Why are Batteries and Cells Rated in Ah (Ampere-hour) and Not in VA or Watts?
A battery stores energy in the form of chemical energy, which later changes into electrical energy when needed. This energy is released gradually over a certain period of time. The total charge a battery can deliver is called its capacity, and it is usually measured in ampere-hours (Ah). This rating tells us how much current the battery can supply for a given number of hours before it becomes fully discharged. For example, a 10 Ah battery can ideally provide 1 ampere for 10 hours or 2 amperes for 5 hours.
Inside a fully charged battery, the number of molecules able to release electrons and keep current flowing in a circuit is fixed. In simple terms, the battery stores a limited quantity of electrons through its chemical reaction. These electrons can move through the circuit only as long as there is stored chemical energy available. Once this energy is used up, the battery reaches its full discharge, similar to how a filled water tank can only supply water until it is empty.
Technically, the capacity of a battery could be described by counting the total number of electrons that flow over a given period of time. But this would be extremely impractical because the number of electrons is unbelievably large. To make things easier, scientists use the unit coulomb (C). One coulomb equals about 6.25 × 10¹⁸ electrons, which is the same as 6,250,000,000,000,000,000 electrons—an amount far too big to handle directly.
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In electrical terms, 1 ampere (A) is defined as 1 coulomb (C) of charge flowing every second. From this definition, we can calculate:
1 hour (h) = 3600 seconds
1 Ah = (1 A) × (3600 s) = (C/s) × (3600 s) = 3600 C
Therefore, 1 ampere is equal to 1 coulomb per second (C/s).
This brings up a common question: why do we use ampere-hours (Ah) to express battery capacity instead of just using coulombs? The answer is simple—practicality. Both units measure charge, but Ah is much more convenient for engineers, technicians, and everyday users because it directly shows how long a battery can provide current.
A helpful comparison is how electricity billing works. Instead of charging households in raw joules, which would be a massive and confusing number, the energy is measured in kilowatt-hours (kWh)—also called a Board of Trade Unit. This makes calculations, bills, and usage much easier to understand, just like Ah does for batteries.
Watch the video below to learn why a battery is rated in ampere-hours (Ah) instead of volt-amperes (VA).
Why Battery is Not Rated in VA (Volt-Ampere) or Watts
The unit VA (volt-amperes) is used to represent apparent power, which is one of the three main types of power in AC circuits. The three types are:
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True (real) power, measured in watts (W)
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Reactive power, measured in VAR (volt-amperes reactive)
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Apparent power, measured in VA (volt-amperes)
The relationship between these three forms of power defines the power factor of a system. In AC circuits, a high power factor means electricity is being used efficiently with minimal energy losses.
On the other hand, batteries supply DC power, where the power factor is always equal to unity (1). This is because DC circuits do not involve frequency or phase differences like AC systems do. For this reason, the concept of power factor is irrelevant in DC applications.
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Another key point is that a battery is not a perfectly constant voltage source. Its voltage gradually decreases as it discharges. Since this value changes over time, rating a battery in watts (W) or volt-amperes (VA) would be misleading. Instead, we measure what truly matters—its capacity, or the total charge it can deliver over a specific period. This is why batteries are rated in ampere-hours (Ah) rather than VA or Wh.
To picture this more clearly, imagine pouring water from a tank. The flow rate might change as the water level drops, but what really matters is the total volume of water available. Similarly, a battery’s voltage may vary, but its Ah rating shows how much usable charge it can actually provide.
