What this specific energy comparison can tell you
Useful capabilities
Compare the same mass boundary and energy definition, then account for pack-level components separately.
The listed units express the same supplied specific energy quantity. The 200 Wh/kg → 720 kJ/kg example makes the selected convention visible before it is used in a longer calculation.
Important limits
Specific energy is energy per mass. Volume-based energy density needs a separate density value.
Align the mass boundary before comparing specific energy. Power per mass is a separate quantity and depends on the time behaviour.
| Possible mismatch | What resolves it |
|---|---|
| Comparing cell and complete-pack figures directly | Align the mass boundary before comparing specific energy. |
| Treating Wh/kg as a discharge power rating | Power per mass is a separate quantity and depends on the time behaviour. |
A cell-only energy-per-mass rating may omit an enclosure, wiring and other pack components. Usable energy can also differ from nominal stored energy. Retain the numerator and denominator boundaries before comparison. Wh/kg and W/kg measure different quantities; Wh/L needs a mass-volume relationship to connect to Wh/kg. A prefix conversion cannot establish the performance of a complete battery design.
As a controlled uncertainty example, suppose the original 200 Wh/kg record had a stated ±1% input interval. Its endpoints would be 198 Wh/kg and 202 Wh/kg. Converting those endpoints gives an output interval from 712.8 kJ/kg to 727.2 kJ/kg. The interval scales with the same fixed unit ratio as the central value. This chosen interval illustrates the arithmetic and is not an accuracy claim for your measurement.
Energy per mass does not tell you the whole system mass
A battery comparison quotes specific energy in Wh/kg. One figure can describe a cell, another a complete pack with housing and electronics. Translating 200 Wh/kg into kJ/kg aligns the energy unit but cannot reconcile those different mass boundaries or usable-energy definitions.
A specific energy of 200 Wh/kg equals 720 kJ/kg. An illustrative 2 kg mass at that value would represent 400 Wh if the specified mass basis applies. The multiplication does not establish that a finished battery pack has the same value as the cells inside it.
Cell-only mass and system mass can include different components. Enclosure, wiring, cooling and electronics can reduce the energy per kilogram of the complete system compared with the cell figure. Usable energy may also differ from nominal stored energy. Identify both the numerator and the mass boundary before ranking specifications.
Volumetric energy density in Wh/L uses a different denominator. Changing Wh/kg into Wh/L requires density or a suitable mass-volume relationship; it is not a fixed unit factor. Specific power in W/kg is yet another quantity and describes a rate rather than stored energy. A good comparison table keeps energy, power, usable fraction and the mass basis separate. The converter normalizes the supplied energy-per-mass quantity without inferring a pack design.
200 Wh/kg = 720 kJ/kg; at the same stated basis, 2 kg represents 400 Wh.
Specific Energy units, side by side
The values below all describe 200 Wh/kg. They provide a comparison of unit sizes for specific energy; they are not separate measurements. Read the complete symbol and convention before copying a number.
- J/kgJoules/kilogram
- 720,000 J/kg
- kJ/kgKilojoules/kilogram
- 720 kJ/kg
- Wh/kgWatt-hours/kilogram
- 200 Wh/kg
- kWh/kgKilowatt-hours/kilogram
- 0.2 kWh/kg
- cal/gThermochemical calories/gram
- 172.084130019 cal/g
Keep the original watt-hours/kilogram record alongside the converted kilojoules/kilogram display. The unit list uses up to twelve significant digits for arithmetic readability, while the measurement precision still comes from its source.
Stored energy per defined mass
Specific energy is energy per unit mass. Convert J/kg, kJ/kg, Wh/kg, kWh/kg or thermochemical cal/g for supplied data. It is useful for comparing like-for-like material or storage measurements, but total system performance may include mass and losses outside the quoted material value.
One Wh/kg equals 3,600 J/kg, while one thermochemical cal/g equals 4,184 J/kg because both energy and mass units change. Specific energy differs from volumetric energy density and from specific power. Converting to energy per volume requires density; converting to total energy requires a clearly defined mass.
Total energy equals specific energy multiplied by the mass to which the figure applies. Volumetric energy density additionally needs a density or volume relationship. Specific power in W/kg is a rate and is not interchangeable with Wh/kg. Test conditions, nominal versus usable energy and system additions should be made explicit in a meaningful comparison.
Compare the same mass boundary and energy definition, then account for pack-level components separately.
Questions about specific energy
Does 200 Wh/kg mean a kilogram supplies 200 watts?
It describes 200 watt-hours of energy per kilogram under the stated measurement basis. Power and duration require additional information.
Can this convert Wh/kg into Wh/L?
A density or equivalent mass-per-volume relationship is needed. The conversion alone cannot supply the material’s density or packing arrangement.
Before using this specific energy result
- Identify whether the denominator is active material, cell, pack or total system mass.
- Keep usable versus nominal energy and the test conditions explicit.
- Distinguish Wh/kg from W/kg; one concerns energy and the other power.
Keep the original specific energy value, its unit and the relevant conditions in the record. A copied answer of 720 kJ/kg is interpretable only when those details remain available. State which of the measurement checks above you verified before using the value in another calculation.
200 Wh/kg in kilojoules/kilogram
200 Wh/kg = 720 kJ/kg
Output = input × source factor ÷ destination factor200 × 3,600 ÷ 1,000 = 720 kJ/kg. The two factors express the source and destination relative to the same internal reference unit. Their ratio is the multiplier.
Both unit factors refer to the same internal reference for specific energy. Multiplying by the source factor and dividing by the destination factor changes the numerical scale while retaining the quantity. The source definition still matters: Align the mass boundary before comparing specific energy.
Reversing the unrounded 720 kJ/kg result returns 200 Wh/kg. Use this as a direction check. A consistent reverse calculation cannot resolve comparing cell and complete-pack figures directly or improve the original observation.
Compare cell and system boundaries
For a storage comparison, keep the mass boundary and usable-energy definition consistent. Convert only comparable specific-energy values and document system-level additions separately.
The following inputs stay in watt-hours/kilogram and the results use kilojoules/kilogram. They illustrate different magnitudes for this measurement. Retain the specific conditions described above when replacing an example with your own value.
| Starting value | Equivalent value |
|---|---|
| 100 Wh/kg | 360 kJ/kg |
| 200 Wh/kg | 720 kJ/kg |
| 300 Wh/kg | 1,080 kJ/kg |
This specific energy unit pair has a constant factor ratio. Changing the input changes the output by the same proportion. If a copied figure differs by an unexpected factor, review the full unit symbol before changing the source measurement. Power per mass is a separate quantity and depends on the time behaviour.
Compare the same mass boundary and energy definition, then account for pack-level components separately.