Battery nominal voltage - can you confirm my understanding?
The capacity of the battery should be specified at some given rate of discharge. If not, look for a battery supplier that will specify the battery correctly.

So, the amount of energy available can be estimated by multiplying the specified capacity by the specified final (terminal) voltage for that capacity rating. If the battery manufacturer says that the battery will provide 6Ah when discharged at 1A to a terminal voltage of 12V, then you can estimate that you will obtain at least 72Wh from the battery if you discharge it at 1A.

Of course this is only an estimate. Rarely is there an application where the discharge is truly a constant current and there will be some variation in the actual capacity of individual battery units. But such an estimate is usually suitable (good enough) for the typical application.