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Battery rated energy vs. capacity

If you’ve ever been shopping for a battery, chances are you’re familiar with the often confusing task of decoding specifications. One question that commonly comes up during battery specifications comparison is, what’s the difference between rated energy and capacity?

It’s actually very important to distinguish between rated energy and capacity, which are technically specified as watt hour (Wh) and amp hour (Ah), respectively. Despite what some may think, these values are not correlated. So, if a decision is made based on capacity alone, you might not end up with a suitable battery for your application

What’s the difference between Ah and Wh?

An amp hour (Ah) is a measure of charge and provides an estimate of how much energy a battery can hold. It is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour. A watt hour (Wh), on the other hand, is a measure of power and indicates the equivalent to one Watt of average power flow over an hour.

How to compare battery systems

Putting this knowledge into practice, let’s take a look at the table below comparing two different battery storage systems. In residential storage solutions there’s a broad range of batteries available, each with specific energy content. Someone can find two commercial battery storage systems with the same rated energy of 9.8 kWh, but different capacities. Let’s call them System A and System B. 


System A

System B

Rated Energy

9.8 kWh

9.8 kWh

Rated Capacity

63 Ah

189 Ah

Internal battery voltage

156 V

52 V

Output Voltage

400 V

52 V


Why do they have different capacities but the same rated energy? Because capacity is equal to the ratio of energy and voltage. System A has an internal battery voltage of 156 V while System B, with the higher capacity, has an internal battery voltage of 52 V.  Furthermore, System A offers an output voltage of 400 V, indicating the presence of an internal DC-DC converter. System B, on the other hand, has no DC-DC converter since the output voltage is 52 V.

Why capacity isn’t everything

The difference is even more apparent when comparing battery systems across applications. Have you ever looked at the battery specifications of your power bank or smartphone battery? Nowadays, everyone makes use of portable energy in their day-to-day life. The batteries of a power bank fit in the palm of your hand and can be used to charge your smartphone or laptop. Consider a power bank with an energy content of 37 Wh and a capacity of 10 Ah. Compared to the residential battery System A with a capacity six times as large, the energy content of the power bank is as much as 264 times smaller. This is due to the difference in internal voltage, as the power bank battery voltage is only 3.7 V.

So the next time you look at specifications of a battery system, keep in mind that the capacity of a battery does not reveal all there is to know about the system. As we have learned, battery voltage is the missing link that allows us for direct comparison between a set of battery systems. But the most important specification for your application will always be the rated energy.


 Jolien Despeghel                      Jeroen Tant

PhD Researcher                        Postdoctoral Researcher

EnergyVille – KU Leuven            EnergyVille – KU Leuven