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Capacity is a unit of measurement for the amount of energy that can be stored in a battery storage system and is specified by default in the unit kilowatt hours [kWh]. It describes how much energy a battery can absorb or deliver. Coupled with the Capacity, it 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 specified in ampere-hours. For example, today's cell phone batteries have an average capacity of 2500-4000 mAh (2.5-4 Ah). A powerbank with about 10 000 mAh can thus charge our current smartphones about 2-4 times. Slightly larger but still in ampere-hours are car batteries. These usually have a capacity of 80 Ah.

However, the ampere-hours can also be easily converted into the usual unit of larger storage units (kilowatt-hour). To do this, the capacity in ampere-hours is multiplied by the voltage of the battery. Car batteries usually have a voltage of 12 V. The capacity of 80 Ah multiplied by the voltage of 12 V results in a capacity of a car battery of 960 Wh, i.e. approx. 1 kWh.
Stationary large-scale battery storage, have capacities of 10 MWh to 200 MWh, 200,000 times that of a car battery.

Capacity & Power - What is the difference?

For batteries, the 2 most important properties are the capacity of the battery and the power. The capacity tells us how much electrical energy can be stored chemically in the battery, while the power tells us how much energy can be converted in one moment. With these two properties it can be determined how long the storage can supply a consumer electrically.

This can be visualized well by the simplified example of a beer barrel with a tap. The capacity of the barrel indicates how many liters fit into the barrel - let's assume 10 liters.
If the tap is now opened so far that approx. 10ml per second flow out, the barrel can therefore provide 10ml of beer per second, which represents the capacity of the beer barrel.
Taking this output and the 10 liter capacity, the barrel would be empty after exactly 1000 seconds, which is a little over 16 minutes.
The capacity can therefore be used to determine how long the barrel can provide the power until it is empty.

If we transfer the example to the storage facilities, we get the same principle. A storage facility can store a certain amount of energy, for example 200 MWh, and has a capacity of 100 MW.
If we divide the capacity by the power, we find that the storage facility with the capacity of 200 MWh can provide the power of 100 MW for 2 hours.

Capacity & Energy Consumption - Both kWh?

The unit kWh is known not only by its capacity, but also by your annual electricity bills, since electricity consumption is also expressed in kWh.  

So what is the difference ?

Both describe a quantity of energy. In the case of the capacity of a battery, however, the kWh describes the amount of energy that is stored and can be used again at a later time as electrical energy.
The electricity consumption describes the amount of energy that your household has used, for example, within a year. This amount of electricity was used by the various household appliances and can not be reused as with the capacity.

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

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