I have an AGM battery that is rated at 12V 225Ah. Using the 50% "rule", the battery should be able to deliver 12 * 225 * 0.5 = 1350 watt hours = 1.35 KWh. However, when the battery is under load, the voltage drops to as low as 10.5V, where many power inverters are set to warn the user. I don't know what the average voltage is during a capacity test, but it slowly drops from as high as 13V (with a very light load at the beginning of the test), to around 10.5V if the load is just right such that it is heavy enough to dip the battery that much at the tail end of the load test.
So my question is, do battery manufacturers expect us to use the nominal voltage when calculating the usable capacity of that battery or the average voltage during the capacity test? I don't want to overestimate this 50% rated capacity (100% usable capacity) and overdischarge my battery. The difference is about 6.25% if I use the average voltage (around 11.25V) vs. the rated voltage (12V). For example, 11.25V (ave) * 225Ah * 50% = 1.265 KWh, not the 1.35 KWh we calculated using the 12V rating. Which is more correct? is it more correct to just use the watt hour rating by the manufacturer and not worry about voltage sag under load, as that is already accounted for by the different capacity ratings such as 10 hour, 20 hour... drain rates?