BMW i3 Confirmed as Most Efficient EV Yet
Several months ago, I published the results of an analysis to rank BEVs according to several key performance measures that included projections for the then unreleased BMW i3. This is an update to that article using the newly released EPA i3 metrics.
One of the conversations that is rarely if ever included in the EV forum discourse is efficiency. There is as much conversation as you could ever want about range, conveniences, charging protocols, acceleration, regen, interior sound levels, battery life, interior capacity and more but very few words are exchanged about how efficiently EVs propel themselves and their occupants from destination to destination.
Even, one might say especially, in the realm of alternative energy vehicles where reducing the carbon footprint is central to the concept, efficiency remains the true premium since in their presently generated form, all alternative fuels have a carbon cost. Possibly someday in the future we will be swimming in gloriously abundant photovoltaic electricity and the story will be different but we are nowhere near that point yet so we must consider efficiency in our analysis of EV performance.
True, EV drive trains are inherently more efficient than their ICE counterparts but they are currently powered by batteries and energy storage density in batteries is much lower than gasoline as we know. For a given vehicle package with current technology, EVs are heavier than ICEs due largely to battery mass. As we also know, mass is the enemy of motion, or more accurately change in velocity – acceleration, braking, turning, suspension response, handling are all negatively impacted by mass.
The lower battery energy density makes the balance between mass and performance more delicate for EVs than for ICEs – a few more gallons of gas won’t penalize you much in mass but it will provide substantially more energy. Obviously energy can be increased by a larger battery as well but batteries are heavy and a bigger battery necessitates a stronger structure (read heavier) and the return is not linear. The heavier structure also requires stronger (heavier) suspension components which then require a bigger battery which mandates a stronger structure ad infinitum.
But mass isn’t the only story, there are other energy sapping factors to consider in addition to old-fashioned mass including overall size, coefficient of drag, drive train losses, circuitry losses, wheel size/geometry, rolling resistance and more. So achieving higher efficiency is a remarkable balancing act, very sensitive to movement in any direction away from the optimized performance target.
So I thought it would be good to look at EV choices relative to efficiency to see where the market is. I compiled a list of the EVs that have been tested by the EPA with their five cycle test, hoping to result in a data set that is more objective then by using OEM numbers or magazine tests. I recorded the efficiency in kWh per 100 miles for each vehicle from the EPA list (I am only analyzing BEVs so I just don’t want to look at MPGe, too much ICE in it for me).
I then calculated Miles per kWh, the metric I like best for BEVs. I added a few other data points that may be of interest to this discussion and populated them from the OEM websites – 0-60 times came from zeroto60times.com except when it was available on the OEM website.
Sorting on Miles per kWh generated the table below:
In this analysis, as we all now know, the BMW i3 is the leader, nearly twice as efficient as the least efficient RAV4. The Bavarians have achieved quite a remarkable result, designing the i3 to the top of the “do more with less” rankings. And of course, this correlates to the lowest annual fuel cost. But does it sacrifice in other important areas? As important as it is, efficiency at the expense of other critical performance criteria does not a great product make so I checked out a few other rankings:
The Teslas dominate this measure as we all know. The i3 holds its own though, high on the rankings coming in right after the Teslas and apparently in line with its most obvious upcoming competition – the Mercedes Benz B-Class ED, said to have a 0-60 time of 7 seconds as well. The RAV4 is a surprise but it does have that big battery.
The i3s are not the lightest of the group which may not be surprising considering the diminutive nature of several of the offerings. As small as the small ones are, including a two-seater, as a group they operate on the lower end of efficiency and acceleration spectrum emphasizing that mass is not the only factor in doing more with less. The Teslas are the heaviest by a substantial margin – the SP85 is literally a full ton heftier than the i3 BEV.
Again the Teslas own this category with their massive battery packs. They do it with brute force, however, claiming the lowest efficiency and highest mass in the group. The i3 is in the middle of the rankings, generally ahead of other vehicles that offer much less in other function and features.
The i3 has clearly established a new bar in efficiency demonstrating what is possible with a clean sheet, purpose built design.
It is not the smallest or the lightest or the least utilitarian or the slowest EV from the set the EPA has tested, which one may expect from the vehicle with the highest efficiency. In fact quite the opposite is true – it also exhibits strong performance by other measures including acceleration and with its low curb weight, it should prove to be very nimble and has already been reported to possess great handling. All with seating for four adults and a cargo area. In my opinion, it is the best all around performer due to all of these considerations, the most significant of which is its EV leading, carbon footprint reducing efficiency.
Footnotes for all four tables:
1. Curb weight, Motor, Torque Battery capacity from OEM websites 2. 0-60 times from www.zeroto60times.com. From their website “0 to 60 times and Quarter mile data is gathered from numerous credible sources. There are a great deal of factors that affect 0-60 stats, so different sources may test the same vehicle and each may arrive at a unique 0-60 mph and quarter mile result. Zero to 60 times .com does not guarantee the accuracy or validity of the any of the 0-60 mph times 3. EPA Combined rating as reported on www.fueleconomy.gov 4. 0-60 times for Tesla from www.teslamotors.com, Smart Electric Drive time is for 0-62mph, Toyota 0-60 from www.toyota.com 5. Tesla Model SP 85kWh Curb Weight from www.teslamotorsclub.com 6. Sport Mode 7. Specs from www.wikipwedia.org/Toyota iQ