Energy density

Energy density refers to the amount of energy per weight or volume of a given substance (e.g. gasoline) or device (e.g. batteries).

Weight is a significant factor in transportation energy storage because the more something weighs, the more difficult it is to accelerate (after a stop), and hence the more of the fuel gets used in order to be able to transport the remaining fuel. This may be less important in some applications (e.g. freight trains, where the weight of the cargo hugely overwhelms the weight of the fuel), but for most transportation it is the key factor. It is especially significant in aviation, where any increase in mass leads to a significant increase in air drag..

Volume is also important, however, in that the storage space for the fuel should not take up more than a small fraction of the vehicle's available volume; a fuel with high energy-per-mass but low mass-per-volume will require a large tank for storage, which is inconvenient and can also weigh a lot and counteract the benefits of the fuel's energy-per-mass.