Energy Density

The amount of energy stored in an object, expressed in watt-hours per kilogram (1 Wh/kg = 3.6 kJ/kg). For example, the energy density of a standard rechargeable battery ranges from 70 to 100 Wh/kg. This rises to 150 to 200 Wh/kg for a lithium-ion battery and up to 1,500 Wh/kg for some lithium or manganese batteries. This is far lower than the energy density of a liquid fuel, which is around 10,000 Wh/kg.