BMW i3 Fuel Cost Comparison

So, yay! My shiny new BMW i3 has been delivered, in all its "solar orange" glory. I've driven around excitedly, and smelled the new car smell, and done all the things that might be expected at such moments. However, despite all my excitement, I did eventually manage to calm down for long enough to wonder how much it was costing me to fuel. This is a much less explicit cost than that of petrol or diesel, as it takes the form of a non-itemised increase to my electricity bill.


This question led to the following spreadsheet, comparing the fuelling cost of my previous car (a diesel BMW 320d) with the i3.

So, the cost per mile to fuel the i3 is comfortably under half of that for the 320d. Call me ungrateful, but I found this a little disappointing as I had guesstimated it to be more like 20% of the cost.

Turns out, however, that calculating the true cost is not that simple.

Firstly, it's winter. The range of electric vehicles is impacted by the effects of the cold on their batteries. BMW have done various clever things on the i3 to try and mitigate this. For example, if you leave the i3 on charge and tell it in advance when you plan to leave your house, then it will use your domestic electricity supply to warm up the battery and the cabin, rather than using battery power after you drive off. This, and other optimisations, help quite a bit, but it remains the case that range is more limited in winter. This makes the i3 look more expensive than it would look if I were able to average the miles per kWh across all four seasons.

Secondly, there's a lot of free charging around, at least for now. I can plug my i3 into the Ecotricity chargers that can be found in an increasing number of motorway service stations, and charge my car with fantastic rapidity, and this doesn't, as yet, cost me a penny. An ever-increasing number of business establishments offer free charging to their customers. This means that when I do longer journeys, or travel to places with chargers, my fuel costs are subsidised. It's hard to see how this situation can go on indefinitely, but it certainly has the potential to save me money in the short to medium term.

Thirdly, Ecotricity give me a discount on my domestic electricity supply for having an electric vehicle. This amounts to £40 per year, and I have not taken it into account in the calculation above. It will further reduce the overall cost per mile. Using the above figures, I calculate that this £40 discount buys me about 79 miles per month – thank you very much, Ecotricity!

Fourthly, world oil prices have dropped recently, and with them diesel prices at the pumps. For most of the four years during which I drove the 320d, the cost of diesel was around £1.30 or even £1.40 per litre. At these prices, the cost per mile for the i3 is closer to a third of the diesel price than a half of it.

So I think that, on balance, I can overcome my initial disappointed reaction, and be pleased with the reduction in fuel costs per mile from the i3 when compared with the 320d.

It's great that I can have all this fun driving, and save money at the same time. In four years time, when the i3's lease expires and I'm looking around again, I'm sure the landscape will have changed. New cars will have 1,000 mile batteries in them rather than the 80-100 miles of the current crop of vehicles, and the cost models will be different again. Time will tell, I suppose.

In the mean time, I love the i3 - it is a great car to drive, and it feels like the future.

Update, 22nd Feb, 6:20pm

I shared this post on the BMW i3 UK Facebook group, which prompted some really useful and interesting feedback.

First off, I've been reminded about Economy 7, which is a split cost tariff offering very cheap overnight electricity rates, offset by more expensive daytime rates. If the car charging occurred almost exclusively overnight (so far it does), then this would bring the per-mile cost down to around 16% of the cost of the 320d.  Much more in line with what I was expecting.

Also, installation of solar roof panels can allow free charging at home too, especially in the summer.  It's not really an option for us at the moment, but I can see that it would be very satisfying to drive around on self generated fuel.

Update, 10th March, 5:00pm

The i3 now shows the average consumption over 1068 miles as being 4.0 miles per kWh, which brings the fuel cost per mile down slightly to around 3.9p/mile.

If I was really dedicated to reducing the cost per mile, I could switch providers away from Ecotricity, and pay as little as 3.2p/mile, but my energy would then come mostly from non-renewable sources which seems to rather defeat the object of the exercise.

However, if I were to remain with Ecotricity but switch to an Economy 7 meter, then the cost per mile would be reduced dramatically. The fuel cost would be down to 1.4p per mile, providing that I timed recharges so that they started at 2am. However, due to the additional cost of daytime and evening consumption under Economy 7, it may not be cost effective overall unless I can shift other consumption to the overnight slot.

I will be doing my own calculation to see whether this adds up or not. Watch this space.