Energy density

Fuel energy density is discussed on this page, for energy density of storage devices like batteries, click here.
Figure 1. An XKCD comic showing the comparative energy density of Uranium.[1]

Energy density is the amount of energy that can be stored in a given mass of a substance or system.[2][3] The higher the energy density of a system or material, the greater the amount of energy stored in its mass.[4] Energy can be stored in many different types of substances and systems.

A material can release energy in four types of reactions. These reactions are nuclear, chemical, electrochemical and electrical.[5] When calculating the amount of energy in a system most often only useful or extractable energy is measured. In scientific equations, energy density is often denoted by U.[6]

Energy density is generally expressed in two ways, although the first is more common:

  • Volumetric energy density - how much energy a system contains in comparison to its volume; typically expressed in watt-hours per liter (Wh/L) or Megajoules per liter (MJ/L).[7]
  • Gravimetric energy density (Specific energy) - how much energy a system contains in comparison to its mass; typically expressed in watt-hours per kilogram (Wh/kg), or Megajoules per kilogram (MJ/kg).[7]

Having a high energy density does not give information on how quickly this energy can be used. This knowledge is contained in a substance's power density, which describes the rate at which its energy can be put out. Typically having a high energy density goes along with a low power density. Visit energy density vs power density for more information and examples.

Fuel energy density

Many different materials can store energy, ranging from food, to diesel, to uranium. These materials are known collectively as fuels, and all of these fuels are used as energy sources for a variety of systems. When the fuels come directly from nature (like crude oil) they are primary fuels; when the fuels have to be modified so they can be used (like gasoline) they're referred to as secondary fuels. The table below shows the energy density for a variety of common fuels.

For a visual representation of these values, Figures 1 and the graph to the right show comparisons of energy densities of different fuels.

Fuel Type Reaction Type Energy Density
Typical uses
Wood Chemical 16 Space heating, Cooking
Coal Chemical 24 Power plants, Electricity generation
Ethanol Chemical 26.8 Gasoline mixture, Alcohol, Chemical products
Biodiesel Chemical 38 [8] automotive engine
Crude oil Chemical 44 Refinery, Petroleum products
Diesel Chemical 45 Diesel engines
Gasoline Chemical 46 Gasoline engines
Natural gas Chemical 55 Household heating, Electricity generation
Uranium-235 Nuclear 3 900 000 Nuclear reactor
electricity generation
Table 1: The energy density(MJ/kg) of a variety of different fuels.[9]

How far can you go?

Energy sources do not give up their energy in the same ways, but assuming they could, how far would each move a vehicle? To find out, coal can be used as a base-line, if the amount of energy in a particular mass of coal equal to 10 meters - the length of a school bus. This makes the energy available in that same mass of uranium equal to the distance between Kelowna, British Columbia to Winnipeg, Manitoba (Figure 2). Below is a list of other fuels compared to coal to figure out the energy per distance to compare other fuels to coal.

  • Wood - 7 meters, about the width of a two car garage
  • Coal - 10 meters, about the length of a school bus
  • Crude oil - 18 meters, roughly the length of a humpback whale
  • 235Uranium - 1,625,000 m (1,625 km), greater than the distance from Vancouver to Saskatoon
Figure 2. The distance from Vancouver, BC to Saskatoon, SK using the energy density of Uranium.

Just to reiterate, this is a comparison test. To see the energy in crude oil, set its distance equal to 20 kilometers, which is a reasonable distance for a kilogram of gasoline in a car, then the comparative distance a kilogram of 235Uranium would take a car 1.77 billion meters. How far is that? That's a trip from the Earth, to the moon, and back. Twice.[10] 235U is an incredibly energy-dense fuel.


  1. XKCD. Log Scale [Online], Available:
  2. C. Dillon. (2009, October). How Far Will Energy Go? - An Energy Density Comparison [Online]. Available:
  3. A. Golnik and G. Elert. (2003). Energy Density of Gasoline [Online]. Available:
  4. Uni. South Carolina. (2003, October). Description of Energy and Power [Online]. Available:
  5. B. E. Layton, "A comparison of Energy Densities of Prevalent Energy Sources in Units of Joules Per Cubic Meter," Int. J. Green Energy, vol. 5, no. 6, pp. 438-455, Dec. 2008.
  6. E. W. Weisstein. (2007). Energy Density -- from Eric Weisstein’s World of Physics [Online]. Available:
  7. 7.0 7.1 C. Simpson, "Characteristics of Rechargeable Batteries," National Semiconductor. Texas Instruments Inc., Dallas, 2011.
  8. Y. Chisti, “Biodiesel from microalgae,” Biotechnol. Adv., vol. 25, no. 3, pp. 294–306, May-Jun. 2007.
  9. I. Hore-Lacy, "Future Energy Demand and Supply," in Nuclear Energy in the 21st Century, 2nd ed., London, UK: WNUP, 2011, ch.1, sec.6, pp.9
  10. Wolfram Alpha, entry: 1772727273 m

Authors and Editors

Jordan Hanania, Braden Heffernan, James Jenden, Ryan Leeson, Tamsin Mah, Jasper Martin, Kailyn Stenhouse, Jason Donev