Electric Vehicle - Mileage: how much 'bang for your buck'?

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?




Hybrid Vehicle - Mileage: Don’t fool yourself with ‘free energy’

With hybrid vehicle the most frequently used statistic is the MPG, or the amount of miles you can travel on a gallon. The European equivalent being the kilometers per liter, or km/l. There are many ways to determine this statistic for a vehicle and even for electric vehicles this statistic is used, even though the electric vehicle does not consume liters or gallons of fuel to travel, but electricity (measured in kWh).  

 

The mileage as it is indicated on the display of your vehicle - Link

 

y easy to derive; one simply fills up the tank, drive till the tank is empty and it is a simple equation of dividing the distance driven by the gallons used. Cars have a different efficiency when driving at different speeds; a conventional car is the most efficient on the highway at high speeds and less efficient in urban traffic (especially in traffic jams, with an idling engine you consume fuel and don’t cover a lot of distance). In order to create equal conditions for vehicles we use a driving cycle. There are various driving cycles around, some are country specific, others are specific for highway conditions, urban traffic or a combination of highway and city driving. The main point here is that you want to test a vehicle at the same driving condition as the other vehicle to be able to make a fair comparison. Examples of driving cycles are the NEDC and the Je05. For more information on driving cycles please refer to the site of For a conventional vehicle, this figure is relatively easy to derive; one simply fills up the tank, drive till the tank is empty and it is a simple equation of dividing the distance driven by the gallons used. Cars have a different efficiency when driving at different speeds; a conventional car is the most efficient on the highway at high speeds and less efficient in urban traffic (especially in traffic jams, with an idling engine you consume fuel and don’t cover a lot of distance). In order to create equal conditions for vehicles we use a driving cycle. There are various driving cycles around, some are country specific, others are specific for highway conditions, urban traffic or a combination of highway and city driving. The main point here is that you want to test a vehicle at the same driving condition as the other vehicle to be able to make a fair comparison. Examples of driving cycles are the NEDC and the Je05. For more information on driving cycles please refer to the site of dieselnet.com.

 

 

Where with a conventional vehicle you have just a fuel tank and the distance to consider to derive the mileage figures. With a hybrid there is also a battery involved and when not considering this in the proper way, one could falsely use this ‘free energy’ to get a better mileage. When determining the mileage of a hybrid car, it is only fair to make sure that the battery is equally charged at the end of the cycle as at the beginning. This does not imply that the battery is not to be used, quite the contrary. The battery will (automatically) be used to recoup energy from regenerative braking, it will discharge some on acceleration, etc. In the hybrid vehicle the battery is considered a buffer and if at the end of the cycle the battery would be fully discharged, the mileage has been influenced by this free energy. The fair thing to consider when looking at the mileage is to ‘convert’ some of the fuel to recharge the battery again to the same level as at the start.

 

Don't be fooled - consider electricity in your mileage; it does not come for free - Link

 

As an example, consider a hybrid vehicle which has a fully electric range of 30 kilometers. If you would take it for a trip of 50 kilometers, you could fully drain the battery for 30 and then use your fuel for the remaining 20. If you want to determine the mileage, taking the fuel consumption of the 20 kilometers and combine it with the total distance of 50 is fooling you a bit with the supposedly ‘free energy’. To determine the correct mileage you would then need to run the engine to recharge the battery again, with the bonus that you can have the engine run at its most efficient operating point to do so.

 

Of course, if this would be an actual trip to just get somewhere in the cheapest possible way, you would just plug it in at the end of the trip to recharge the battery and not run the engine. That would not just be cheaper, but also more environmentally friendly.