Plug-in hybrids often burn more fuel than expected

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Plug-in hybrids are meant to bridge the gap to full EVs, but real-world fuel use is often far higher than official figures suggest. According to Empa studies funded by the Swiss Federal Office for the Environment (FOEN), the biggest factor is simple: whether drivers actually charge them.

If someone owns a plug-in hybrid and does not charge the vehicle regularly, that person is effectively driving a heavier vehicle with a combustion engine. Due to the additional weight of the battery and electric motor, consumption can even be higher than with a comparable conventional gasoline engine.

Until now, much of the evidence came from On-Board Fuel Consumption Monitoring (OBFCM) data collected from vehicles in everyday use across Europe. Those data showed a clear gap between official type-approval numbers and real consumption, but not the cause. To dig deeper, Empa tested 12 current plug-in hybrid passenger cars on a roller test bench at 23°C (73°F), -7°C (19°F), and -7°C (19°F) with the heating switched on, along with more dynamic driving profiles.

The pattern was consistent. Under ideal conditions, plug-in hybrids can cover long stretches on electricity and keep emissions low. But cold weather, cabin heating, and harder driving sharply reduce electric range, making the combustion engine start sooner and run more often. That pushes up fuel consumption, CO₂ emissions, and pollutant emissions, sometimes significantly.

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Empa also found that vehicle design matters. Elser said the best results did not automatically come from the smallest cars, but from lighter vehicles with moderate engines and balanced battery sizes. Heavy vehicles need more energy per kilometer, and in a plug-in hybrid the extra mass of the battery and electric drivetrain can make the penalty worse.

A larger battery is not a guaranteed fix either. It only helps if it is charged regularly and if typical trips fit within the electric range. Otherwise, the added weight raises energy use all the time.

On regulation, Empa examined the utility factor used in European type testing — the assumed share of distance driven electrically — which sits between 70% and 85% for plug-in hybrids. The EU originally based that on older U.S. commuter data, then updated the calculation in 2025, with another revision planned for 2027 after European OBFCM data showed lower real electric driving shares.

Using Switzerland’s Microcensus Mobility and Transport, Empa estimated that a Swiss-specific utility factor would likely be higher than the European factor because average trips are shorter and Switzerland has a lower share of company vehicles, which in Europe are charged less often than private plug-in hybrids. But Elser cautioned that uncertainty remains because the analysis assumed all users charge every day.

The takeaway from both studies is that country-specific and realistic utility factors matter, because they directly affect official values for fuel use, electricity demand, and emissions. Empa argues that plug-in hybrids still have a role — but only if people charge them consistently and policy supports that with reliable home and workplace charging, fleet charging rules, and incentives that favor electricity over fuel.