What is a mild hybrid car and should you buy one?
Many cars have mild-hybrid engines, but how does the tech work and how are they different to hybrids and plug-in hybrids? Here's everything you need to know...
Hybrid cars – ones that combine an engine and an electric motor to improve fuel economy and reduce emissions – have been a familiar concept since the launch of the Toyota Prius back in 2000.
In recent years, we've seen the arrival of plug-in hybrids (PHEVs), which can be plugged in and charged up. Another variation on the theme is the mild hybrid electric vehicle (MHEV), which uses a battery-powered electric motor to support a conventional petrol or diesel engine to improve efficiency and reduce emissions.
Many car makers now offer models fitted with MHEV (or mHEV) technology. But what is an MHEV, how does it work and should you buy one? Read on to find out more...
What is a mild hybrid car?
MHEVs are more common than you might expect, because mild hybrid tech is relatively inexpensive to fit, and reduces the fuel consumption and CO2 emissions of both petrol and diesel-engined cars.
It might not be immediately obvious from a model's name that it's a mild hybrid. For example, the mild-hybrid VW Golf is badged the eTSI, while the Fiat 500 Hybrid and Suzuki Swift Hybrid are MHEVs rather than regular hybrids.
Generally, a mild hybrid has a small electric generator instead of a traditional starter motor and alternator (the device that keeps the 12-volt battery charged), plus a lithium-ion battery.
Most mild hybrids run on a 48-volt electrical system, which is a higher voltage than the electrical systems of traditional combustion-engined models. The 48V system powers components that would have previously been powered by the engine, enabling the engine to operate more efficiently.
How do mild hybrid cars work?
As the name suggests, a mild hybrid car provides only gentle electrical assistance to the engine. While regular hybrids (also known as full hybrids) and plug-in hybrids can be driven on electric power alone, an MHEV cannot.
Mild hybrid systems work in slightly different ways depending on which car manufacturer has developed them. In general they assist the engine under hard acceleration and help make the car's stop-start system operate more smoothly.
MHEVs also use regenerative braking to harvest energy when the brakes are applied or when the car is coasting. That energy is converted into electricity that's fed into a lithium-ion battery and stored until it's needed to provide extra assistance. On average, a car with mild-hybrid technology can be around 15% more efficient than its conventional counterpart.
Read more: The best full hybrid and plug-in hybrid cars
One prime example of mild hybrid tech being used to enhance the economy and performance of a car is the Ford Puma.
Most versions of the model come with Ford’s 1.0-litre three-cylinder mild hybrid petrol engine, which is also used in versions of the Ford Focus and Ford Kuga. Ford says the system continuously monitors how the car is being driven to determine how intensively to charge the battery and when to assist the engine.
The system on the Puma can give the engine an extra 15lb ft of pulling power, so it doesn’t have to work as hard under normal loads, and can deliver a 10% improvement in fuel economy. The same boost can be applied when the engine is under heavy load to improve performance, delivering up to 20% faster acceleration.
Are all mild hybrid systems the same?
No. There's a wide range of mild hybrid systems, and some are more sophisticated than others. One of the cleverest is in the Audi Q8 luxury SUV.
It goes to even greater lengths to improve economy by working with the car’s adaptive cruise control system to slow it down as it approaches corners and roundabouts, reaping the energy from the regenerative braking. It can even deactivate the engine and let the car coast for up to 40 seconds at a time to save fuel.
Are mild hybrids the same as hybrids?
Mild hybrids are the same as other hybrids in the sense that they use a combination of a petrol or diesel engine alongside some electrical assistance. But unlike full hybrids, they’re not able to run on electric power alone because the assistance the motor provides is, as their name suggests, mild.
Mild hybrids vs plug-in hybrids: which is best?
The main differences between a mild hybrid and a plug-in hybrid are that mild hybrids cannot be driven purely using electric power and they can't be plugged in to top up their batteries. A PHEV may be the most suitable car for you if you do lots of short journeys and have access to a cheap way to recharge the car. If you can charge up the batteries frequently you may be able to do all your driving just using the car's electric motor, which will be cheaper than using the engine.
However, if you're not able to charge the car up often and end up driving on the petrol engine all the time, this will work out more expensive. So, for those without easy access to a home EV charger who are looking to minimise their fuel bills, a mild hybrid could be a sound choice.
Read more: The best hybrid cars you don't have to plug in
Are mild hybrid cars cheaper to tax?
Mild hybrids aren’t as economical as many full hybrid cars and PHEVs, and they don’t benefit from the lowest rates of company car tax.
However, for private buyers, the mild-hybrid road tax rate is a little cheaper than for a pure petrol or diesel, because most are termed as "alternative fuel", so they get the same £10 annual discount enjoyed by owners of full and plug-in hybrid cars in the 2023-24 tax year.
Why should you consider a mild-hybrid car?
For a start, they’re less complex than other hybrids, so in many cases they’re cheaper to buy. They’re also easier to live with than PHEVs, which need to be plugged in regularly to get the best efficiency.
If a PHEV is too expensive or you don’t have access to a charger, a mild hybrid is a cleaner option than a conventional petrol or diesel. They’re also no different to drive than conventional petrol or diesel models and work with cars with manual gearboxes, so they’re a good option for those who want to stick with a more traditional driving experience.
