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SESK: Stuff Everyone Should Know · sesk.info/eng-specific-energy-vs-power

A battery's specific energy is in Wh/kg. What does its specific power in W/kg tell you?

How fast it can deliver energy per kilogram, not how much it stores.

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  1. How much total energy it stores per kilogram
  2. The maximum power it can deliver per kilogram ✔
  3. How many charge cycles it lasts

Why: Energy is the size of the tank; power is how fast you can empty it. Specific energy helps determine range for a given battery weight, while specific power helps determine acceleration capability.

Huh, didn't know that: The same ideas per liter instead of per kilogram are called energy density (Wh/L) and power density (W/L).

The dad joke
My battery has lots of energy but no power. It is basically me on a Monday.
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References

Answer: The maximum power it can deliver per kilogram. Energy is the size of the tank; power is how fast you can empty it. Specific energy helps determine range for a given battery weight, while specific power helps determine acceleration capability.

  1. MIT Electric Vehicle Team, A Guide to Understanding Battery Specifications web.mit.edu/evt/summary_battery_specifications.pdf
    Specific Power (W/kg) - The maximum available power per unit mass.
    Checked 2026-10-10.

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