Tesla Model S – how does battery size influence eRating?

Those following Tesla Motors over the past two years have witnessed an often unpredictable ride. With a media-savvy CEO not afraid to Tweet his mind and a stock price that has more than tripled over the last 6 months, the ensuing excitement has almost overshadowed the introduction of Tesla’s flagship Model S. The seven passenger luxury sedan, the first to be completely internally developed, has earned critical acclaim including prestigious “Automobile of the Year” awards from no less than Motor Trend and Automobile Magazine. Add an almost unheard of 99/100 rating from Consumer Reports and it becomes clear that the wild ride is far from over.

2013 Tesla Model S

Part of Tesla’s plan is offering consumers a choice of different battery packs. Among the most expensive parts of an EV, the battery is a key determinant of both driving range and purchase price. Tesla originally offered 40, 60, and 85 kWh battery packs at prices of $59,900, $69,900, and $79,900 respectively. The EPA estimates that the 60 and 85 kWh packs will provide an average range of  208 and 265 miles. The 40 kWh hour pack, offering 139 miles of electric propulsion, has since been discontinued due to a lack of demand.

Larger batteries have the upside of increasing vehicle range but also inevitably lead to a heavier vehicle that requires more energy to get moving as well as greater environmental impacts during manufacturing. To see how the extra mass affects the Model S’s environmental performance, we computed eRatings for the 60 and 85 kWh models.

Make and Model$MSRPSpecsCity / Hwy / Comb
Tesla Model S (60 kWh)69,400302 hp / 317 ft-lb electric motor,
208 mile range
94 / 97 / 95
Tesla Model S (85 kWh)79,400362 hp / 325 ft-lb electric motor,
265 mile range
88 / 90 / 89

As it turns out, the eco-penalty for carrying around the larger pack is slight. The Model S with the 60 kWh pack scores 211 on the eRating scale. Although upgrading the Model S to an 85 kWh battery adds 183 pounds, the car’s eRating only drops by 13, to 198, implying just seven percent more pollution per mile than the 60 kWh version. The combined miles-per-gallon equivalent drops by 6 mpge.

Both Model S variants carry eRatings well above those earned by the comparably sized and priced Audi A6 and BMW 740i (98 and 100, respectively). The Tesla Model S 85 kWh version’s eRating of 211 implies 53% less overall pollution per mile than the average eRating of these similarly sized European luxury sedans.

While a larger battery pack may produce a slight dent in environmental performance, those who opt for access to more electrons will find a nice bump in performance and range. Larger batteries are able to not only store more energy, but release that energy at a higher rate, providing the electric motor with more power.

While Tesla does not release monthly sales projections, estimates based on weekly production rates put Model S sales slightly ahead of the Nissan Leaf. This would make it the best selling electric vehicle in America which, given what we’ve seen in the last two years, wouldn’t be so surprising.


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