Here's the generic formulas to use:
Let's assume equipment of 400 watts (or 3.3 amps @ 120V), so Steve's request is not far off. I'm also assuming that the heater is on 100% of the time.
3.3 amps x 120 volts = 400 watts
To keep this running for 8 hours,
400 watts / 12 volts x 8 hours = 267 amp-hours from the battery.
We also have some efficiency factors to consider,
- the typical inverter efficiency is 90%
- battery ratings are based on a 20 hour discharge, and they provide less energy output when discharged faster. Using a battery discharge curve, a typical battery might provide 85% if discharged in 8 hours (if discharged in 1 hour, the rating drops to around 50%).
With these corrections,
267 amp-hours / 0.90 / 0.85 = 350 amp-hours
.... much more than the Canadian Tire unit.
If we only have the 60 amp-hour unit, we don't even get 1 hour
to discharge the battery.
i.e.
400 watts / 12 volts x 1 hour = 33 amp-hours
33 amp-hours / 0.90 / 0.50 = 75 amp-hours required
Further, Andy's concern about the sine wave is correct. Depending on the quality of the wave output (square wave, modified square wave, sine wave), some devices may not work at all and others (typically motors and pumps) will consume 25% more amp-hours than their rating, potentially running hotter, and shortening their life.
I'd definately check out the type of sine wave output you'd be getting.