The transport methodology used has been developed by us, Department for Transport and Department for Environment, Food and Rural Affairs. It’s compliant with the UK’s streamlined energy and carbon reporting if the data used is from the company’s current or previous financial year. 

The methodology was established to determine carbon dioxide emissions. You can adapt it to assess your energy use as required for the energy saving opportunity scheme.   

This guidance is based on the Department for Business, Energy and Industrial Strategy (DBEIS)/Department for Energy Security and Net Zero (DESNZ) GHG fuel conversion factors for carbon dioxide equivalents (CO2e/units in litre, kg, kWh and CO2e/kWh). These are published annually.  

The factors used must reflect the data year. For example, if your organisation is reporting for the financial year 2020/21, it would use the 2020 greenhouse gas (GHG) conversion factors published by the Department for Business, Energy and Industrial Strategy. The complete data sets are available from 2014 and new data sets are published every June.    

The methodology covers all vehicles up to 3.5 tonnes. Refer to ‘table 2 – GHG & kWh calculation methods’ for a detailed overview of the five methods used. 

•  Methods one and two are based on data about the actual fuel used or the actual vehicle used. 
•  Methods three to five are based on data from the UK national fleet with information about the vehicle used. 

You can use methods three to five to estimate GHG emissions and energy use if you do not have specific data for the vehicle. However, any improvements made to the fuel efficiency or vehicles on fleet will not be accurately tracked. 

Method one: Fuel use

This method is the most accurate way to determine energy use and GHG emissions. Refer to ‘table 3 – GHG factors’ for more information on GHG emissions and energy use from fuels. 

If you own a directly operated fleet vehicle – an LCV (van) or HDV (lorry) – you should already have access to your fuel data. This data is sometimes available for company or lease cars using fuel cards, where staff get private fuel as a benefit or are reimbursed by the company for their private mileage.  

For opt-out, salary sacrifice or grey fleet, fuel data will not usually be available. In these cases, you can use method five. 

When you have accurate mileage driven, fuel use can give you a measure of energy efficiency. This is known as miles per gallon in the UK, but it can also be represented as miles/kWh, litres/100km and kWh/km. Using kWh is independent of the fuel type or its delivery measurement (litre, kg or kWh). Electric vehicles use Wh (Watt hours) per mile (Wh/mile) or miles per kWh. 

Method two: mileage and published g/km

If you don’t have fuel data, you can use the published GHG emissions of the vehicle in g/km. These are the next most accurate estimate of energy use and GHG emissions. This data has been available for all cars since 2001, for some cars since 1997, and for all vans since 2009.  

You’ll need to submit a P11D form to HMRC for every employee you’ve provided benefits in kind or expenses to. You can get this data from the DVLA via agents such as CarweB using the vehicle registration mark.    

You must apply an uplift factor to the emissions manufacturers publish based on WLTP tests. This is because these are significantly understated relative to real-world driving emissions.  

The uplift factor relates to real world values and considers vehicle types and behaviour variations. For example, CO2 emissions in changing traffic and weather conditions are higher than indicated by the manufacturer.  

In the car fleet, the difference between published data and actual emissions has been increasing steadily. This has been reflected in the DBEIS/DESNZ GHG factors since 2014.  The most accurate method is an age-related uplift based on the DBEIS/DESNZ data. Refer to ‘table 4 – DBEIS/DESNZ real world uplift by year of registration’ for more information.  

The calculation is based on:   

Published g/km x (1+% uplift for year) x distance travelled (km) = gCO2e  

The GHG fuel factor (Refer to ‘table 2 – overview of GHG & kWh calculation methods) for kg CO2e/kWh is used to convert the calculated GHG emission into kWh of energy used. You must also know the fuel type to make this calculation. Where the year is not known, you can use an appropriate average uplift.  

Method three: mileage plus fuel and a size factor

You can use this method when you don’t have any published emission data about your vehicles but know the fuel type, engines, or vehicle sizes.

Method four: mileage plus fuel type

If you only know the fuel type, the following factors are used:

Method five: UK fleet average

Without data other than mileage driven, the only option you have is to use UK fleet data. National data for England, Scotland and Wales has yet to be made available.