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  • Writer's pictureThe Climate Coach

EV Shorts: 1. Charging at home

Updated: Mar 26, 2023

In this short series of posts, I'll cover the basics of running an electric vehicle (EV) instead of a petrol or diesel vehicle - an important step in the transition away from fossil fuels. In this post, we'll cover the most popular way to charge an EV - at home.

A quick note on battery sizes

Before we talk about charging, an electric car obviously contains a large battery - with its capacity measured in kWh - a unit of energy. A kW on other hand, without the 'h' on the end, refers to the transfer of energy between two places - in this case from the grid to your car. Most EV batteries are between 40 and100 kWh which indicates how much energy they can store when they are fully charged. The most popular size for the "magic" c.300 mile range seems to be around 75 to 80 kWh.

At time of writing this includes the "long range" versions of the following electric vehicles:

  • VW ID3, ID4 and ID5

  • Audi Q4 etron

  • Škoda Enyaq

  • Hyundai Ioniq 5

  • Kia EV6

  • Tesla Model 3

  • Tesla Model Y

For the purposes of this post and to help keep things simple, we'll use an example of a 78kWh usable battery pack, which is similar to the typical battery size of the models above.

Charging at home

Charging your car is fundamentally the same as using any appliance that uses electricity at home. However, the approach to charging may change depending on how fast you want or need to charge.

1. Standard plug - often referred to as a "Granny charger" (sorry Granny!)

Most EV's actually come with a charger which can be used on a standard 3-pin plug - and if you not you can buy one separately. You shouldn't use this in combination with an extension lead - unless you have one which can handle sustained high power running through it (very few do). Charge speed is low and will take a long time - but if you don't do many miles and are just topping up between short trips then this solution could work just fine. Clearly if you are somewhere without a charger but with electricity - then this is sometimes the only option.

  • Installation cost: N/A

  • Charge speed: 1.5-2kW

  • Time taken to charge a 78kWh EV from 0-100%: 39-52 hours

  • Time taken to charge a 78kWh EV 20-80%: 23-31 hours

  • Cost for a full charge (@34p p/kWh): £26.52

  • Cost per mile: 9.4p (assuming 250 miles from 78kWh)

2. Single phase home car charger

For a single phase home - which is most UK homes, a dedicated EV home charger can usually provide charge speeds of 7kW - although some may only be able to support a 3.5kW charger. It's usually possible to make some upgrades to your supply, consumer unit and/or cables to allow a 7kW charger though - but there may be additional costs associated beyond the quoted "standard installation" cost. Charging speeds are much faster than on a granny charger, but will still take a relatively long time.

  • Installation cost: £500-1500

  • Charge speed: 7kW (sometimes limited to 3.5kW)

  • Time taken to charge a 78kWh EV 0-100%: 11 hours (22 hours for 3.5kW)

  • Time taken to charge a 78kWh EV 20-80%: 6.7 hours (13.4 hours for 3.5kW)

  • Cost for a full charge (@34p p/kWh): £26.52

  • Cost per mile: 9.4p (assuming 250 miles from 78kWh.

3. Three phase home car charger

If you have a three phase supply to your home then you can usually have a 22kW charger installed. Some cars (including some of the ones listed above) can only actually support a maximum of 11kW AC charging so check first before you even consider this option. Also, if you don't have a three phase supply - an upgrade from a single phase can be very costly.

  • Installation cost: £500-1500

  • Charge speed: 22kW

  • Time taken to charge a 78kWh EV 0-100%: 3.5 hours

  • Time taken to charge a 78kWh EV 20-80%: 2.1 hours

  • Cost for a full charge (@34p p/kWh): £26.52

  • Cost per mile: 9.4p (assuming 250 miles from 78kWh.

4. What kind of car charger though?

To be honest, there are a lot on the market and more being added all the time so it's hard to recommend one specific brand. If you have solar panels - or are planning to add solar panels in the future - then it might be worth looking for a charger which can use excess solar energy to charge your car rather than export to the grid. The myenergi Zappi is the most well known charger to do this but there are also others who are launching this feature or are planning to in the near future. If you are planning to charge at specific times too (which I'll run through next) then you will want to make sure the charger you choose has the ability to set scheduled charging via an app or interface or some kind. The goods news is that the vast majority of home chargers now support this.

5. Off peak charging

You'll have noticed the costs listed above are all the same regardless of charging option and that's because they all provide the same amount of energy to your EV - just at different speeds. They are based on charging at anytime of day and assuming you are on a standard variable rate electric tariff at time of writing (February 2023). So at that rate, you are paying just over £26 per "tank of fuel" which will get you around 250-300 miles for that size battery. Although some of the EV's listed above claim nearly 400 miles, I've used a more real world figure which should be achievable all year round.

So worst case, it works out about 9.4p per mile for charging your EV at home.

The benefit of having a 7kW car charger is that you can condense your car charging into certain periods of the day, when electricity may be abundant and therefore cheaper.

Depending on how often you need to charge your car at home - and how much electricity you use at home excluding your EV, you will likely want to explore one of the dedicated EV tariffs (e.g. Octopus Go) which allow you to charge your car off-peak when demand on the grid is at it's lowest. These tariffs work by offering a discounted rate for a short period overnight but increasing your day rate to offset this. As an example, Octopus Go at time of writing offer a discounted cheap rate of around 10p p/kWh overnight - with a day rate of about 40p p/kWh. Assuming you don't use too much electric during the day at home - and you do a decent number of miles in your EV, this will probably work out much cheaper for you overall. Most companies advise that if at least 20% of your total household electric usage will be for your EV then it will work out cheaper. In this scenario, the cost to run your car are significantly reduced by about 70%.

  • Cost for a full charge (@10p p/kWh): £7.80

  • Cost per mile: 3.1p (assuming 250 miles from 78kWh.

  • Cost per mile of most efficient EV: 2p (assuming 370 miles from 78kWh

So best case scenario - using your car charger in combination with a EV electric tariff can reduce your car charging costs by up to 70% to just 2p per mile.

That 2p per mile example is based on a Tesla Model 3 Long Range achieving 370 miles on a full charge - and charged overnight at 10p p/kWh. I do appreciate 370 miles is not achievable all year round and on certain types of journeys (motorway), and that a Tesla Model 3 is an expensive car - but it does demonstrate what is possible with this relatively new technology which is only going to improve! This is despite the UK currently having the highest electricity price per kWh in history at the moment. Imagine how much cheaper this could be if prices reduce in the future and EV's continue to get more and more efficient.

OK, got it - anything else?

One additional benefit of charging EV cars overnight from a climate perspective is that it helps to "balance" the electricity grid. Usage is heavily skewed towards the hours when most people are awake - so around school and work time and then the evenings when most people are at home. When most people are asleep usage is significantly reduced because not as many people are using electricity. Generation infrastructure is designed to meet peak loads - e.g. during the daytime and evening and thus there is often excess generation at off peak times. In other words, the wind doesn't stop blowing!

So in summary, it's actually fairly easy for the grid to support some extra demand during these off peak periods - and this is why energy companies are able to offer these cheaper rates. When it's windy, it's very likely a high percentage of this energy overnight will be wind too - making it even better still from a climate perspective.

If you have any thoughts, feedback or ideas you wish to contribute on this or any other topic covered by The Climate Coach - please get in touch, we'd love to hear from you.

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