What is Energy Capacity?

• A. The rate at which energy can be drawn from a system.
• B. The total amount of energy that can be stored or delivered by a battery system or energy storage system, typically measured in kilowatt-hours (kWh).
• C. The maximum voltage the system can produce.
• D. The number of cycles a battery can undergo.

Answer: (B)

Energy capacity is the total amount of energy a battery or energy storage system can store and later deliver. It’s a measure of stored energy, not how fast you can draw it. The common unit is kilowatt-hours (kWh), which means the system could supply a certain power for a certain time (for example, 100 kW for 1 hour equals 100 kWh). In real use, the usable energy is less than the rated capacity due to depth of discharge limits and charging/discharging losses. So a 100 kWh system might provide around 72 kWh of usable energy if you typically discharge to 80% with about 90% round-trip efficiency. The other characteristics—rate of energy flow (power in kW), maximum voltage, and the number of cycles—describe different aspects and aren’t the total stored energy.