Public EV charger users get just 37% of the expected rate

Some EV owners who use public EV charge points are getting electricity at just 37% of the rate promised, according to exclusive What Car? research. 

We took a variety of cars to 15 EV charging stations around England operated by five different EV charging networks. At eight of the sites, we found that the chargers delivered power at less than two-thirds of the rate we should have received, markedly increasing the time we had to spend plugged in.

The slowest charging speed we saw was 48kW in a Renault Scenic E-Tech at an InstaVolt charger in Twickenham, Middlesex. The advertised maximum charging rate was 125kW and, although our test car was capable of charging at up to 130kW while its batteries were at 4% (according to charging curve data from independent EV data website evkx.net), the fastest speed we saw of 48kW was just 37% of the expected speed. 

We also received power at 49% the promised rate at a Shell Recharge site in Tytherington, near Macclesfield, where we charged a Peugeot e-208. The site’s maximum advertised speed was 150kW and our test car should have achieved 98kW with its batteries at 29%. However, it, too, topped out at 48kW during our charging session. 

Not all sites provided slower charging than expected, though. Two Fastned sites in Basildon, Essex and St Albans, Hertfordshire, both provided power at 100% of the expected rate for our Vauxhall Astra Electric hatchback, and an InstaVolt charger in Bromley, Kent, gave our Vauxhall Astra Sports Tourer a top speed of 98kW. 

EV public charging site test results

Why do EVs charge at less than their maximum possible rate? 

There are a number of reasons why EVs may not charge up at the maximum stated speed, including issues with the car, charger and infrastructure. One commonly cited reason for slower than expected charging is that the vehicle restricts the amount of charge it accepts from a charger in line with parameters set by its ‘charging curve’. 

This is dictated by an onboard system that generally lets the car charge faster when its batteries are more depleted and then slows down the rate of charge as they fill up. It may also stop the batteries from accepting a high-speed charge if they are too depleted or too cold. Looking at a car’s charging curve will tell you approximately how quickly its batteries should charge at varying states of charge.

To take into account the charging curves of our five test vehicles, we compared the maximum charging speeds we achieved with the fastest speed each one should have been able to accept when we started charging, according to charging curve graphs published by evkx.net. Charging curve data should also be available from car makers, but not all publish it and it’s not always readily available online.  

What factors affect EV charging rates? 

Other reasons why cars won’t necessarily charge as fast as the advertised maximum rate include how warm the car’s batteries are, the outside temperature and the age of the car and the health of its battery pack. 

The temperature of a car’s battery pack can have a big influence on charging speed and, according to many industry experts, the best way to achieve the fastest possible charge is to pre-condition the car’s batteries to ensure they’re at the optimum temperature when you reach the charger. However, this requires the driver to set a charging station as a location on the car’s sat-nav system or to manually select pre-condition in the car’s settings. 

While doing this may be simple on some vehicles, it’s not straightforward on all, and it’s not common practice among all EV drivers, especially those who use Android Auto or Apple CarPlay for navigation. So, to ensure a level playing field among our test cars, we didn’t pre-condition any of them prior to charging. Instead we drove each test car for 1.5-2hrs prior to charging. 

The results of our testing show that it is not essential to pre-heat batteries to gain the maximum charge. When charging our Vauxhall Astra hatchback at the Fastned sites in Baslidon and St Albans, we achieved the car’s top rate for its state of charge: with 31% in its batteries, it charged at 100kW, and when it had 36% in its batteries it achieved 83kW; these figures are in line with the charging curve for the model. 

Does the electricity supply hamper EV charging speeds? 

Whether or not you pre-condition your car’s batteries, you can still have difficulty getting the best charging rates. The electricity supply can also have a massive effect on charging rates, as one network manager from an unnamed nationwide charging provider explained.

“Load balancing (the process of distributing power across multiple chargers), can drastically reduce charging speeds. Static load balancing means that if, for example, you have four chargers, each will receive 25% each regardless of how each is being used,” he said. So, if you have a site with four chargers and a maximum charging rate of 100kW, each charger will only be able to provide 25kW.

