[JRTJH]

Here's the "general rule" :

1 watt-hour is defined as 1 watt of power expended for 1 hour. 1 amp hour is defined as 1 amp of current expended for 1 hour.

So, at 13.6 VDC, there are 13.6 watts in every amp.

A 100 amp hour battery that's fully charged and providing 13.6 VDC is equivalent to a 1360 watt hour battery. (13.6 x 100 = 1360)

Two 100 amp hour batteries that are fully charged and providing 13.6 VDC is equivalent to a 2720 watt hour battery. (13.6 x 100 x 2 = 2720)....

Where the formula gets "wonkey" is when you start trying to calculate amp/hour or watt/hour variations as the battery voltage drops as the charge drops.... Since lead acid battery voltage drops at a different rate from a lithium battery, the "direct correlation" is, at best, hard to calculate. So many (probably most) don't attempt to "calculate the slight differences between battery bank technology.