MPGe of an electric car, revisited

Last year in MPGe of an electric car I attempted to compare the carbon emissions of the BMW Mini-E, and my own Ford Sport Ka. The result was 212 g/km (30 MPGe) for the Mini-E charged from coal, and 172 g/km for my Sport Ka (37 MPGe). In retrospect, the comparison was far too rough to draw conclusions from. This post is an attempt to correct that. This morning the Today Programme on Radio 4 covered the news that the Nissan Leaf electric car is to be built in Sunderland. In a piece Evan Davis asked the BBC Business Reporter whether the electric car would really reduce carbon emissions. Here is a partial transcript:

Evan Davis:
Coming briefly back to the electric car in Sunderland. You talked about the rollout of a way of getting it recharged. Nissan will make much of the idea that it is a low carbon car, but of course it relies on electricity, which comes from the National Grid, and the National grid is powered by… whatever, coal fired power stations. Is it really a low carbon car, or is it masquerading?
John Moylan:
Well I suppose it depends where you see our wider energy system in the decade to come. The government has decided that it wants nuclear to be a big part of that, it has also announced major plans for off shore wind. So in the future the shape of electricity generation could look very different in the UK, it could have a more low carbon base, and that would link into making people who buy electric vehicles more comforatable about where the energy is coming from.

This provoked a quick AudioBoo response from Robert Llewellyn. Clearly annoyed at Radio 4 he said

The Toyota Prius which I drive regularly and record CarPool in produces between 80 and 90 grams of CO2 per kilometer. The next cleanest cars are all in the 120-150 mark, that’s most small saloon cars. Anything with a slightly bigger engine it’s up in the 200 - 300 grams per km. An electric car like the Nissan Leaf plugged directly into a coal burning electricity plant, somewhere like Didcot or Drax B up in Yorkshire. Those huge coal burning plants that burn millions of tonnes of coal every day. They burn that coal incredibly efficiently. It is on a massive scale, it’s not like 10 million tailpipes from lots and lots of clanking internal combustion engines. It’s very simple and that electricity goes into the car where it is used incredibly efficiently. So even if you charge an electric car from a coal burning, unfiltered power station you release between 40 and 45 grams per km, in comparison to the 90 of the Prius, and the 120-150 of the very small econobox diesel/petrol car, to 350 grams/km for the Porsche Cayenne (oh how I love that car).

Today listeners might have have left thinking that electric car and conventional car emissions are on par, but Robert’s numbers put electric car emissions (even on coal) at half that of a Prius. There is a clear discrepancy. In Februrary Mike Boxwell (author of Owning an Electric Car) performed a direct test. He drove 2 conventional and 2 electric cars along the same route and measured their respective energy use:

In his test the electric cars charged from the National Grid both emitted 50 g/km, whilst the conventional cars emitted 137 and 155 g/km tank-to-wheel. The same electric cars emitted 131 and 140 g/km charged from coal. I compared Mike’s numbers to my own to arrive at the following tables:

  REVA G-Wiz Mitsubishi i-Miev BMW Mini-E
Distance travelled (km) 22.6 22.6 100.0
Electricity used (kWh) 2.99 3.19 20.63
Electrical eff. (kWh/km) 0.13 0.14 0.21
Elec. Emissions, grid (g/kWh CO2) 375.2 351.2  
Elec. Emissions, coal (g/kWh CO2) 1056.2 927.9 1028.8
Car emissions, grid (g/km CO2) 49.7 49.7  
Car emissions, coal (g/km CO2) 140.0 131.2 212.2
  Toyota Aygo Fiat Panda Ford Sport Ka
Distance travelled (km) 22.6 22.6 492.4
Fuel used (l) 1.33 1.51 37.60
Fuel eff. (l/km) 0.059 0.067 0.076
Fuel emissions, ttw (g/l CO2) 2315.0 2314.9 2246.0
Fuel emissions, wtw (g/l CO2) 2789.6 2789.4  
Emissions, ttw (g/km CO2) 136.5 154.9 171.5
Emissions, wtw (g/km CO2) 164.5 186.7  

The difference came down to how many units of electricity (kilowatt hours/km) were used by the electric car. Not having one to test myself I had used the EPA combined cycle figure for the Mini-E, which represents a very harsh driving style with many stop-starts. Mike’s direct measurements, however used his own driving style in all cases. The EPA figure for the Mini-E (0.21 kWh/km) is 1.5 times Mike’s highest (0.14 kWh/km). Mike’s test produces the fairest comparison, and it’s his numbers that should be used over my flawed comparison. It does show though, how critical driving style is to carbon emissions. Driving an electric car very aggressively, will make it perform worse than a conventional car driven conservatively. Coming back to the Nissan Leaf, why did the BBC interviewer suggest it wouldn’t reduce carbon emissions? Most likely because that’s the public’s impression, that the power to charge electric cars comes mostly from coal. This impression is wrong, as I write this only 29.5 % of electricity in the UK is coming from coal, you can see the current carbon footprint of the National Grid for yourself. Robert’s number of 45 g/km is correct for an electric car charged from the UK national grid, but not for a fictional car plugging straight into Drax or Didcot coal power station. It was 7:30 though, so such slips are forgivable.