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Charging an EV can be really straightforward—have a charger (EVSE) installed, plug the car in and wait until the battery is full. But there is a lot more to it than that—the technical constraints, the varying costs, the desire for sustainability all play a role.
Technical Constraints
Grid Connection Capacity
Many European single-family houses will have a hard limit on on grid connection capacity. For my own home this amounts to 230V, 3-phase, 20A per phase, i.e. 13.8kVA. Plug in any more than that and the distribution fuse blows, leaving you in the dark with an expensive addition to your next electrical bill.
The usual car chargers (EVSE) for family homes in Europe will have a 3x 16A (11kW) charging capacity. This is enough to charge a car at home, but it’s not much more than the grid connection capacity of my house. So if the charger is dumb and I try to charge my car while baking some cookies, the combined current on one phase will shoot to 16A (charger) + 8A (oven) = 24 A and I’m going to end up in the dark, with an empty battery and no cookies 🫠
Solutions
Buy More Capacity
The old way of solving this would be to install bigger distribution fuses, but this is not an optimal solution and it will be expensive:
- Your monthly capacity cost will be bigger.
- You might have to rewire a part of your house wiring for bigger current.
- Depending on the utilization of the transformer in your neighborhood, you might not get the extra capacity even if you are willing to pay for it.
- You will be paying for mostly unused capacity.
Use a Smart Charger
A much better solution is to simply look at the current you are taking from the grid and take action when it exceeds the limit.
You can do this manually to some extent, you just need to be the nagging presence that scolds anyone who even thinks about turning on any big load while you charge your car. This should also immensely improve your relationships! 🤪
A much better solution is to automate this. It’s called “dynamic charging” and most modern home charging stations are able to do it at least to some extent. It works like this: You have a current meter installed on the main electrical connection to your home and a mechanism of some sort regularly checks the load. If the load exceeds your limit, this mechanism (a smart home automation or a function of the charger itself) commands to charger to temporarily reduce the charging current until everything is back to normal. Your car will charge a bit slower sometimes, but you get to eat your cookies, stay warm and keep the lights on. 😃
These days, you can buy an off the shelf home car charger with dynamic charging support and there are a lot of models available. But if you are interested in DIY solutions, I wrote an article about building one yourself.
Varying Costs
Time-Based Energy Costs
Depending on where you live, the cost of electric energy will vary through the day. Where I live, daytime electrical energy is about 50% more expensive, so charging your car will be much cheaper at night and on weekends here.
Grid Utilization Costs
Depending on where you live, you might have time-slot based grid utilization costs charged extra. We just got these added to our system and they can make a major difference, a couple hundred Euro or more per year.
It works like this: you choose a connection capacity for each time slot and pay an upfront fixed cost for that. The busy time-slots (during office hours and daytime in winter) are really expensive per kilowatt, so it makes sense to book just enough capacity during the expensive time-slots and all the available capacity during the cheap ones. But there is a catch: if you don’t book enough capacity, you will pay a couple of Euros for each 15 minutes you have exceeded it. It will add up quickly!
At first glance, this boils down to the same thing as varying energy costs—charging from grid on off-peak times is cheaper. But this cost basically forces you to really think about your energy use. If you book low capacity during expensive time-slots, you can save hundreds of Euro per year, but if you then blow through those limits, you will pay much more in penalties.
Solution
Both of these issues can be taken care of manually by being careful about when you decide to charge your car. Of course, having a bit of automation can help a lot, especially with the grid utilization costs and the varying capacities in individual time slots.
A good automated solution will automatically limit the charging power to the capacity limits of the individual time-slots and give you an option to allow or disallow daytime charging. Most cars also have built-in charge schedulers that can help you charge them when the energy is cheaper.
Sustainability
If you care about the climate emergency, you care about carbon intensity of the electric energy you use. In an ideal world, this would have been taken care of by the utilities for you, but we are still a long way from that.
Of course, many utilities are happy to sell you an “All Renewables” package for your electricity bill, where you pay a little extra so the energy is bought from renewable sources. I support this wholeheartedly, because it gives the energy producers the right incentives to build more renewables. On the other hand, we have to be realistic and acknowledge that you are always consuming electric energy with the average carbon intensity your grid is producing at any given time. There is no way to select for less-carbon intensive electrons, unfortunately 😞
This means you must think about the energy mix in your area and this is something your utility is compelled by law to report transparently. You should get a diagram showing the percentages of energy that come from coal, natural gas, nuclear, solar, hydro, wind etc with all your electricity bills.
But keep in mind, this mix is an average and the actual values vary:
- Depending on time of day: If you have a big nuclear backbone, like my country does, you will be consuming mostly nuclear at night, which is low carbon. If you don’t, the cheaper night-time energy will probably have a much bigger carbon footprint, coming from coal and natural gas. But some countries have a lot of hydro…
- Depending on the weather: If your country has a lot of solar, daytime electricity will be almost carbon free unless the weather is bad. A rainy day can cut solar production by 80% or more.
- Then there are the seasons: Hydroelectric plants produce much more energy when the water is plentiful, e.g. in spring and fall. Solar is less plentiful in winter and the opposite holds for wind power.
The good thing is, you don’t really have to work all of this out yourself. There are services that will give you a good estimate of current carbon intensity in your area, like Electricity Maps, which also has an API and there is even a Home Assistant integration for it.
Solution
You can do a lot here if you just behave rationally. Find the data and act accordingly. But there is also the potential to automate things using the publicly available services like Electricity Maps mentioned above.
Conclusion
EVs have the potential to strongly reduce your carbon footprint and cost per kilometer driven, if you use them rationally. But this is not trivial and there are many factors to consider.
If you want to do EV-charging right, you need to think at least about:
- Dynamic charging
- Time-based energy costs
- Grid utilization costs
- The energy mix in your area
- The variations of the energy mix because of time of day, weather and seasons
Keep in mind that you can still think about your energy mix even if you are not able to charge at home. You will, of course, make sure to keep yourself mobile first, but once you take care of that, you can start thinking about other things like carbon intensity.
To keep this article somewhat manageable, I did not discuss other very important factors, like your driving habits, choosing an efficient car and, most importantly, alternative mobility solutions. But let’s face it, these are way more important than any charging strategy and the one thing you should think about at the very beginning. Our dependence on cars is toxic to us and the planet, so the best car is still the one that never got built.
The Second Conclusion
You may have noticed I did not mention rooftop solar or home energy storage. This is intentional, for two reasons: Most people do not have rooftop solar systems. And of those that do, only few have storage. I wanted to focus on a regular home owner buying their first EV, trying to help them understand the potential and behave rationally, both in terms of costs and environmental concerns.
But rooftop solar, especially combined with storage, opens a whole new dimension of options. I explored these in the next article, EV Charging With Rooftop Solar and Storage.
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