The shift away from fossil fuels in transport has created two prominent low-carbon pathways: biodiesel for existing vehicles and grid electricity for the growing electric vehicle fleet. For energy professionals advising clients or shaping policy, understanding how these pathways compare on carbon intensity is essential. Based on current UK data, grid electricity for electric vehicles typically demonstrates carbon intensity in the range of 45 to 55 grams of CO₂ equivalent per megajoule delivered to the wheels, whilst UK-produced biodiesel spans approximately 30 to 80 gCO₂e/MJ depending heavily on feedstock choice. The margins are closer than many assume, and the nuances matter considerably. This comparison depends on feedstock sourcing, grid decarbonisation trends, calculation methodologies, and the temporal dynamics of both systems. Understanding these complexities enables more informed strategic thinking about the UK’s transport transition.
Understanding Carbon Intensity in Transport Context
Defining Carbon Intensity for Energy Professionals
Carbon intensity, expressed as grams of CO₂ equivalent per megajoule of energy delivered, provides a standardised metric for comparing different energy pathways on a level playing field. This approach proves far more useful than simple tailpipe emissions because it captures the full lifecycle environmental burden. When we discuss carbon intensity in transport, we are employing a well-to-wheel framework that encompasses extraction or generation, processing and refining, distribution and delivery, and finally the conversion of fuel to motive power. This comprehensive boundary ensures we are comparing complete energy systems rather than cherry-picking favourable segments.
The CO₂ equivalent notation accounts for all greenhouse gases, converting methane, nitrous oxide, and others to their CO₂-equivalent warming potential over a hundred-year timeframe. For biodiesel, this becomes particularly important because agricultural production can involve significant nitrous oxide emissions from fertiliser application. For electricity, it captures the methane leakage from any gas-fired generation in the mix. The well-to-wheel approach also reveals why electric vehicles appear to “emit nothing” at the point of use but still carry a carbon burden from electricity generation, whilst biodiesel vehicles emit CO₂ at the exhaust but may have captured biogenic carbon during feedstock growth.
Why the Comparison Matters Now
This analysis carries particular urgency within the context of UK climate policy. The Transport Decarbonisation Plan sets ambitious targets for reaching net zero, and the government has committed to ending sales of new petrol and diesel cars and vans by 2030. However, the practical reality is that biodiesel serves the existing vehicle fleet, which will remain on our roads for many years, whilst electricity serves new electric vehicles entering the market. Both pathways matter simultaneously during this critical transition period.
The Renewable Transport Fuel Obligation, which requires fuel suppliers to incorporate a growing percentage of renewable fuels, uses carbon intensity calculations directly. Suppliers earn tradable certificates based on the carbon savings their renewable fuels deliver compared to fossil fuel baselines, with lower carbon intensity fuels earning proportionally more valuable certificates. This creates powerful economic incentives that flow directly from the technical carbon accounting we are examining. Getting these comparisons right affects investment decisions worth billions of pounds across both the biofuels sector and the electric vehicle charging infrastructure landscape.
The UK Grid Electricity Pathway
Current Grid Carbon Intensity and Rapid Decarbonisation
The UK electricity grid has undergone a remarkable transformation in carbon intensity over the past decade. From around 500 gCO₂e per kilowatt-hour in 2012, the grid has declined to approximately 170 to 200 gCO₂e/kWh in 2024, representing one of the fastest decarbonisation trajectories of any major economy. This dramatic improvement stems from coal phase-out, which has seen coal generation drop from roughly 40 per cent of electricity in 2012 to effectively zero in recent years, combined with rapid expansion of renewables, particularly offshore wind, and a stable nuclear baseload.
What makes this especially interesting for transport applications is the significant temporal variation in grid carbon intensity. On a windy spring afternoon when demand is moderate and wind generation is high, the grid can operate at 50 gCO₂e/kWh or even lower. Conversely, on a cold winter evening when gas generation ramps up to meet peak demand, intensity might exceed 250 gCO₂e/kWh. This variation creates opportunities for smart charging strategies that preferentially charge electric vehicles during low-carbon periods, though for this analysis we focus on annual average figures that represent typical charging behaviour without sophisticated optimisation.
The trajectory matters as much as the current position. National Grid’s Future Energy Scenarios project grid carbon intensity falling to around 50 gCO₂e/kWh by 2030 and approaching near-zero by 2035 as offshore wind, solar, and potentially hydrogen-fired generation displace remaining gas capacity. This improving trend means that electric vehicles purchased today will automatically become cleaner over their operational lifetime as the grid decarbonises, a dynamic benefit that biodiesel cannot match.
From Grid to Wheels: Accounting for System Losses
To calculate the true carbon intensity of electric vehicle operation, we must trace electricity from generation through to useful work at the wheels. Transmission and distribution losses across the grid network consume approximately 5 to 7 per cent of generated electricity before it reaches the charging point. Charging itself introduces further losses, typically 10 to 15 per cent depending on charging speed and equipment efficiency, with faster DC charging generally less efficient than slower AC charging. Battery discharge and the power electronics within the vehicle add another small loss factor, around 5 to 10 per cent.
