It depends how you drive them.

Currently accounting for 3% of new car sales, plug-in hybrid electric vehicles are sold as low-carbon alternatives to fossil fuel and conventional hybrid cars. But a new report threatens to shatter their green credentials.

Research from the pressure groups Transport and Environment and Greenpeace has claimed that CO₂ emissions from plug-in hybrid cars are “two and a half times” higher than tests by manufacturers suggest.

While these official figures place the average emissions from plug-in hybrid vehicles at 44g of CO₂ per kilometre, the new report argues that it’s more like 120g on roads.

How carmakers measure emissions

A plug-in hybrid electric vehicle has both a petrol engine and battery, either of which can power the wheels.

Unlike a normal hybrid vehicle, where all the energy comes from the fuel, a plug-in hybrid has a larger battery and can be plugged into the mains to charge. Because a plug-in hybrid has two energy sources, petrol and electric, its emissions will vary widely depending on how much time is spent in full electric or petrol mode.

Read the full version by Dr Ashley Fly on the Conversation website.