That's not how it works. Since it is in a series (At least this is what it looks like given the 1 page manual) the capacity is the same at whatever the AAA's are rated at but the voltage is doubled. 2 AAA's in a series would pump about 3v vs 1 AA cell which would be 1.5v. The AA cell would have the better capacity (runtimes) in this situation, given the same load is put on the two units. You cannot tell which is better really, unless you have a multimeter and check amps to see usage.