The cost of using a public charge point can vary by as much as nine times depending when and where the vehicle is charged.
The RAC Foundation analysis shows overnight charging at home can typically be done for as little as 8p per kWh but can be as high as 69p per kWh at a public rapid charge point.
This means that the fuel costs for a 100-mile journey in a 2018 Nissan Leaf could be anywhere between £2.67 and £23.
By comparison – based on official fuel consumption figures – the same journey in a 1.5-litre petrol-engine Ford Focus would cost around £12 in fuel, while a similar Ford Focus with a diesel engine might do it using only around £10 of fuel.
The RAC Foundation research comes as What Car? reveals its own analysis, finding that a driver would pay £45.89 to charge an Audi E-Tron from 10% to 80% at an Ionity ultra-rapid charger.
However, performing the same charge on a domestic charger at an average night-time energy tariff of 7p per kWh it would cost only £4.66.
Comparing with the Ionity charging network makes the E-Tron even pricier to run than an equivalent diesel Audi Q7, says What Car?.
The E-Tron costs 34p per mile, while a Q7 50 TDI, which averages 27.2mpg, costs 22p per mile.
Ionity is one of a small number of extremely fast 350kW charging networks, capable of replenishing EVs batteries in 30-40 minutes.
However, some slower public charging networks are also far pricier than charging at home, says What Car?
Getting the same battery boost for an E-Tron at both a 50kW Shell Recharge point and a 50kW Ecotricity socket costs £25.94, although Ecotricity rates are cheaper for its home energy customers.
What Car? found that electric vehicle owners who regularly need to use public charging networks could save money by signing up for a scheme with a one-off or a monthly fee because these often have a lower energy usage rate.
Sign up for a Source London Full plan and it will cost £4 per month, but just £6.32 every time you charge up your vehicle.
Consumers also need to watch out for the hidden costs of using public chargers. Some EV-charger equipped car-parks in London charge £9 per hour for parking with no discount for those using the chargers and What Car?’s research found overstay fees levied by charging networks to discourage that ranged from £10 to £21 per hour.
Steve Gooding, director of the RAC Foundation, said: “Consumers are so sensitive to the cost of filling-up that petrol and diesel prices are routinely displayed to the tenth of a penny.
“Even at the extremes, there is unlikely to be more than a 30-40% price differential between the keenest supermarket and the most expensive motorway service area. Not so with electricity as the cost of recharging your battery-powered vehicle can differ dramatically, with prices highly dependent on where and when you plug-in, which charge speed and who is operating the facility & providing the power.
“The good news is that overnight charging at domestic rates at home can cost as little as a few pence per kilowatt-hour.
“However, contrast that with the dizzying news that you could pay as much as ten times that if you decide to ‘fill-up’ at certain ultra-rapid EV charging points on the motorway network."
The canny consumer is going to have to do some more research to make sure their electric vehicle delivers the expected savings of driving an Electric Vehicle.
What Car? editor Steve Huntingford added: “Although there are still a lot of slow (3kW) public charging points that are free to use, you’ll have to pay if you want a quick energy fix. And this is where the costs can rack up if you don’t research the various networks in advance.”
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