What is bi-directional charging? And which cars in the UK can support it?

Shock me, what is bi-directional charging?

Basically, it’s two-way electric vehicle charging. At the moment, almost all EV chargers are one-way (uni-directional). Bi-directional tech gives your car the option to become a massive power bank, a home back-up generator and, potentially, a money maker.

Sounds lucrative. How does it work?

Let’s go back to GCSE Physics for a quick refresh. There are two types of electric current. EV batteries store DC current, but domestically the UK runs on AC. EVs are built with onboard chargers to handle the conversion from one to the other.

Rapid chargers, operating on DC supply, top your batteries up faster because there’s no conversion needed.

Anyway, if you’re going to send juice back out of the car’s battery, it needs to be in the right form for its destination, so it’ll need to be converted back again.

‘The right form for its destination’? Might need to explain this…

There are different types of bi-directional charging, determined by where the electrons from the car’s battery is being sent onto.

Powering your home is creatively known as vehicle-to-home, or V2H, charging. Powering devices or appliances? That’s vehicle-to-load (V2L) support. If you’re sending power back to the National Grid, that’s (somewhat wildly) called ‘vehicle-to-grid’ (V2G) functionality.

Hmm, surely I want the energy in my car battery, so I can, er, drive?

You do, of course.. But think about how often your car sits on your drive/in your garage, doing nothing.

Just like Nick Heidfeld in the 2011 F1 season/the designer of Ford’s small cars division*, cars sit unused about 96 per cent of the time – 73 per cent at home, the rest at work, according to statistics from the RAC Foundation.

*delete as appropriate

So I’m now going to AirBnB the energy in my car?

Kind of, but not quite. Kaluza – a division of OVO Energy – partnered with Nissan and Indra (one of two bi-directional EV charge point manufacturers in the UK) for the world’s biggest V2G study, involving over 300 bi-directional devices.

In addition to the excessive amount of time cars sit parked unused, it calculated that an average of 40kWh of energy would power a modern home for two days. For context, the VW ID.4 has a battery capacity of 77kWh, the Polestar 2 one of 82kWh and Mercedes EQS‘s battery is 107.8kWh. How’s your mental arithmetic?

Poor, but I get it. Instead of paying the Grid for energy, my car would power my home and I can still drive to the supermarket.

Exactly. At least in a V2H model, which is very possible right now. V2H is like a back-up generator for your home. Charging the storage (in this case the EV’s battery) when energy prices are low, then drawing down from the car’s battery means you can reduce your home energy bills. Plus there’s the added peace of mind that in the event of a blackout, you’ve got a way of keeping the lights on.

Kaluza’s report reckons a UK household could save on average £420 a year, just by keeping their EV plugged in.

Tidy enough for some holiday spend. How does V2L differ from V2H?

V2L is all about rejuicing-away-from-home and being on holiday. Essentially, it turns the car into a giant power bank, as TG’s Jack Rix did with the Ford F-150 Lightning when he hooked up a projector on a camping trip.

Ok, that’s straightforward enough. Let’s go back to the money-making: does V2G earn me money?

V2G might help you save some money in the long term. The same Kaluza report reckons some £3.5 billion could be saved in Grid reinforcement if the ESO could tap into a smidge of energy from each EV.

The ES-who?

The Electricity System Operator, or ESO for short, is the bunch of brainiacs keeping the lights on across the whole country.

It has to toggle the release of energy into the Grid when everyone gets home, starts cooking tea (that’s ‘dinner’ for you, southerners), watching the telly and making a brew.

If there’s not enough energy in the system when we all draw from the Grid simultaneously, the ESO is responsible for switching the tap on (getting a power station to turn up the generators, or open a dam to send the water past the hydro turbines). If it doesn’t, there’s a blackout.

So how would V2G help the ESO?

You charge your car when energy prices are low, you sell it back to the Grid when the ESO is needing to top up the system. It’s a pretty sweet solution. Firstly, the ESO doesn’t need to tap into the power stations and thus, uses more renewable power.

Plus, you’d pocket the difference between how much you paid for the energy overnight, and how much you sold it for (on demand) – and all of us would benefit from smaller energy bills, too. At least, in theory.

What’s the catch?

In practice, effectively functioning V2G charging is quite a few years off. And by quite a few, we mean ‘many’.

Firstly, (as mentioned) the choice of chargers is very small. Secondly, they’re not cheap with a cost of around £6,000 – a pricey mark-up on unidirectional chargers which top out about £1,500. You’d have to sell a lot of energy to make back that investment.

Thirdly, these early adopter designs aren’t the daintiest for the wall on your drive because they have to house more components. Finally, you have to have permission from the Grid to send energy back, too, so it’s not a widely used tech right now.

Right, presuming we’re sitting many years in the future, will one charger achieve all of this?

That’s where it gets a bit more complicated. You need both your home charger and your car to be bi-directionally capable for the right task, too. 

So the EV I drive right now isn’t compatible with bi-directional charging?

Probably not. There are only a handful of models capable of bi-directional charging. Below is the best list we have of the models on the UK market right now and what they’re capable of:

Is my smart charger bi-directional?

Smart charging and bi-directional charging are different. In 2023, the government mandated that all EV home charging points had to be ‘smart’. That makes them connected to the Grid, your energy provider, your car and the companion app so the charging time and rate can be controlled remotely (albeit not without permission). Just because the car is plugged in, doesn’t necessarily mean it’s charging.

Is there anything else I should know?

Smart Home Charge told us that the majority of bi-directional enquiries it’s receiving are about vehicle-to-home charging, which suggests that plenty of people are considering using their EV as an energy storage solution, as well as a car.