Wh/kg describes energy per unit mass. Read it together with the energy test, mass boundary and whether it refers to a cell or a complete pack.
Cell
Cell energy ÷ cell mass
Cells + hardware
Connections, management, enclosure
Complete pack
Pack energy ÷ complete pack mass
Capacity and energy answer different questions
Ah measures charge capacity. A voltage is also needed to convert that capacity into a nominal energy value. For a measured discharge, energy follows the changing voltage and current over the test.
E_nom (Wh) = V_nom (V) × Q (Ah) E_test (Wh) = ∫ V(t) × I(t) dt · t in hours
| Quantity | Unit | What to confirm |
|---|---|---|
| Charge capacity | Ah / mAh | Rating and discharge method; 1,000 mAh = 1 Ah. |
| Energy | Wh | Nominal calculation or measured discharge energy. |
| Specific energy | Wh/kg | Energy and mass of the same object. |
| Volumetric energy density | Wh/L | Volume boundary and measurement state. |
Reference: IATA · Battery Guidance Document 2026
Cell mass and pack mass
A pack adds connections, protection, management hardware and an enclosure; some systems also add cooling or heating equipment. The denominator changes when these parts are included.
For an illustrative 74 Wh cell weighing 0.25 kg, the nominal calculation is 296 Wh/kg. Six such cells total 444 Wh and 1.5 kg. Adding 0.4 kg of pack hardware gives about 234 Wh/kg at the pack boundary. These are arithmetic examples, not catalogue specifications.
Specific energy (Wh/kg) = Energy (Wh) ÷ Mass (kg)
A documented figure keeps its conditions
A lower-bound specification such as >270 Wh/kg is different from a typical value or an individual test result. Nominal-voltage arithmetic is useful for a first comparison, while the applicable specification and discharge test define the supplied product.
Questions to prepare
- Do you need a cell-level or complete-pack Wh/kg target?
- What total mass and installation volume can the application accept?
- Under which discharge rate, temperature and cutoff conditions is energy required?
Common questions
Does a higher Ah value mean more energy?
Compare Wh when voltages differ. A higher Ah value alone does not establish a higher energy value.
Can a calculated Wh/kg replace the documented figure?
Keep calculations labelled and preserve the original test basis. The two may use different energy, mass or measurement conditions.