When we compound these factors, the overall grid-to-wheel efficiency sits around 70 to 75 per cent. This means that electricity generated at 200 gCO₂e/kWh translates to roughly 270 gCO₂e/kWh of useful energy delivered to the wheels. Converting to the standard megajoule basis for comparison with liquid fuels, recognising that 1 kWh equals 3.6 MJ, yields approximately 75 gCO₂e/MJ before accounting for vehicle efficiency. However, electric powertrains themselves operate at 85 to 90 per cent efficiency converting stored electrical energy to motive power, far superior to internal combustion engines at 20 to 30 per cent efficiency. The final well-to-wheel calculation for current UK grid electricity therefore delivers approximately 45 to 55 gCO₂e/MJ of useful work, though this figure continues improving as the grid decarbonises.
UK Biodiesel Production and Carbon Accounting
Feedstock Diversity and Lifecycle Emissions Variability
UK biodiesel production draws from a remarkably diverse feedstock base, and this diversity drives enormous variation in carbon intensity. Used cooking oil and tallow, categorised as waste or residue materials, can achieve extraordinarily low carbon intensities of 15 to 30 gCO₂e/MJ when assessed under waste methodology. This approach allocates minimal or zero carbon burden to the feedstock itself since it would otherwise require disposal, attributing emissions only to collection, processing, and distribution.
Crop-based biodiesel from UK-grown rapeseed presents a different picture. The lifecycle includes emissions from fertiliser manufacture and application, agricultural machinery operation, oilseed processing, and transesterification to produce biodiesel. Typical values range from 50 to 70 gCO₂e/MJ, though this depends significantly on farming practices, nitrogen use efficiency, and processing energy sources. When indirect land use change factors are included, as required under some regulatory frameworks, these figures can rise further to account for the displacement effects of dedicating agricultural land to fuel production rather than food.
The UK’s implementation of the Renewable Transport Fuel Obligation has created strong incentives favouring waste-based feedstocks through double counting mechanisms and higher certificate values for lower carbon intensity. This has shifted UK biodiesel production increasingly toward used cooking oil and animal fats, making the UK fleet’s average biodiesel carbon intensity lower than it would be under a purely crop-based system. Understanding this feedstock mix is crucial when generalising about UK biodiesel performance.
The RED II Framework and Default Values
The EU Renewable Energy Directive II, which the UK has substantially retained post-Brexit through domestic regulation, establishes the calculation methodologies and default values that underpin carbon accounting across the biofuels sector. This framework provides default lifecycle carbon intensity values for various biodiesel pathways, offers typical values representing current average production, and allows producers to claim actual values through verified sustainability certification schemes.
For rapeseed biodiesel, the RED II default stands at 52 gCO₂e/MJ, whilst used cooking oil biodiesel defaults to 13 gCO₂e/MJ. These values provide regulatory certainty and comparability, though producers who can demonstrate better performance through improved agricultural practices, renewable process energy, or efficient logistics can claim lower actual values. The framework also establishes the fossil fuel comparator at 94 gCO₂e/MJ for diesel, creating the baseline against which renewable fuels must demonstrate carbon savings. To qualify as sustainable under RTFO, biodiesel must deliver at least 50 per cent carbon savings compared to this baseline for production facilities operational before 2021, rising to 65 per cent for newer facilities.
The Head-to-Head Comparison
Well-to-Wheel Carbon Intensity: The Numbers
When we bring these pathways together for direct comparison, the picture that emerges is more nuanced than headlines often suggest. Grid electricity for electric vehicles currently delivers approximately 45 to 55 gCO₂e/MJ to the wheels based on average UK grid carbon intensity and system efficiencies. Biodiesel spans a wide range from 30 to 80 gCO₂e/MJ, with waste-based variants at the lower end potentially outperforming grid electricity, and crop-based biodiesel generally showing higher intensity especially when ILUC factors apply.
The crucial insight is that both pathways deliver dramatic improvements over conventional diesel at around 94 gCO₂e/MJ, representing carbon savings of roughly 40 to 70 per cent depending on specific parameters. The difference between the pathways is often smaller than the difference between either pathway and fossil fuels. However, the directionality matters for strategic planning. Grid electricity’s carbon intensity is on a steep downward trajectory, with credible pathways to near-zero within a decade, whilst biodiesel’s intensity is largely determined by feedstock constraints and processing efficiency limits that offer less scope for dramatic improvement.
It is also worth noting the variability within each pathway. An EV charged predominantly from renewable energy during off-peak periods might achieve 20 gCO₂e/MJ, whilst one charged during high-demand periods could reach 70 gCO₂e/MJ. Similarly, used cooking oil biodiesel achieves vastly different performance compared to palm oil biodiesel with ILUC factors included. The comparison is therefore not a single data point but overlapping distributions with significant tails.
