In discussions related to renewable fuels you often come across the terms well-to-wheel, tank-to-wheel or well-to-tank, or their respective abbreviations WtW, TtW, or WtT. These are terms that describe the scope of emissions that are considered in a calculation. For example, in the context of the Renewable Energy Directive, the Well-to-wheel emissions are considered.

But what do these terms mean and how do they compare?

The figures and text below explain the differences between them.

• Well-to-Wheel (WtW): A complete life-cycle perspective of energy use and emissions, covering the entire chain from resource extraction (oil, gas), feedstock growth (biomass) or feedstock generation (electricity generation) through fuel production, distribution, and finally consumption in the vehicle. It combines both well-to-tank and tank-to-wheel phases.
• Well-to-Tank (WtT): Focuses on the upstream part of the chain—energy and emissions associated with extracting, refining/producing, processing, transporting, and distributing a fuel until it is available at the vehicle’s tank or battery.
• Tank-to-Wheel (TtW): Covers the downstream use phase—this means the combustion of the fuel in the vehicle. This is also often referred to as tailpipe emissions.

Note: If we talk about energy use or emission in aviation or in shipping, we do not refer to a wheel, but instead refer to a wing or wake, respectively. This means the abbreviation WtW could mean either Well-to-Wheel, Well-to-Wing or Well-to-Wake depending on the context.

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While biofuels emit carbon when combusted in a vehicle, these emissions are equalled out by the uptake of carbon from the atmosphere during the growth of the biomass. Therefore, the Tank-to-wheel or tailpipe emissions are often considered as zero. This gives biofuels a considerably advantageous GHG performance compared to fossil fuels, which make them a solid climate solution.