What Is Energy Density and How Do I Compare Batteries?

Energy density measures how much energy a battery stores relative to its size or weight, and it is the main reason runtime improves in thinner, lighter devices.

Two common figures

Gravimetric energy density is expressed in watt-hours per kilogram (Wh/kg); volumetric energy density is expressed in watt-hours per liter (Wh/L). Energy itself is calculated as capacity (Ah) multiplied by average voltage (V), so a cell with the same mAh but higher voltage stores more energy.

Compare like with like

Numbers are only meaningful when compared on the same basis. A high-voltage cell looks better than a conventional one partly because of its voltage; pouch cells often beat cylindrical cells on volumetric density; and measured values depend on discharge rate, temperature and end-of-life point. Always compare the chemistry, format, test conditions and whether the figure includes the protective circuit or just the bare cell.

What to prioritize

For wearables and thin devices, volumetric density (Wh/L) usually matters most; for drones and weight-critical systems, gravimetric density (Wh/kg) dominates. Ask the supplier for the test conditions behind any headline number.