From some real tests on one of our UPSs (designed to work with 12 or 24V batteries, with only very small changes in some components) and with 2x 9Ah batteries (in series=24V or in parallel=12V), the runtime can be even 40% higher with 24V than 12V.
If the load is not too high, the difference is much lower.
Those differences are mainly due to:
- much more resistitive losses with lower voltage (you have to double your supply current to get same power)
- less strain to the batteries: half current when they are put in series. If you take a look at the discharge curves you'll understand what I mean
This is one of the reasons why it is always suggested to work with at least 24V when your power requirements are higher than 5-600W.
P.S. if the capacity of your battery is much higher than 7.2/9Ah (80Ah), the main difference will be due to the resistitive losses because the internal resistance of the 80Ah battery will be low enough to minimize the second point.