Plug-in hybrid electric vehicles (PHEVs) are supposed to be the perfect compromise between practicality and carbon emissions, with an electric motor to handle your commute and an engine for range. But they might not be as eco-friendly as they appear.


A new report from the campaign group Transport and Environment (T&E) has shown that PHEVs emit, on average, over 2.5 times more CO2 than their official test values. The group recommends that the UK government commits to the ‘earliest feasible phase-out date’ for all cars with engines: internal combustion engine (ICE) cars, conventional hybrids and PHEVs.

How much CO2 do PHEVs emit?

Laboratory tests for PHEVs found that they emit, on average, around 44g of CO2 per kilometre. However, T&E analysed fuel efficiency data from 20,000 PHEV vehicles around the world and found that the ‘real world’ value is 117g of CO2 per kilometre.

For comparison, a conventional hybrid – or ‘self-charging’ hybrid, which draws electric power from the brakes – emits only slightly more, at 135g of CO2 per kilometre. A new ICE car emits between 164 and 167g of CO2 per kilometre.

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Why do PHEVs emit so much more than expected?

Plug-in hybrids have the potential to run on their electric battery only, often allowing a range of at least 32 kilometres (20 miles) with zero emissions. However, the driver does need to plug it in regularly to keep it charged – something which T&E claims that most real-world users don’t do.

Dr Ian Walker, who studies environmental psychology at the University of Bath, isn’t surprised by this behaviour. “I think we have to ask, is anybody actually buying these because of their environmental concern?” he says. His research has found that environmental considerations are low down the list of factors people consider while buying a car – he calls it a ‘tertiary consideration’.

“The people who genuinely are concerned about the environment are not driving a car at all or they're certainly not using them the way most cars are being used,” he adds. He points out that many PHEVs are SUVs, which use “dramatically more” energy than small cars.

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So, why do people buy PHEVs at all, if not to use its electric motor? A big reason, Walker says, is the government’s grant scheme which reduces the cost by up to £3,000. “Nobody ever checked whether you were actually using it as intended,” he says. “You're merely getting a car with several thousand pounds off the cost.”

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But even if you do use your PHEV as intended, that doesn’t guarantee that it will drive on battery mode. “[…] even when a PHEV is driven in the supposed zero-emission mode, most continue to use their engine, burning fuel and emitting CO2,” the T&E report claims.

They found that many models reverted to engine mode when outside temperatures were too cool or the speed was too high. “The reality is it is almost impossible for the car to drive in zero-emission mode even for short distances on a regular basis,” it adds.

Should we still buy PHEVs?

Walker believes that the debate over the best type of car is asking the wrong question. “It feels like the discussion starts with the assumption that everybody is going to have a car and use it all the time,” he says. Instead of finding the best car for our lifestyles, we should be changing our lifestyles so we don’t need to drive everywhere.

“Where should people live and work? How should they get between those places? Should urban planning not trap you into a car-dependent lifestyle?” he asks.


“We're carrying on with 1960s lifestyles and just fretting about what the motors are going to look like. And it's the wrong question. We need to be looking at our lifestyles, our goals, what we want to accomplish and how we want to live and who we want to be, and then working out how to get there rather than just saying, is it petrol, diesel, or battery?”

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.