[ELI5] Why are battery capacities expressed in Ah (Amperes-hour) and what does it mean, for example, a AA battery that is 2500 mAh?
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Normally you would express it in Wh (Watt-hours), but since Watts are the Volts times the Amperes (W=V•A) and a battery only has one constant output voltage, it is easier to show the numbers in Ah instead.
You can easily find out how many Watt-hours are in a battery by multiplying the Ampere-hours with the output voltage.
Take your example of a regular AA-battery. It has an output voltage of 1.5V and a capacity of 2500 mAh (miliAmpere-hours)(a thousands of an Ampere-hour) which is 2,5 Ah. If we multiply the two, we get that the AA-battery has a capacity of 3,75 Watt-hours, meaning it will power a 1W LED for 3 hours and 45 min.
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