[Cuog]

That's a pretty large efficiency gap. I did this calculation using the efficiency of the inverter I have(83% at full load) And still come up with a bit over 2 hours.

When calculating anything which can be highly variable you will of course need to build in a fudge factor. For example, we don't really know how close a battery after being used for FRC is to its original 17.2AH rating.

There is a reason that battery meters on laptops can be so inaccurate a lot of the time. There's a lot of variables to account for and trying to pin down an exact length of time something will last is only going to a better chance of being wrong.

Do your math for the best case scenario and figure out what worst case scenario is acceptable, and see if that leaves a realistic fudge factor.