Capacity is a unit of measurement for the amount of energy that can be stored in a battery storage system and is specified as a standard unit of kilowatt hours [kWh]. It describes how much energy a battery can absorb or release. Coupled with the power, can be used to specify how long a battery can supply other consumers with the given power.

Examples and comparisons:

For small batteries (e.g. cell phone batteries), the capacity is often given in amp-hours. For example, today's cell phone batteries have an average capacity of 2500-4000 mAh (2.5-4 Ah). A power bank with approx. 10,000 mAh can charge our current smartphones around 2-4x. Car batteries are slightly larger but still given in ampere hours. These usually have a commercial capacity of 80 Ah.

However, the ampere hours can also be easily converted into the usual unit of larger storage systems (kilowatt hour). For this purpose, the capacity in ampere hours is multiplied by the voltage of the battery. Car batteries normally have a voltage of 12 V. The capacity of 80 Ah multiplied by the voltage of 12 V results in a car battery capacity of 960 Wh, i.e. approximately 1 kWh.
Large-scale stationary battery storage systems have capacities of 10 MWh to 200 MWh and thus 200,000 times that of a car battery.

Capacity & power — What is the difference?

When it comes to batteries, the 2 most important characteristics are the capacity of the battery and the power. Capacity tells us how much electrical energy can be chemically stored in the battery, while power says how much energy can be converted in a moment. With these two properties, it is possible to determine how long the storage system can supply a consumer with electricity.

This can be easily visualized using the simplified example of a beer keg with tap. The capacity of the keg is how many liters fit in the keg — suppose 10 liters.
If the tap is now opened so that approx. 10 ml per second flow out, the keg can therefore provide 10 ml of beer per second, which represents the output of the beer keg.
If you take this power and the 10 liter capacity, the keg would be empty after exactly 1000 seconds, which is just over 16 minutes.
The capacity can therefore be used to determine how long the keg can provide power until it is empty.

If you transfer the example to the memories, the same principle results. A storage system can store a certain amount of energy, for example 200 MWh, and has an output of 100 MW.
If you now divide the capacity by the power, you find that the storage system with a capacity of 200 MWh can provide the output of 100 MW for 2 hours.

Capacity & energy consumption — both kWh?

However, the unit kWh is not only known for its capacity, but also for your annual electricity bills, as electricity consumption is also expressed in kWh.

So what is the difference here?

Both describe an amount of energy. However, when it comes to the capacity of a battery, the kWh describe the amount of energy that is stored and can be used again as electrical energy at a later point in time.
Electricity consumption describes the amount of energy that your household has used within a year, for example. This amount of electricity was used by various household appliances and cannot be reused as with capacity.

Capacity in kWh:
Describes the amount of energy stored in a battery, which can be converted back into electrical energy at a later point in time.

Power consumption in kWh:
Describes the amount of energy that your household has used, for example, and which cannot be used again.