Editing Fuel efficiency (section)

==Energy efficiency terminology==
[[Energy efficiency (physics)|Energy efficiency]] is similar to fuel efficiency but the input is usually in units of energy such as [[megajoules]] (MJ), [[kilowatt-hours]] (kW·h), kilocalories (kcal) or [[British thermal units]] (BTU). The inverse of "energy efficiency" is "[[energy intensity]]", or the amount of input energy required for a unit of output such as MJ/passenger-km (of passenger transport), BTU/ton-mile or kJ/t-km (of freight transport), GJ/t (for production of steel and other materials), BTU/(kW·h) (for electricity generation), or litres/100&nbsp;km (of vehicle travel). Litres per 100&nbsp;km is also a measure of "energy intensity" where the input is measured by the amount of fuel and the output is measured by the [[distance]] travelled.  For example: [[Fuel economy in automobiles]].

Given a heat value of a fuel, it would be trivial to convert from fuel units (such as litres of gasoline) to energy units (such as MJ) and conversely. But there are two problems with comparisons made using energy units:
* There are two different heat values for any hydrogen-containing fuel which can differ by several percent (see below).
* When comparing transportation energy costs, a [[kilowatt hour]] of electric energy may require an amount of fuel with heating value of 2 or 3 kilowatt hours to produce it.