A watthour represents a unit of energy equal to one watt drawing power for one hour. Since batteries are storage units for electrical energy, watthour specifications equal battery capacity. For Energizer batteries, the manufacturer chooses milliamp hours rather than watthours. To convert milliamps to watts, it becomes necessary to convert milliamps to amps (1,000 milliamps in one amp), and then use the formula watt = amp x volt.
AA Batteries

A standard Energizer AA battery, like all AA batteries, has 1.5 volts. With the constant voltage determining the watthours, specification becomes a matter of obtaining the Energizer specifications in milliamp hours and performing a conversion. Data sheets for Energizer's technical information can be searched on their website. For their AA battery, it looks to have a capacity of 2800 milliamps hours, or 4.2 watthours.
9Volt Batteries

The voltage on the industry standard 9volt battery is, obviously, 9 volts. Energizer's 9volt battery's milliamp hours equal 610, or 5.49 watthours. Thus, the 9volt battery has a greater capacity than the AA, which means that it is likely a 9volt Energizer battery will outlast a AA battery.
AAA Batteries

All AAA batteries, a smaller size battery than AA, are 1.5 volts. Energizer's AAA battery has a capacity of 1250 milliamp hours, or 1.87 watthours, giving the AAA battery considerably less capacity than a AA battery.
C Batteries

With the same voltage of a AA or AAA battery (1.5 volts), the C battery differs in that it has a milliamp capacity of 8,200. When converted to watthours the number becomes 12.3 watt hours, demonstrating the difference in battery capacity among the smaller AA and AAA as opposed to the larger C.
D Batteries

With a larger size than a C battery but the same 1.5 voltage, D batteries should have a greater capacity. A specification of 21,000 milliamp hours, or 31.5 watthours, gives the D battery more than two and onehalf times the capacity of the C battery.
References
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