Electric battery capabilities are measured in three different ways:
- Power capacity or power rating: The maximum amount of power that a battery can instantaneously produce on a continuing basis. This can be compared to the nameplate rating of a power plant. Power capacity or rating is measured in megawatts (MW) for larger grid-scale projects and kilowatts (kw) for customer-owned installations.
- Energy storage capacity: The amount of energy that can be discharged by the battery before it must be recharged. This can be compared to the output of a power plant. Energy storage capacity is measured in megawatt-hours (MWh) or kilowatt-hours (kWh).
- Duration: The length of time that a battery can be discharged at its power rating until the battery must be recharged.
The three quantities are related as follows:
Duration = Energy Storage Capacity / Power Rating
Suppose that your utility has installed a battery with a power rating of 10 MW and an energy capacity of 40 MWh. Using the above equation, we can conclude that the battery has a duration of 4 hours:
Duration = 40 MWh / 10 MW = 4 hours
This means that if the battery is fully charged, and discharged at its maximum power rating, it will provide energy for four hours before needing a recharge. Of course, if it is discharged at less than its maximum rating, it could provide energy for a longer period of time.