How many coulombs does one ampere-hour equal?

One ampere-hour equals 3,600 coulombs. The conversion follows directly from the definitions of the ampere and the hour: one ampere is one coulomb per second, and one hour contains 3,600 seconds.

Therefore, a steady current of 1 ampere flowing for 1 hour transfers 1 ampere-hour, or 3,600 coulombs, of electric charge. The unit is written Ah or A·h and is widely used to describe the charge capacity of batteries. A 2 Ah battery, for example, theoretically stores enough charge to deliver 2 amperes for 1 hour or 1 ampere for 2 hours, subject to real-world limitations.

Ampere-hours measure charge, not energy. To estimate battery energy, voltage must also be included: watt-hours equal ampere-hours multiplied by volts. A 12-volt, 10-Ah battery has a nominal energy rating of about 120 watt-hours.

The smaller milliampere-hour is common in portable electronics. Since 1 Ah equals 1,000 mAh, a 2,500 mAh battery contains 2.5 Ah, or theoretically 9,000 coulombs of charge.