When choosing or designing a battery system, many people subconsciously believe that the larger the capacity, the stronger the load it can carry. So, when the device can no longer power high-power appliances, the first reaction is to “replace it with a battery of a larger capacity.”
This intuition is often wrong. What truly determines whether it can drive or not is the power (kW). It is the capacity (kWh) that determines “how long it can last”. Confusing the two can lead to minor issues such as incorrect selection and cost overruns, and in severe cases, the system may fail to work properly or even damage the equipment.
Two core concepts
- Capacity (kWh) -kWh (kilowatt-hour) is a unit of electrical energy, indicating the total energy that a battery can store. 1 kWh is equivalent to the amount of electrical energy consumed by a 1-kilowatt appliance in one hour of operation.
Typical applications: 5 to 20 kWh for residential energy storage batteries, 40 to 100 kWh for electric vehicles.
- Power (kW) – kW (kilowatt) is a unit of power, indicating the speed at which a battery charges or discharges electrical energy instantaneously. It determines how much load the battery can drive simultaneously.
The Relationship between the Two: The C rate is the key link
Capacity and power are not independent; they are linked through the C-rate:
Power (kW) = capacity (kWh) × C rate
The C rate indicates at what speed the battery discharges. For example:
- 1C: All energy is discharged within 1 hour. At this point, the power value equals the capacity value (a 10 kWh battery discharges at 10 kW).
- 0.5C: Discharged in 2 hours. Power = capacity × 0.5
- 2C: Discharged in 0.5 hours. Power = capacity × 2
Here comes the key point: For batteries with the same capacity of 10 kWh, some can only output 5 kW (0.5C), while others can output 20 kW (2C). The “carrying capacity” of the two differs by four times, but the “total energy” is the same.
How to select the right type?
- Clarify the load requirements
- Calculate the required capacity
- Verify whether the power is sufficient
- Check the heat dissipation and BMS
To understand a battery specification data sheet, the key is to think about “energy” and “power” separately. Next time you see a battery labeled “10 kWh”, ask yourself: What is its continuous power? What is its peak power? How long can it last? The answers will tell you whether it is a truck that can carry heavy loads or a fuel-efficient car that can only carry light loads.
