> a pointer to a battery datasheet or website would have been helpful

Actually, common sense suffices.  For a given battery chemistry,  
capacity is approximately proportional to cell volume (or mass.)  For  
a linear regulator, extra voltage doesn't buy you extra capacity.   
Now compare the size of a 9V battery with the size of 3 AA cells.    
Actually, compare HALF the size of the 9V battery with the 3AA cells,  
since half the 9V is "wasted" in the regulator.

http://data.energizer.com/ seems to have pretty good data, including  
capacity in mAH at different discharge rates (Duracell has curves  
that are more difficult to interpret.)

A 9V alkaline seems to have about 250mAH at 300mA discharge, while an  
AA has about 1500mAH (6x more).  Note that you still need 2x of the  
AA to reach a reasonable voltage for a PIC; otherwise you need a  
switchmode boost regulator that throws more variables into the equation.

I think it was Linear Technology who first pointed out in  
advertisements that 2AA boosted to 5V (1500mAH*1.5V*2 = 4.5WH) would  
result in considerably more run-time for 5V devices than a 9V battery  
linearly regulated to 5V (250mAH*5V = 1.25WH).  Back in the dawn of  
swicthmode regulator chips.

BillW

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