Electric Car Carbon Comparison Calculator

(Skip straight to the Electric Car Carbon Comparison Calculator)

It's generally true that if you buy a new car, buying and running an electric car will lead to lower overall CO₂ emissions than buying and running a fossil-fuel car. The energy cost used to manufacture an EV is higher than a fossil-fuel car but the emissions from using the electric car are lower. NB: This means that if you drive very few miles your new electric car has a worse carbon footprint than a new hydrocarbon-powered car.

Here's a short review from Volkswagen in 2019, explaining how the energy sources used to manufacture and run an electric vehicle make a big difference to how far it needs to be driven before the EV pays back the higher carbon emissions required to build it: https://www.volkswagenag.com/en/news/stories/2019/04/from-the-well-to-the-wheel.html.

I used to worry that, by ditching my ageing petrol car and buying a new electric car, I might be wasting embodied carbon/energy used to manufacture that car. And that therefore I should wait until the end of the old car's working life before switching to electric. So I decided to work out whether there is a point during the lifetime of your fossil-fuelled car when it makes sense (in terms of lower carbon emissions) to replace it with a new electric car.

I think there are two cases to consider:

  1. A not-so-old car that can be sold and used by a new owner - see Resaleable Cars
  2. A car nearing the end of its life, where it's unlikely anyone else will take it on - see Clapped-out Cars

Clapped-out Cars

Let's look at this case first, as I think it's the most interesting. I think the answer depends largely on how much you use your car and how long the battery in a new electric car might last, and is independent of how many more years you think your old car will keep working.

New Peugeot e-208...

Let's work through the example of my friend Alex's electric Peugeot e-208. He says it's specified as having a 200-mile range starting from a full 50kWh battery, but that on the motorway the range is more like 125 miles. Let's say on average the range is 160 miles.

Alex says he typically drives 6,000 miles per year. That's 37.5 full battery charges ie 1.88MWh.

To work out the carbon cost of running the Peugeot e-208 for a year, we need to know the carbon intensity of the electricity used to charge it. I can't find an exact figure but chart "History of Carbon Intensity of Generation" on https://www.nationalgrideso.com/future-energy/our-progress/carbon-intensity-dashboard suggests the average during 2021 was around 200gCO₂/kWh.

That means Alex's use of his electric car generates 375kgCO₂ per year.

NB: In reality, rather than the average carbon intensity, I think we should consider the marginal carbon intensity ie the highest carbon intensity in the generating mix at the time we charge: typically the dirtiest generating capacity, that has to be turned up (because all lower-carbon generating capacity is already running flat out) in order to charge my car in addition to all other demands. Conversely, this does mean that by charging an electric car at periods of low carbon intensity, ie after midnight when there is supply outstrips demand, almost no extra CO₂ is emitted.

We also need to consider the carbon emitted to manufacture the new car and share that out over the lifespan of the car. I can't find a figure for the e-208 but for the (similar-sized) e-Golf the chart "Climate footprint: e-Golf versus Golf Diesel" on https://www.volkswagenag.com/en/news/stories/2019/04/from-the-well-to-the-wheel.html estimates 12teCO₂. We don't have to worry about the 'e' in eCO₂ because where I've used gCO₂ the figure for geCO₂ is the same.

Let's suppose the Peugeot e-208 will have a working life of 10 years. That's a typical length of a battery warranty. The carbon emitted during manufacturing can be spread over 10 years at 1,200kgCO₂ per year.

So the grand total for the e-208 is about 1,580kgCO₂ per year.

... versus old VW Golf

I don't think we need to worry about the embodied energy/carbon in Alex's old car. The embodied carbon has long gone: it's floating around the atmosphere. We only need to look at the ongoing carbon emissions from driving 6,000 miles per year in the old Golf. The embodied carbon matters only for the new car we're causing to be built.

