Specific Energy Converter
Convert specific energy between J/kg, kJ/kg, BTU/lb, kcal/kg, kWh/kg, and MJ/kg.
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Specific energy is the amount of energy stored per unit mass of a substance. It is sometimes called gravimetric energy density and is expressed in joules per kilogram (J/kg) or equivalent units.
Why specific energy matters:
When comparing fuels, batteries, or food, we care about how much energy is packed into each kilogram — not just the total energy. A fuel tank full of gasoline has more energy than a tank of water because gasoline has much higher specific energy.
Key reference values:
| Material | Specific Energy |
|---|---|
| Hydrogen (gas) | ~142 MJ/kg |
| Gasoline | ~44 MJ/kg |
| Diesel | ~43 MJ/kg |
| Coal | ~24–35 MJ/kg |
| TNT (explosive) | ~4.6 MJ/kg |
| Lithium-ion battery | ~0.5–0.9 MJ/kg |
| Wood | ~15–20 MJ/kg |
| Food fat | ~37 MJ/kg (≈ 9 kcal/g) |
| Food carbohydrates | ~17 MJ/kg (≈ 4 kcal/g) |
Unit conversions:
- 1 kJ/kg = 1,000 J/kg
- 1 MJ/kg = 1,000,000 J/kg
- 1 BTU/lb = 2,326 J/kg
- 1 kcal/kg = 4,184 J/kg
- 1 kWh/kg = 3,600,000 J/kg
Applications:
Specific energy is used in aerospace engineering (fuel selection for rockets and aircraft), battery technology (comparing lithium-ion vs. solid-state vs. hydrogen fuel cells), nutrition science (caloric density of foods), and military applications (explosive yield comparisons). The higher the specific energy, the more “bang per kilogram” you get.