Beyond the Numbers: Real-World Considerations
Several practical factors complicate this straightforward carbon comparison. Energy density creates fundamental differences in vehicle range and refuelling behaviour. Diesel contains roughly 36 MJ per litre, whilst battery energy density, though improving, requires substantially more mass and volume to store equivalent energy. This affects vehicle design, payload capacity, and the practicality of different applications. Long-haul lorries and rural operators often find biodiesel blends more practical than battery electric alternatives, at least for now.
Infrastructure represents another asymmetry. The existing fuel distribution network accommodates biodiesel blends seamlessly, requiring minimal adaptation for blends up to B20 and moderate upgrades for higher concentrations. Electric vehicle charging infrastructure, whilst expanding rapidly, requires substantial investment in grid reinforcement, charging point installation, and management systems. The drop-in nature of biodiesel provides immediate emissions reductions across the existing fleet without requiring vehicle replacement, whilst electrification demands new vehicle purchases with attendant embodied carbon from manufacturing.
Scalability ultimately limits biodiesel’s role in deep decarbonisation. The UK’s sustainable biodiesel feedstock base, even optimistically assessed, cannot support wholesale replacement of road transport diesel demand. Waste cooking oil collection is already intensive, tallow supplies are finite, and dedicating agricultural land to fuel crops at scale conflicts with food security and biodiversity objectives. Grid electricity faces no comparable hard limit; renewable generation capacity can scale to meet transport demand as it grows, constrained primarily by infrastructure investment rather than fundamental resource availability.
Strategic Implications for the UK Energy Transition
RTFO Credits, Carbon Trading, and Economic Signals
The Renewable Transport Fuel Obligation translates these carbon intensity values into concrete economic incentives through its tradable certificate mechanism. Suppliers earn certificates based on the carbon saving their renewable fuels deliver, calculated precisely from these lifecycle assessments. A litre of used cooking oil biodiesel earning certificates based on 85 per cent carbon saving generates far more value than rapeseed biodiesel with perhaps 45 per cent saving.
This creates market forces that drive feedstock choice toward lower carbon options. As waste feedstock supplies tighten and prices rise, the economic premium for low carbon intensity helps justify the higher feedstock costs. The system also double-counts certain wastes and residues, providing additional incentives for developing collection infrastructure and processing capacity for materials that might otherwise go to landfill or incineration. These economic signals have successfully shifted UK biodiesel production toward better-performing feedstocks, demonstrating how carbon accounting directly shapes market outcomes.
For electricity, whilst not directly covered by RTFO, the same carbon accounting principles influence corporate sustainability strategies, fleet procurement decisions, and investor assessments of charging infrastructure projects. Companies setting science-based targets must account for Scope 2 and Scope 3 emissions from electricity use, creating preference for demonstrably low-carbon charging solutions and driving demand for renewable electricity supply contracts.
Portfolio Thinking: Both Pathways in the Transition
Rather than framing this comparison as an either-or choice, strategic thinking demands we recognise both pathways as complementary during the transition period. Biodiesel serves the existing vehicle fleet immediately, delivers emissions reductions today rather than waiting for fleet turnover, and addresses hard-to-electrify applications including heavy goods vehicles, agricultural machinery, and potentially marine and aviation through sustainable fuel variants. Its role may be time-limited by feedstock constraints and the improving performance of electric alternatives, but it remains valuable for bridging the critical decade ahead.
Electrification serves new passenger vehicles and light commercial fleets with superior long-term decarbonisation potential, benefits from continuous grid improvement without requiring fuel replacement, and aligns with the broader energy system transition toward renewable electricity. The challenge lies in charging infrastructure deployment, grid capacity management, and supporting the transition for drivers without off-street parking.
Both pathways deserve continued policy support tailored to their strengths. Biodiesel policy should focus on feedstock sustainability, avoiding perverse incentives that drive crop-based production, and directing limited sustainable supplies toward applications where electrification faces genuine barriers. Electric vehicle policy should prioritise charging infrastructure in underserved areas, grid investment to support transport electrification, and ensuring the electricity supply itself continues decarbonising rapidly.
Conclusion
The carbon intensity scorecard reveals that grid electricity for transport currently holds a modest advantage over typical UK biodiesel, delivering around 45 to 55 gCO₂e/MJ compared to biodiesel’s range of 30 to 80 gCO₂e/MJ depending on feedstock. However, both pathways achieve substantial emissions reductions compared to fossil diesel, and the variation within each pathway often exceeds the difference between them. What matters most is recognising that the UK’s transport decarbonisation strategy appropriately leverages both approaches during this transition period.
Looking forward, the trajectories diverge. As the grid accelerates toward 2030 and beyond with expanding renewable generation and declining fossil fuel use, electricity’s carbon advantage will widen significantly. Meanwhile, biodiesel’s role will likely shift toward specialised applications where electrification faces genuine challenges and toward sustainable aviation fuel where liquid fuel energy density remains essential. The critical insight for energy professionals is that we need not choose between these pathways in absolute terms but rather deploy each where it delivers the greatest impact during the decisive transition decade ahead.