And I don't think we need to speculate on the possible future lifespan of the old Golf. What holds for the carbon-emissions comparison between cars over one year holds equally true for every further year the old Golf could have run for. That's true even if the Golf had 20 more years of life left in it and Alex had had to replace his e-208 with a new EV over the same period.

Let's suppose, as we did for the e-208, that the old Golf (although specked higher) in practice, on average delivered 50mpg.

Continuing to drive 6,000 miles per year would have used 120 gallons of diesel ie 545 litre.

Diesel produces 2.68kgCO₂/litre - according to https://en.wikipedia.org/wiki/Diesel_exhaust. So 6,000 miles driving the old Golf would have generated about 1,460kgCO₂.

Uh oh: scrapping Alex's old Golf and buying a Peugeot e-208 has lead to an increase of emissions of 120kgCO₂ per year!

However, Alex's Golf had only 50,000 miles on the clock. And this alters the maths significantly. Find out how in Resaleable Cars.

Resaleable Cars

Imagine the car market as a pipeline with new cars entering at one end and "uneconomic-to-repair" cars exiting at the other end some years later. Most people choose how many cars they own based on their needs and their budget. That number may change over time as their circumstances change, which may change the overall number of cars in the pipeline, but the total number of cars changes much more slowly than the flow of cars through the pipeline. So the number of cars entering and exiting the pipeline are approximately equal.

If we sell our second-hard fossil-fuel car and buy a new electric car, it simply increases the number of electric cars entering the pipeline. The person who buys our second-hand car will buy it instead of buying a slightly more expensive (probably less old) car, and so on until that chain ends with a new car not being bought. This logic works until we reach the point where new fossil-fuel cars are only being bought by a smull number of people who would never consider buying a second-hand car no matter how cheap it was. If too many second-hand cars are entering the pipeline, there's a chance another car will be ditched and exit the pipeline a year or two sooner than it otherwise would, but I don't think that affects the following calculation much.

Let's return to the example of Alex's old Golf considered in Clapped-out Cars above. Alex expects it to continue running for a total of 150,000 miles. It had already clocked 50,000 miles. That means it can run for another 100,000 miles before exiting the pipeline and being replaced by a new car entering the pipeline. That means by selling our old fossil-fuel car we have delayed the production and purchase of a new car.

Let's assume further that (although cars are getting bigger) the old car is replaced at the end of its life by a similar-sized car using the same fuel, ie assuming that the distribution of car sizes within the pipeline changes much more slowly than the flow of cars through it.

How much carbon have we saved by delaying the production of a new Golf? 100,000 out of 150,000 miles is two-thirds of a Golf! (If we imagine diesel Golfs being replaced like for like over time, the saving is the same no matter how miles per year are driven; and the fuel used per year is the same an old or a new car). The same source as above (https://www.volkswagenag.com/en/news/stories/2019/04/from-the-well-to-the-wheel.html) estimates 6teCO₂ to build a fossil-fuel Golf. So by releasing the car for reuse we've saved 4teCO₂.

I can't think of a better way to share out the carbon savings other than splitting it over the lifetime of Alex's new electric car: a saving of 400kgCO₂ per year.

Thank goodness: Alex selling his old Golf and buying a Peugeot e-208 leads to a verall decrease of emissions of 280kgCO₂ per year!

Electric Car Carbon Calculator

This calculator generalises the example above, to help you work out whether you can reduce your carbon footprint by switching to an electric car.

Please enter real or estimated values into the boxes below and click on 'Calculate'. For help see Notes on the Calculator.

Old Fossil-Fuel Car
Real-world fuel efficiency: mpg
Fuel:
Distance driven per year: miles
Destination:
How many miles driven: miles
Max expected mileage: miles
Footprint of delayed new car: teCO₂
New Electric Car
Carbon footprint of manufacture: teCO₂
Battery capacity: kWh
Real-world range: miles
Carbon-intensity when charging: gCO₂/kWh
Expected lifespan: years

Notes on the Calculator

TODO: add a comparison between a new electric car and a new fossil-fuel car



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