How many watt-hours in two AA cells?
2 AA cells with a nominal capacity of 2 Ah and a nominal voltage of 1.5V  each in series, I'll get 3V with 2 Ah, right? Is that, thus 6 Wh?  


The 1.5V of an AA is not like the nominal 1.2V of a NiMH. The estimation of Wh by V x capacity works much better for NiMH and Li-ion where the nominal voltage is mid point in the discharge curve. NiMH fresh charge voltage is 1.6V and Li-ion 4.2V, and cutoffs 1V and 3V.  AA fresh voltage is 1.5V and cutoff is 0.8V, very unlike NiMH and Li-ion.    

So no, 8 Wh would not be correct for an alkaline AA. It would be 6 Wh if the battery voltage was a constant 3V over the lifespan of the battery.  Notice, in the MilliAmp-Hours Capacity bar chart below, it says "Continuous discharge to 0.8V".  The 2 Ah is not at a constant 3V.  An AA has a 2 Ah capacity only with a specific load.   

Both the capacity and voltage are dependent on the load and therefore the Wh is also load dependent.  

More information is needed. Need to know the instantaneous current and voltage over the lifetime of the battery.  Even at that it is not easy to calculate the watt hours used. 

An empirical approach would be easier.  Continuously record the current and voltage of the battery while your device is operating.  Then calculate the Wh for small periods of time and sum the results. 





The capacity of an AA varies with load's current draw and whether or not it is a continuous load.   


Source: Energizer E91 AA Datasheet   



The voltage and discharge curve is also load dependent. 


Source: Energizer E91 AA Datasheet



If the load is not continuous the discharge curve is much different than a continuous curve. 


Source bossmanbattery.com