Plug-in hybrids continue to promise on paper emissions far lower than what they produce in everyday use. Transport & Environment’s analysis of official real-world driving data for PHEV models sold in 2024 quantifies a gap that is now difficult to consider marginal: average actual emissions turned out to be nearly six times higher than the figures stated in the WLTP cycle.
The issue is not just the gap between laboratory and road, which is natural for any powertrain. In PHEVs, type approval depends decisively on how much distance is assumed to be driven with a charged battery and the way electric drive and the internal combustion engine alternate. If these assumptions do not reflect real-world behavior, the result is a CO₂ figure that is formally correct according to procedure, but barely representative of the vehicle’s climate impact once delivered to the customer.
According to the analysis, plug-ins are driven in all-electric mode just 27% of the time. For the remainder, the combustion engine kicks in, or hybrid management takes over where fuel consumption and vehicle mass take a significant toll. This is a critical factor for a category that in recent years has benefited in many European countries from tax and corporate perks built around very low homologated emissions.
The crux lies in how emissions are calculated
WLTP uses a parameter called the utility factor, or UF, to estimate the share of distance a PHEV covers using battery electricity. This factor directly affects the CO₂ value assigned to the car: the higher the assumed share of electric driving, the lower the final official figure will be. The European Union has already revised this parameter for the 2025-2026 period and plans a further adjustment from November 2026, with implementation in 2026-2027.
According to T&E, keeping the factor introduced for 2025-2026 unchanged would still result in underestimating real-world emissions by 56%. The upcoming revision does not change what comes out of the tailpipe, but it updates how those emissions are accounted for regarding manufacturer fleets, taxation, and public policy. It is precisely this accounting that has made PHEVs an attractive option for companies and brands: a heavy, powerful model can achieve a very low WLTP rating if the algorithm assumes frequent charging.
The 2024 snapshot suggests that the mechanism has drifted even further from reality as the market has shifted toward larger, higher-performance vehicles with higher-capacity batteries. The average electric range of PHEVs increased from 58 to 75 kilometres between 2021 and 2024. In theory, this should be good news; in real-world use, however, increased range alone does not guarantee a proportional drop in emissions.
More range is not enough if the electric motor remains marginal
Models with over 70 kilometres of electric range showed the largest discrepancy between on-road emissions and official ratings: 52%, considering the corrected utility factor, compared to 35% for PHEVs with under 70 kilometres. Longer-range models accounted for more than half of the overall gap observed in 2024.
Part of the reason lies in the technical setup. A large battery can improve laboratory figures, but it does not solve the problem if the electric motor lacks sufficient power to continuously drive a vehicle that is often a large SUV or a premium saloon. Under these conditions, the combustion engine kicks in more frequently, especially during acceleration and on extra-urban journeys, while the added weight of the battery and hybrid components continues to penalise fuel consumption.
According to the analysis, the most useful indicator would therefore not be electric range in isolation, but the ratio between electric power and combustion engine power. PHEVs where electric power exceeds combustion power by 25% emit 95 g of CO₂ per kilometre in real-world use: roughly a third of the average for the entire category. Even during the battery discharge phase, when the car alternates between electric and hybrid operation, these versions top out at 20 g/km, a level three times lower than the average recorded for that mode.
The takeaway for the industry is not that every PHEV is technically incapable of delivering a benefit. Rather, it is that the category encompasses vastly different products, and that the homologation system has so far failed to adequately reward the quality of the electric powertrain. A plug-in hybrid engineered to still rely on its petrol engine can secure regulatory perks similar to those of a model designed to genuinely cover most journeys on plug power.
The adjustment may drive new designs, but it also alters the economics
The signals following the initial revision of the utility factor point in the direction shown by the data. To offset the rise in official emissions caused by the new calculation, some manufacturers have increased the electric range of their models from 63 to 117 kilometres, an 85% jump. In the updated 2026 variants, average battery capacity grew from 14 to 24 kWh, up 68% compared to previous versions.
For those models, roughly three-quarters of the increase in homologated emissions caused by the factor adjustment was offset by the larger battery. Many have thus remained below the 50 g/km CO₂ threshold, an important benchmark across various tax schemes and corporate fleet policies. T&E also estimates that the price increase tied to the higher battery capacity was in the region of 2%.
The next phase, however, will be more challenging. To maintain or achieve emissions of 50 g/km or less by 2030, the analysis points to electric ranges between 140 and 220 kilometres. This implies larger battery packs, improved efficiency and, above all, architectures where the electric motor does not play a secondary role. This is not a simple catalogue update: it alters the trade-off between cost, weight, packaging and the vehicle's commercial positioning.
Charging remains the decisive factor. Even a more advanced PHEV retains its advantage only if it is plugged in regularly. For corporate fleets and private buyers, access to home or workplace charging, energy reimbursements and usage policies therefore become an integral part of environmental performance. Without these conditions, a larger battery risks turning primarily into dead weight to transport.
The European overhaul thus puts pressure on both manufacturers and national governments. The former will have to decide whether to invest in plug-ins that are genuinely more electric or accelerate the transition towards battery-electric models; the latter will need to assess whether incentives and taxation sufficiently differentiate between the most efficient versions and those that, despite favourable homologation, approach conventional vehicle emissions in real-world use. The 2024 data makes this choice less theoretical: the credibility of PHEVs as a transition technology will depend on their engineering and actual use, not merely on the figure printed on the WLTP spec sheet.
