Why is a Battery Rated in Ah (Ampere hour), Not in VA?

Electrical Technology

Why are Batteries and Cells Rating in Ah (Ampere hour) and not in VA or Watts?

A battery stores energy in the form of chemical energy, which is later converted into electrical energy for use over a specific period. The amount of charge a battery can deliver is known as its capacity, typically expressed in ampere-hours (Ah). Furthermore, in a charged battery, the number of molecules available to support electron flow in an electric circuits is limited. Therefore, the battery contains a finite number of electrons that can be driven through the circuit until the battery is fully discharged.

Related Post: Difference Between a Battery and a Capacitor

Technically, we could express a battery’s capacity in terms of the number of flowing electrons over a specific period. However, this would be impractical due to the extremely large number of electrons involved. Instead, we use a more convenient unit (the coulomb (C)) where 1 coulomb equals approximately 6.25 × 10¹⁸ electrons, or 6,250,000,000,000,000,000 electrons.

In addition, 1 ampere (A) is equal to 1 coulomb (C) of charge per second, and:

1 hour (h) = 3600 seconds

Therefore:

1 Ah = (1 A) × (3600 s) = (C/s) × (3600 s) = 3600 C

∴ 1 ampere = 1 coulomb per second (C/s).

But then, you might ask: Why invent a new unit like ampere-hours (Ah) for battery capacity when we already have coulombs? Well, of course, it’s just to make life more “interesting” (read: confusing) for technicians and students! 😉

Just like they do with electricity billing: 1 unit of electricity = 1 kilowatt-hour (kWh) = 1 Board of Trade Unit.

Why Battery is Not Rated in VA (Volt Ampere) or Watts?

VA stands for volt-amperes, which is a measure of apparent power (one of the three types of power in AC circuits). The other two are:

True (or real) power, measured in watts (W)
Reactive power, measured in VAR (volt-amperes reactive)

The relationship between these three determines the power factor of a system. A high power factor is desirable in AC circuits because it indicates efficient power usage.

On the other hand, batteries supply DC power, where the power factor is always unity (1), since there is no frequency involved in DC circuits. Therefore, power factor is not applicable to DC systems.

Moreover, a battery is not a constant voltage source, its voltage drops over time as it discharges. For this reason, we are primarily interested in its capacity, which is rated over a specific period. This is why batteries are rated in ampere-hours (Ah) rather than watt-hours (Wh), watts (W), or volt-amperes (VA).