As a little follow up from my previous post on the mileage for a hybrid vehicle, this post will try to detail what it takes to derive the mileage for an electric vehicle. The first and most obvious question is: “Mileage? But an electric vehicle does not burn any fuel?”. While that is true, since the electric vehicle carries a battery around and runs on electricity, currently the majority of the world’s electricity is still derived from some sort of fossil fuel in power plants. Despite the successes of solar and wind, we are still some time off from 100% clean energy for all our needs, unfortunately.
In a similar process as with the hybrid vehicle, an electric vehicle is charged up fully and given a certain driving cycle to drive. This standardized road trip consumes power from the battery and at the end of the trip one has to determine how much power (or charge actually) is left in the battery.
As an example, let’s take the Nissan Leaf, it has a battery of 24 kWh and a driving range of 100 miles or 160 kilometers. Suppose we’d drive it empty for this given distance (Note: it is not advisable to completely drain your battery, it can cause serious damage!); in that case 24 kWh was used to travel 100 miles / 160 kilometers. Doing the maths this leads to 4.16 miles / kWh, or 6.66 kilometers / kWh. An interesting figure, but hard to compare to something that represent a distance per a volumetric unit (gallons or liters). How to compare these figures with the fuel based MPG in order to see what is more efficient?
For the Nissan Leaf the EPA has determined a figure that is an MPG value for the electric vehicle. In order to achieve this, they looked at how much energy is in a gallon of fuel and derived how much fuel they would need, to get an amount of electricity that will be used to travel. This method uses the MPGe, or MPG equivalent value. It uses the theoretical amount of energy that is available in fuel and does not account for the conversion process from the fuel to electricity. Even in a high efficient power plant, converting fossil fuels to electricity comes with losses, a lot of energy is lost there as heat, same as with the internal combustion engine.
Fortunately, there is also the method that is referenced to as the well-to-wheels analysis. This is a very thorough analysis which starts at the source of the energy (for example the oil well). It takes into account the extraction of the fossil fuels, processing and transportation, a conversion up to the final stage where the energy gets converted into the actual motion to drive, the wheels. Whereas this method is very thorough and can very clearly define how much energy it required from the source to travel a given distance, the average driver is not so much concerned by this. For an example of a nice well-to-wheels analysis, here is a link of such an analysis by Stanford for the Tesla Roadster.
Link to the video from Alef Arendsen on TEDx Amsterdam on Youtube - Link
The well-to-wheels analysis is a great tool for policy makers and the industry, but the average driver is more interested in what it will cost him to travel. Alef Arendsen from The New Motion worded this very well during his Ted X talk in Amsterdam last year; “Me first, then the world”. If it costs people less to travel, they’ll go that way. If it is also beneficial for the world because it is more energy efficient, that is considered an added bonus. Luckily the electric vehicle is such a product that meets these ends.
I think in the end the mileage figures are nice to have and compare, but for most people they will speak a lot more of they are converted to the (local!) energy prices (and perhaps with the associated CO2 emissions). If vehicle A uses 1 liter to travel 25 kilometer, and vehicle B uses 1 kWh to travel 25 kilometer, there is no obvious winner for most people. If you convert it to money, it becomes a lot clearer.
Table to to compare to hypothetical cars - Liters vs kWh, or Euro vs Euro?