When we shove things into our vehicles and turn them from boring old liquids or tubs of chemical goop into sweet, sweet motion, we soon realize not all fuels are created equal. Why is it that a gasoline car, for example, can go so much farther than an electric? And why can nuclear subs go without refueling for decades? Specific energy is why.

Specific energy is how much energy is in a given amount of something. It's sometimes referred to as energy density, which is a bit more self-explanatory but technically refers to volume, not mass. This infographic here uses a given unit of mass, a kilogram (2.2 lbs for us backward Americans) so sticklers may have issue with some of my labeling. Sorry.

The energy units used are in joules, specifically megajoules, where each joule is a unit of energy/work that has a number of different definitions, but a handy reference is to think that a thousand of those little joules all packed up into a megajoule has about the same amount of kinetic energy as a one-ton car barreling around at 100 MPH. So, it's a bunch of energy.

One of the big things to realize from this chart is that we got pretty lucky that gasoline exists. Gasoline and Diesel fuel both have very similar specific energy, and it's a good leap over what previous fuels like wood and coal had, which allowed early cars to be the sloppy, inefficient things they were while still being able to do useful work.

And while electric car drivetrains are very efficient, the ranges of electric cars is still low compared to gas cars because chemical storage of electrical power, even in advanced Li-ion batteries like the Tesla uses, has far less specific energy than gas has, or, for that matter, fat has.


That fat has a much higher energy density than TNT is one of those facts that's hard to wrap your head around, but there it is. So be extra careful the next time you decide to smoke and fall down some stairs if you're a bit out of shape, like most of us.

Anyway, try not to dwell on the not-really scaled nature of the chart, and I hope this helps!


Oh, and please note that for compressed air, the specific energy is so low partially because of factoring in the weight of the high-pressure vessels needed to contain and use it as an energy source.