Understanding energy density
Watt-hours per kilogram measures energy density as a function of mass, making it particularly valuable for comparing different energy sources and storage solutions. The unit quantifies how much energy a given weight of material can hold, which directly impacts payload capacity, fuel efficiency, and overall transportation costs.
While Wh/kg measures mass energy density, there’s also volumetric energy density measured in Watt-hours per cubic meter (Wh/m³), which accounts for the space a material occupies rather than its weight.
Scale variations
The Wh/kg unit scales up depending on the magnitude of energy being measured:
- 1 Kilowatt-hour per kilogram (kWh/kg) equals 1,000 Wh/kg
- 1 Megawatt-hour per kilogram (MWh/kg) equals 1,000 kWh/kg or 1 million Wh/kg
- 1 Gigawatt-hour per kilogram (GWh/kg) equals 1,000 MWh/kg or 1 million kWh/kg
Practical applications in logistics
Mass energy density is critical for the transportation industry when selecting fuels and battery systems for vehicles. Understanding the Wh/kg rating helps logistics professionals make informed decisions about:
- Vehicle range and refueling requirements
- Payload capacity versus fuel weight
- Battery selection for electric fleet vehicles
- Cost-efficiency of different energy sources
Real-world examples
Different materials exhibit vastly different energy densities:
Lower density (Wh/kg):
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- Capacitors: 1 Wh/kg
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- Super-capacitors: 27.7 Wh/kg
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- Alkaline batteries: 163 Wh/kg
- Lithium batteries: 500 Wh/kg
Medium density (kWh/kg):
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- Wood: 4.5 kWh/kg
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- Coal: 6.6 kWh/kg
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- Kerosene: 11.9 kWh/kg
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- Diesel: 12.6 kWh/kg
- Gasoline: 13.1 kWh/kg
High density (GWh/kg):
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- Uranium-235: 22 GWh/kg
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- Plutonium-239: 23.2 GWh/kg
- Tritium: 93.7 GWh/kg
For logistics operations, conventional fuels like diesel and gasoline offer significantly higher energy density than current battery technology, which explains why long-haul trucking still predominantly relies on liquid fuels despite the growth of electric vehicles in shorter-range applications.
