romSearch=true
There was a trend regarding these powerboxes and Chad indicated he was going to test his 300A powerbox out. I've PMed him and he said he was going to test it. Looking forward to those results Chad

The 1200A box comes with a 60amp-hour battery and a 1200w inverter. Is there a generic formula to calculate how long a certain wattage appliance will last with the different batteries alone before having to use the plug in the inverter and start up the car?