Understanding fuel emissions in shipping
Shipping’s fuel transition is not just about swapping one fuel type for another. To judge whether fuel is truly low-carbon we must look at its entire life cycle - from feedstock production through refining and transport to combustion on board. That is the “well-to-wake” (WtW) approach endorsed by IMO and used in regulations like FuelEU Maritime (FuelEU).
Figuring out the WtW emissions for sustainable marine fuel (SMF) can be tricky. While it is tempting to look only at the emission value included in the Proof of Sustainability (PoS), that value is not reflective of the complete well-to-wake emissions for shipping companies and their customers.
Fortunately, the recently published FuelEU “calculation methodologies” report (ESSF SAPS WS1) elaborates how to calculate complete well-to-wake emissions. The European Sustainability Shipping Forum (ESSF) is a platform for collaboration between the European Commission, EU Member States, and maritime stakeholders, including the MMM Center. The Sustainable Alternative Power for Shipping (SAPS) sub-group has undertaken a series of documents with best practices to support industry implementation of the FuelEU. These documents can be found on the European Commission’s FuelEU webpage.
This article aims to break down examples of how WtW emissions can be calculated for different fuel types and makes available the simplified WtW calculator that the Katalist team uses as a secondary check on voyage data that is booked.
What does “Well-to-Wake” actually mean?
Every fuel’s greehouse gas (GHG) footprint has two main stages:
Well-to-Tank (WtT): emissions from extraction, production, processing, and distribution.
Tank-to-Wake (TtW): emissions when the fuel is used onboard a ship.
Put simply:
Well-to-Wake (WtW) = Well-to-Tank + Tank-to-Wake
This full picture matters because some fuels — like hydrogen-based fuels — might be low-emission at combustion but require inputs during production that generate emissions. Others, like conventional fossil fuels, could have low production emissions but high carbon emissions when burned.
Fuels which are derived from biological sources such as bio-diesel or which have a biological input, such as e-methanol will have Tank-to-Wake emissions from combustion which are balanced by the negative emissions due to their biogenic source.
The ESSF’s recent paper breaks down clear, harmonized calculation methodologies based on FuelEU annexes as well as other related EU legislation. The report was written by industry exports for the purpose of providing greater clarity to practitioners and to enable consistent calculation of GHG emissions. Importantly, it also offers concrete examples of how these calculations should be done for different SMF types like biomethane and bio-oils.
Calculating WtW based off input from POS
When shipping companies receive POSs, there is an “E” value listed that corresponds to the total GHG emissions from supply and use of that fuel. However, this value often excludes the “Eu” value for non-CO2 gases (N20 and CH4) emitted from the consumption of that fuel, and also does not include the methane slip that occurs during consumption of that fuel. Both of these values need to be added in by the shipping companies in order to accurately capture the WtW emissions for that fuel type.
The ESSF report has several examples of what these calculations should look like in Sections 1.2.3 (page 17) and Sections 3.4.1 (page 78).
Katalist WtW Calculator
The Katalist team used these examples to create an excel tool where companies enter the following input to calculate WtW based off of this methodology:
1. SMF type
2. E value from the POS
3. Methane slip assumptions for engines.
4. SMF consumption per engine
Based off these parameters the WTT and TTW factors are calculated. This tool also allows for scenarios when one batch of SMF is associated with multiple POSs, allowing you to create a weighted average WtW for one batch of fuel.
The calculations are straightforward and based on the FuelEU. The report illustrates how the “E” value in a POS is not reflective of the actual WtW emissions for that fuel. For example, if you are a carrier who has received a batch of B100 bio-diesel and the corresponding POS lists the emission intensity as 15 gCO2eq/MJ, the actual WtW intensity is 16.33 gCO2eq/MJ. This difference is driven by the non-CO2 emissions from fuel use that are not included in the POS. While a 1.33 gCO2eq/MJ difference this may seem minor, data accuracy and completelness is essential when comparing fuels.
In FuelEU these values are provided as defaults in Annex II. While the FuelEU uses global warming potential values (GWP), the values that convert non-CO2 gases into CO2eq, form the Intergovernmental Panel on Climate Change (IPCC)’s 4th Assessment Report, Katalist uses the more recent value from IPCC’s 5th Assessment Report.
What does this mean for shipping
Understanding the full Well-to-Wake picture helps cargo owners, ship operators and fuel suppliers make fair comparisons between energy options. Not all “green” fuels are equal — their production method defines their true footprint. At the same time the POS is needed for specific information, but offers an incomplete picture for lifecycle comparisons.
By looking at all emissions from a fuel, the industry can make better-informed decisions and avoid shifting emissions from one stage to another.
References
European Sustainable Shipping Forum (ESSF) - FuelEU Calculation Methodologies Paper, 2025.
European Sustainability Shipping Forum (ESSF) - Sustainable Alternative Power for Shipping sub-group
International Council on Sustainable Fuels (ICSF) — Lifecycle emissions guidance, 2024.
FuelEU Maritime Regulation (EU) 2023/1805.