Specific Energy Converter

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, what matters is how much energy is packed into each kilogram, not the total. It is the number behind why an electric car is heavy and a petrol one is not: gasoline carries roughly 44 MJ/kg, a good lithium-ion cell about 0.8, so on mass alone petrol wins by a factor of fifty. An electric drivetrain claws a lot of that back by being three times as efficient at turning stored energy into motion, but not all of it, and that gap is the whole engineering problem.

The number this page does not give you, and why it matters

Specific energy is per kilogram. Volumetric energy density is per litre, and for anything that has to fit in a tank the second one often decides the argument.

Hydrogen is the clearest case. At 142 MJ/kg it is the best fuel on this page by a wide margin, nearly three times gasoline. But hydrogen gas is extraordinarily light, so a litre of it at 700 bar holds about 5 MJ against gasoline’s 34. Per kilogram hydrogen wins easily; per litre it loses badly. That single fact is most of the reason hydrogen cars have not displaced anything.

Key reference values:

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

The 4,184 there is the thermochemical calorie, which is the one nutrition labels and chemistry use. There is also an International Table calorie of 4,186.8 J, and that is the one our specific heat converter uses, because it makes the calorie-to-BTU identity exact. The difference is 0.07%, which never matters for a food or fuel figure, but it does explain why two perfectly good sources can quote slightly different calorie conversions.

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.

A sanity check you can run on this page. Propane is listed at 50.3 MJ/kg. Our propane BTU converter puts a gallon at 91,502 BTU and 4.24 lb, which is 96.5 MJ in 1.923 kg, or 50.2 MJ/kg. The two pages were built from completely different sources and land within 0.2% of each other, which is about as much agreement as published fuel figures ever give you.