Energy Density Converter: MJ/kg, kWh/kg, BTU/lb

Energy density is the amount of energy stored per unit mass (gravimetric energy density) or per unit volume (volumetric energy density). It determines how much usable energy a fuel, battery, or explosive can deliver for a given weight or size. High energy density is critical for transportation fuels, portable electronics, and aerospace applications where weight and space are constrained. For example, petrol has a gravimetric energy density of about 44.4 MJ/kg, while lithium-ion batteries achieve only about 0.72 MJ/kg, explaining why electric vehicles need much heavier energy storage than petrol vehicles.

Hydrogen has the highest gravimetric energy density of any fuel at around 141.8 MJ/kg, which is roughly three times that of petrol. However, hydrogen gas at standard conditions has very low density, so its volumetric energy density is poor: compressed hydrogen at 700 bar stores about 5.6 MJ/L, compared to petrol at about 34 MJ/L. Liquid hydrogen at -253 degrees Celsius achieves about 8.5 MJ/L. The US Department of Energy publishes targets and data for hydrogen storage technologies. The high gravimetric density makes hydrogen attractive for aviation and long-distance shipping where weight matters most.

Petrol has a gravimetric energy density of about 44.4 MJ/kg (12.3 kWh/kg), while lithium-ion batteries currently achieve around 0.5-0.9 MJ/kg (140-250 Wh/kg). This means petrol stores roughly 50 to 90 times more energy per kilogram than a lithium-ion battery. However, internal combustion engines are only about 20-25% efficient in converting fuel energy to motion, while electric motors achieve 85-95% efficiency, so the effective energy advantage of petrol is reduced to roughly 10-15 times per kilogram of total drivetrain weight. Battery energy density is improving by about 3-5% per year, according to US DOE data.

Gravimetric energy density (also called specific energy) is measured in energy per unit mass, for example MJ/kg or kWh/kg. Volumetric energy density is measured in energy per unit volume, for example MJ/L or kWh/L. The two measures often point in different directions: hydrogen has excellent gravimetric density but poor volumetric density; coal has moderate gravimetric density but can be stored densely. For vehicle applications, both matter: weight affects efficiency and payload, while volume determines tank or battery size.

In electric vehicles, battery energy density is typically expressed in Wh/kg (watt-hours per kilogram) or kWh/kg. A 100 kWh battery pack at 250 Wh/kg would weigh 400 kg. Higher energy density means a lighter battery for the same range, or more range for the same weight. Vehicle range is approximately: battery capacity (kWh) divided by energy consumption (kWh per km). The US DOE Vehicle Technologies Office publishes annual data on battery energy density trends and vehicle performance metrics.