BMW 330e

What is a plug-in hybrid (PHEV)?

Getting a plug-in hybrid car is a great option if you want to reduce your environmental impact and fuel costs, but how do they work and should you buy one? Our guide has the answers.

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By Cazoo editorial team

Hybrid cars are becoming increasingly popular as buyers demand a more environmentally friendly alternative to pure petrol and diesel cars. However, several types of hybrid cars are available. Here, we explain what a plug-in hybrid car is, why they are sometimes referred to as a PHEV, and why one might be the right choice for you.

What is a PHEV?

You can think of a plug-in hybrid car as a cross between a conventional hybrid (also known as a self-charging hybrid) and a pure electric car (also known as an EV). So PHEV literally stands for plug-in hybrid electric vehicle.

Like other types of hybrid, a plug-in hybrid has two power sources – a petrol or diesel engine and a battery-powered electric motor. The engine is usually the same as those found in regular petrol or diesel cars, while the electric motor is similar to those in other hybrids and electric cars. However, unlike other hybrids, a plug-in’s battery can be recharged by plugging it into a power outlet – hence the name.

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What’s the difference between PHEVs and conventional hybrids?

Conventional hybrids work in much the same way as plug-in hybrids but rely on built-in systems to recharge their batteries, which is why they’re sometimes called ‘self-charging’. The difference is that you can’t plug them into a power outlet.

A plug-in hybrid has a bigger battery and a more powerful electric motor than most conventional hybrids, allowing it to go much further using electric power alone. That means the official fuel consumption and emissions figures for plug-in hybrids are far lower than for conventional hybrids, although you need to keep the battery charged to get the full benefit.

How does a PHEV work?

Depending on the circumstances, the engine or the electric motor in a plug-in hybrid can power the car independently, or the two can work together. Most of the time, the car’s internal computers work out which to use, depending on what’s most efficient at the time. Pure electric power is generally the default option when you start the car, and while you drive at low speed.

The latest plug-in hybrids often have several selectable driving modes that alter how the engine and motor work. For instance, if you’re driving around town and don’t want the car to create pollution, you can select ‘EV’ mode, so your car only uses the electric motor. Some cars will use journey information from the built-in sat nav to make sure that they leave some charge in the battery when you drive through urban areas.

There also may be a ‘power’ mode in which the engine and motor prioritise maximum power over efficiency. This can be useful for overtaking on a country road, or when towing a heavy trailer.

How are a PHEV’s batteries recharged?

The main way to recharge a plug-in hybrid’s batteries is by plugging it into a home or public charging point. How long it takes to recharge depends on the size of the car’s battery and what sort of charger you use. As a rule of thumb, a completely flat battery should fully recharge overnight.

Plug-in hybrids also have several built-in systems that recharge the batteries as you drive. The main one is called regenerative braking. When you apply the brakes, it reverses the direction in which the electric motor spins. That turns the motor into a generator and the power that is generated goes back into your battery. This also happens when you ease off the accelerator pedal, which can have the effect of slowing the car, even if you haven’t pressed the brake pedal. Plug-ins can therefore feel slightly different to drive compared to traditional cars, although the differences are small and you can easily get used to them.

Plug-in hybrids can also use their engine as a generator to recharge the batteries. This happens without any input from the driver because the car’s computers constantly use the systems to keep as much charge in the battery as possible. If the batteries run flat while driving, the car simply continues on engine power.

What happens if you don’t plug in a PHEV?

The worst that’ll happen is that the electric motor’s battery will run flat, so you won’t be able to use the electric motor until you recharge it. The car will still be perfectly driveable because it can use its engine instead.

The car’s built-in power generation systems generally prevent the batteries running completely flat, but there are some situations where it might happen – on a long motorway drive, for instance.

How far can a PHEV go on electric power?

Most plug-in hybrids give you an electric-only range of between 20 and 40 miles when fully charged, although some can do 50 miles or more. That’s enough for many people’s daily needs, so if you're able to keep the battery topped up you might be able to make many journeys using zero-emissions electric power.

How far a plug-in hybrid can go before its fully charged battery runs flat depends on the size of the battery and how you drive. Travelling at higher speeds and using lots of electrical features such as the headlights and air con will drain the battery more quickly.

What fuel economy will a PHEV do?

Official figures show that many plug-in hybrids are capable of going hundreds of miles on a single gallon of fuel. But in the same way that most petrol or diesel cars don’t match their official mpg figures in the real world, neither do most plug-in hybrids. The discrepancy isn’t dishonesty by the car’s manufacturer – it’s how those average fuel economy figures are obtained in laboratory testing.

Even so, most plug-in hybrids can give you very good fuel economy. The BMW X5 PHEV, for instance, can give you better fuel economy than you get from a diesel X5. For the best fuel economy from plug-in hybrids, you need to plug in to recharge as often as possible so you can maximise the distance you can go on electric power.

What’s a PHEV like to drive?

When the engine’s running, a plug-in hybrid feels much the same to drive as any other petrol or diesel car. When it’s running on electric power, it feels like an electric car – which can be a little eerie if you’ve not experienced one before because there’s so little noise and acceleration can be quicker than you might expect.

The way a plug-in hybrid’s engine starts up and shuts down while driving, often seemingly at random, can take a bit of getting used to. The same is true for how the brakes feel when the regenerative braking is working. And it’s worth noting that some plug-in hybrids are very fast. Indeed, the fastest version of some cars are now plug-in hybrids – the Volvo S60, for example.

Are there any downsides to PHEVs?

Plug-in hybrids can give you great fuel economy, but their efficiency is very dependent on how you use them: they can use more fuel than you might expect when running on engine power alone. The batteries, electric motors and other components in the hybrid system are very heavy, so the engine has to work harder to move the extra weight. Thus the car uses more fuel. So if you do a lot of longer journeys, which will deplete the battery and require more engine power, you might actually get better fuel economy from a diesel car.

Plug-in hybrids also cost quite a bit more to buy than the same car with just a petrol or diesel engine. And, as with an electric car, if you live in a flat or house without off-road parking, you might not be able to install a home charging point.

What are the upsides to plug-in hybrids?

According to official figures, most PHEVs emit very little carbon dioxide (CO2) from their exhaust. Vehicle Excise Duty (car tax) charges are based on CO2 emissions, so VED rates for plug-in hybrids are usually very low.

Similarly, benefit-in-kind tax rates are tied to CO2 emissions, so drivers of company cars could save thousands of pounds a year by getting a plug-in hybrid. Most plug-in hybrids are also exempt from charges for driving in low-emissions or clean-air zones in cities like London. These two factors alone could be enough to persuade many people to buy a plug-in hybrid.

And, because plug-in hybrids have both engine and battery power, the ‘range anxiety’ that can surface when driving an electric car isn’t an issue. If the battery runs flat, the engine takes over and your journey continues.

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