Energy is transported over land primarily by electric power transmission lines and by pipelines carrying gas or liquid fuel. Pipelines today carry fossil fuels, but in future will carry renewable fuels. The fuel power density is $F=ρhh$, where $ρ$ is the density and h the specific enthalpy of combustion. It is a figure of merit by which candidate renewable fuels can be ranked for suitability as energy vectors. This figure of merit is more useful than the alternatives: specific energy and energy density. It underscores the advantage of high-density liquid renewable fuels such as ammonia and methanol over gaseous fuels such as hydrogen and natural gas.

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