To make things simple take your Voltage X Amperage and you will get Watts. In our 'amperage' figure there is a time, the hour, so our final figure is in Watt Hours.

12V X 255AH = 3060Wh or, converting to kWh, 3.06kWh at 100% discharge.

Now note if we had worked with 6V Batteries it would have been 6V X 255AH = 1530WH and we would have needed to multiply by the umber of batteries (2) and we end up at the same place.

The 'real' way to calculate power is a lot more interesting and involves a bit of higher math than we want to go into here. It also yields similar results. The power in the bank is the number of batteries X the capacity of each battery.

Don't forget, that is at 100% discharge. The figures need to be lowered for any practical application.