Still not sure whether to buy a mild hybrid? These pros and cons should help you decide:
The benefits of driving a mild-hybrid car
– No different to drive than a regular petrol or diesel-powered car
– They don’t need to be plugged in to save fuel, as is the case with plug-in hybrid models
– They’re generally cheaper to buy than a full hybrid or plug-in hybrid car
– Fuel economy and CO2 emissions are improved, reducing bill bills and VED tax
– The technology works with manual and automatic, and petrol and diesel models
The drawbacks of mild hybrids
– They can’t run on electricity alone
– They produce more tailpipe emissions than other kinds of hybrids
– Full and plug-in hybrids are more economical
What cars are available as mild hybrids (MHEVs)?
Many of the most popular cars on sale are available with mild-hybrid technology. In many cases "mild hybrid" or "MHEV" does not feature on any badging to advertise the tech is fitted, so they often fly under the radar.
Models available with mild-hybrid technology in the UK include:
Audi A3
Audi A6
BMW 1 Series
BMW 2 Series
BMW 3 Series
BMW 4 Series
BMW 5 Series
BMW 7 Series
BMW iX
BMW X1
BMW X3
BMW X4
BMW X5
BMW X6
BMW X7
Citroen C4
Citroen C4 X
Citroen C5 Aircross
Cupra Formentor
Cupra Leon
Dacia Duster
Dacia Jogger
Fiat 500 Hybrid
Fiat 500X
Fiat Panda
Ford Focus
Ford Kuga
Ford Puma
Hyundai i30
Hyundai Kona
Hyundai Tucson
Jaguar E-Pace
Jaguar F-Pace
Jaguar XE
Jeep Compass
Jeep Renegade
Kia Ceed
Kia ProCeed
Kia Rio
Kia Sportage
Kia Stonic
Kia XCeed
Land Rover Defender
Land Rover Discovery
Mazda CX-60
Mercedes A-Class
Mercedes C-Class
Mercedes E-Class
Mercedes GLS
Mini Countryman
Nissan Qashqai
Peugeot 3008
Peugeot 5008
Range Rover Evoque
Range Rover Sport
Range Rover Velar
Renault Austral
Seat Leon
Skoda Octavia
Skoda Superb
Suzuki Swift Hybrid
Vauxhall Astra
Vauxhall Corsa
Vauxhall Grandland
Vauxhall Mokka
Volkswagen Golf
Volkswagen Passat
Volvo XC40
Volvo XC60
Volvo XC90
(Click on the links to be taken to our new car reviews of each model.)
What are the other types of hybrids?
All hybrids have a conventional engine, an electric motor and a battery, although the size and capacity of these varies. As well as mild hybrids, there are three other types of hybrids, and each works in a different way. They are:
Parallel hybrids
This is where the hybrid story started, and the (now off-sale) Toyota Prius is the most widely known example of a parallel hybrid. These are sometimes, rather erroneously, called 'self-driving hybrids'.
The car’s wheels can be powered directly by the engine, by the electric motor alone, or by both power sources working together. When pulling away and at speeds of up to 15mph, it uses only the electric motor for power, making it very frugal in city driving. The petrol engine cuts in as speed increases and under hard acceleration.
Whenever you decelerate or use the brakes, the regenerative braking system harvests electricity and stores it in the battery for use later on, but in the Prius, the car can run solely on electricity for only up to 1.25 miles. The system is also used in the Toyota Yaris and Toyota Corolla hatchbacks. Models from premium sister brand Lexus work on the same basis.
Range-extender hybrids
Range-extender hybrids – also known as serial hybrids – differ from regular hybrids in that their combustion engines are there mainly to act as generators, producing electricity to power the electric motors, which drive the wheels.
The latest Honda Jazz is one popular example. It uses two electric motors, powered by a compact battery, working alongside a frugal 1.5-litre petrol engine. The wheels are almost exclusively driven by the electric motors, and the only time the wheels are actually driven by the engine is at higher speeds when it is directly connected to the wheels.
The Honda Civic, Honda CR-V e:HEV and Mazda MX-30 R-EV plus the E-Power versions of the Nissan Qashqai are range-extender hybrids too. According to Nissan, the range-extender system gives the Qashqai better performance than you’d get from an equivalent parallel hybrid.
Plug-in hybrids (PHEVs)
These are a halfway house between regular hybrids and full electric cars, and are being offered by a growing number of brands. As the name implies, they can be plugged into an electrical outlet to recharge their batteries.
Although PHEVs have conventional engines, they also have larger batteries than regular hybrids and can go further on electric power alone, greatly reducing running costs if they’re charged regularly.
One of our favourite PHEVs is the Lexus NX 450h, which has an official electric range of 43 miles – 13 miles longer than the BMW X3 xDrive30e. According to our testers, if you can plug the NX in regularly and mostly do short journeys, you could achieve as much as 313mpg. When the batteries have run out, it still has fuel economy of close to 40mpg.
About the report author
Claire Evans has been a motoring journalist for more than 30 years, and has focussed on consumer issues for much of that time. She was the advice columnist for Carweek magazine in the 1990s, helping car owners with faulty cars get the right level of reparation from car makers.
She also spent six years working on motoring content for Which?, and it is here she oversaw the running of the charity's annual used car reliability survey.
Claire launched the What Car? Reliability Survey in 2017, and since then has helped thousands of buyers choose the most reliable new cars and SUVs, as well as the most dependable used cars.
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