ok, let's be more specific.
Let's say this battery is specified to be 12V and 100Ah at 20hr discharge rate, and it's being charged/discharged at exactly that rate as specified - this should take care of the Peukert Effect.
I am still interested in the theoretical fully-charged energy capacity in Wh. For now I disregard all losses, the fact that you should not discharge a battery excessively, and I've taken care of the Peukert Effect.
The problem with the straight formula that gives 1200 Wh is that it does not seem to fit what we are trying to calculate. The units certainly add up, but the scaling may or may not be correct.
It is for calculating energy stored in the existing electric field as you move a charge Q in that field through a potential difference V. It's like the potential energy in the gravitational field, and you take the whole Q * V (that's how Damon got 1200 Wh).
For charging a battery, some people claim you should take ½ of that to calculate the theoretical capacity. I've seen them explain that the charge you put in battery distributes evenly: ½ positive and ½ negative. I've seen this applied to the electric vehicles, I guess they want to be conservative and not leave anyone stranded.
Also, if you start with another energy-storage device - capacitor, the energy stored in it's electric field is E = ½ * C * V^2.
But, C = Q / V, so E = ½ * Q * V (note the scaling by ½ here).
So, as far as the final result, how different is a capacitor from a battery, both accumulate charge Q, and both create a potential difference V? And, in calculating the stored energy the battery is typically scaled by 1, the capacitor by ½
This makes a big difference, like can you drive your electric car on a full charge 100 miles, or just 50 miles.