There is also a process called local load balancing, which varies the power according to usage. This means that if only one 150kW charger in a bank of four is being used it’ll get 100% of the power, but if a second car plugs in, each will get 50% of the maximum output.

Finally, dynamic load balancing, which looks at the whole site and can deliver unused energy from elsewhere on the site to boost the charge, can work in favour of EV chargers if they are at a standalone site, but not if they’re sharing the electricity with other facilities at a particular location. EV chargers at a retail park could lose out when the shops need more power, meaning there’s less left for the chargers. 

EV drivers have no way of knowing if any of these measures are in place at any site, and the only way they can see if they’re getting the expected charging rate is to look at the real-time kW speed on the charging unit or their car’s display screens. This is why the unnamed source said he believes that charge point operators should publish the expected charging speed at each charging site. 

This leads to another potential difficulty for EV owners. Many slow-speed chargers don’t show the actual charging speed as it’s being delivered, and a growing number of new EVs no longer state the charging speed in kWs on the car’s dash or infotainment screen. Instead the vehicle states the charging speed in ‘miles per hour’. 

This information is useful in one way because it shows how many miles of charge you’re getting for each hour of charging. But it’s not an accurate reflection of the amount of power being pumped into the batteries, because the mileage being calculated is based on the car’s official range, which is rarely achievable in real-world driving. So if your car tells you it’s getting 100 miles of charge in an hour, you’re unlikely to be able to get this range from the car if you leave the charging station and drive it along the motorway. 

What do the charge point operators say about charging speeds? 

We put our findings to the EV charging companies we’d visited. A Gridserve spokesperson stated: “Peak charging speeds are only available for a period of time during the charging curve. Peak charging speed and the overall speed of the charge is dependent on a number of factors including vehicle specification, state of charge, battery temperature, weather conditions and the charger you are using.

“Based on the charging sessions that we have reviewed, the charging speed was limited by the vehicle. In these situations, the charger was able to offer higher speed but the vehicle declined, choosing to charge at a lower speed.

Delvin Lane, CEO of InstaVolt commented: “We recognise that, in some locations, grid capacity can influence the power available to chargers. Where this is the case, we optimise power distribution to ensure the best possible charging experience for all users. The Twickenham site What Car? visited has 125kW chargers operating on a dynamically shared power system, meaning power is intelligently distributed between vehicles based on demand.”

What Car? verdict

Our results are a concern for all EV drivers because they highlight the fact that it’s very difficult to predict how fast a car will charge up at a public charging site. 

The number of vehicles using the charge point and the site can affect charging speed, and so can the overall amount of electricity available and how it is shared between chargers and other facilities at a site. While we understand that issues with grid capacity are being addressed by charging companies, it seems they are still an issue at some locations.

There are also a number of factors relating to the cars, including the battery management system, and external conditions, such as colder temperatures, which can slow down charging speeds. 

The bottom line is that the information provided to EV owners, from both the charging companies and car makers, needs to be improved. There should be more transparency from both about the charging speeds that are actually achievable, both from the cars at various states of charge and in different weather conditions, and from the units supplying the electricity to them and whether they are constrained in any way by the electricity supply. 

Car makers should also provide buyers with information up-front on the charging curves of their cars and other factors that could affect charging, such as pre-conditioning batteries and external temperatures. 

Some EV charge point operators show the average charging speed to the user at the end of each charge, and we think this is something that all providers should do for all locations because it gives a fairly good indication of whether or not you’ve gained your car’s optimum charging speed.

We also think the charging companies should be compelled by the Government to publish the annual average charging speeds their sites have achieved. This should be added to the requirements of the Public Charge Point Regulations, to be published alongside annual reliability data, which will become mandatory in 2026.  

These are just some of the measures we believe need to be introduced to improve the public charging experience for EV owners, and ensure motorists have a better understanding of the capabilities of their cars and the service being provided by the charging companies. They are among the issues highlighted in our EV Manifesto, which outlines the measures we believe the Government should take to encourage more people to switch to EVs. 

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