Calculating energy in battery
As a first very rough approximation, yes you can multiply the capacity and nominal voltage to get energy.

However, batteries are quite complicated with lots of variables.  The output voltage will certainly vary over the discharge life.  What does the 100 Ah spec actually mean?  What is the output voltage at the end of those 100 Ah?  To truly get delivered energy, you have to integrate the instantaneous product of the voltage and current over one full discharge.

But wait, it's not that simple either.  That was one cycle at one temperature.  Batteries can generally deliver less energy when cold, sometimes significantly so.  Overall capacity generally decreases as a battery ages or as it is cycled.  How deeply the battery has been cycled in the past can matter. The output voltage and capacity are a function of the current.  100 Ah doesn't necessarily mean that you can draw 100 A for one hour.  It might be specified to deliver that charge at 10 A for 10 h, and if you draw 100 A you only get 80 Ah, for example.

How does your load actually use this energy?  If you have a switching power supply, then maybe you can actually use the integral of voltage times current.  If you have a linear regulator, then you really only care about the delivered current as long as the voltage stays above some mininum.  Any extra energy the battery delivers in the form of higher voltage will just be dissipated as heat by the regulator.  In that case, the Ah capacity figure is more relevant that a energy figure.

Due to all these variables, just multiply the capacity by the nominal voltage to get a rough idea of the energy, and realize a rough idea is all you're going to get.  Good design will derate from there to a figure you can rely on over the range of temperature, age, and cycle lifetime you want the battery to work over.