April 25, 2020

Tesla Vehicle Efficiency: The EV Gold Standard

In 2008, Tesla released their original Roadster. The 2-seat sports car had an EPA range of 231 miles and a 53kWh battery pack, making it the longest-range EV ever built. The car also established Tesla as the gold standard in EV efficiency. For 11 years now, EV automakers have been trying to improve on Tesla’s range and battery performance, without much success. Today, the Tesla Model S boasts a range of 600km (373 mi), roughly 125km (80 mi) more than any non-Tesla EV. Even the 11 year old Tesla Roadster has better range than Audi’s flagship EV, the E-Tron.

However, these are just anecdotes. We must dig deeper for a complete picture of the range and battery improvements Tesla keeps making, and the EV efforts of other automakers around the world.

The most commonly-cited metric for EV efficiency is Wh/km (Wh/mi in the USA). This is a quick and dirty way to figure out how much energy (Wh) is required to move a car 1 unit of distance (1 km or 1 mi). That’s even the metric that appears on the energy tab of every Tesla today.

And it’s a great metric for measuring the relative efficiency of a single car while driving under different conditions. It’s easy to compare mountain roads, cold temperatures, and highway driving to city driving. But the metric isn’t very useful when comparing different EV models to one another.

There are many additional factors that influence the amount of energy required to move a car over a certain distance. Things like vehicle weight, ride height, tire size, drag coefficient, and surface area can all influence the Wh/km of a vehicle without any changes to a vehicle’s battery.

Among the factors listed above, weight is probably the most influential since variances are so wide. Cars like the Tesla Model S weigh more than twice as much as the smaller EVs like the Smart EQ fortwo. Therefore, it requires far more energy to move a Tesla the same distance as a Smart EQ. Using Wh/km to measure battery efficiency doesn’t help us understand efficiency here, vehicle weight skews the results in favour of the Roadster.

So in order to better isolate the effects of battery performance, vehicle weight must be considered. This makes the metric useful across the entire EV market, and helps normalize the artificially high efficiency of light cars using small EV batteries.

Instead of using Wh/km, we can instead use Wh/km/weight to measure battery efficiency. Here, weight is expressed as (weight in kg/1000) so that the final output of the equation isn’t an obscenely small decimal.

Using this formula, let’s examine how Tesla vehicle efficiency has improved over time. More accurately, let’s measure the energy required to move different Tesla vehicles 1km down the road, per 1000kg of body weight.

The 2008 Tesla Roadster had a 53kWh battery, 372km of EPA range, and a weight of 1305kg. Therefore, it had an ‘efficiency rating’ of 53000/372/1.305 = 109.17. For every 1000kg of body weight, it took 109.17Wh of energy to move the Tesla Roadster 1 km down the road.

Meanwhile, the 2019 Tesla Model S Performance has a 100 kWh battery, 600.3km of EPA range, and a weight of 2215kg. Therefore, it has an ‘efficiency rating’ of 100000/600.3/2.215 = 75.20. For every 1000kg of body weight, it takes only 75.20Wh of energy to move the Tesla Model S 1 km down the road.

This ‘efficiency rating’ clearly shows that the Model S battery and energy management system is better than that of the 2008 Roadster when we consider vehicle weight. But if we had ignored vehicle weight, and simply measured efficiency using Wh/km, it would appear that the 2008 Tesla Roadster’s battery system was actually 14.5% *more* efficient than the 2019 Model S. That’s simply not true.

The Model S is lugging around an additional 910kg of vehicle weight, and accounting for that added weight makes the Model S 45% more efficient than the Roadster. That’s an efficiency improvement of roughly 3.1% per year since 2008.

It’s important to add a disclaimer here. While this ‘efficiency rating’ helps normalize vehicle weight and gives us a way to compare efficiency across vehicles, it’s not a perfect metric. There are other variables that are more difficult to measure like drag coefficient, rolling resistance, vehicle and tire surface area which may contribute to efficiency variance too.

It seems very likely that most of Tesla efficiency improvements are due to battery and energy management improvements, but we can’t rule out the possibility that other improvements are contributing to overall efficiency too.

Either way, it’s fascinating to see how steady Tesla’s efficiency improvements are. Every passing year Tesla cars use roughly 3% less energy than in the previous year to travel the same distance (normalized for vehicle weight).

To better observe this trend of steady efficiency improvements, the chart below compares the ‘efficiency rating’ of all of Tesla’s most efficient vehicles over time. The Model S has been in production the longest of all Tesla vehicles, and has become more efficient in 5 of the last 7 years.

And while this chart is illuminating, it doesn’t show us how the rest of the EV industry is improving. Cars like the Nissan Leaf, BMW i3, and Chevy Bolt have been around for many years now, giving us lots of historical efficiency data to compare against.

It’s clear that while other automakers are generally improving, none of the 3 non-Tesla EVs above have even matched the efficiency of the 2012 Model S. They’ve got a long way to go if they’re going to become more efficient than Tesla’s current vehicles.

Even the newcomers to the EV market, dubbed the “Tesla killers” can’t compete with Tesla on efficiency. Among the Jaguar I-Pace, Audi E-Tron, and Hyundai Kona, only the 2019 Kona is more efficient than the 2008 Tesla Roadster. Although even the Kona isn’t yet as efficient as the 2017 Tesla Model 3.

The importance of efficiency in an increasingly competitive EV market can’t be overstated. A less efficient vehicle means using extra batteries to compete on total range. That adds overall weight to the vehicle, requiring even more battery cells to move the now-heavier vehicle.

And because batteries are among the most expensive components of an electric vehicle, less efficient vehicles are more costly to consumers and less profitable for companies. Lower vehicle gross margins means fewer available dollars to reinvest into R&D, and less efficient batteries in the following year’s vehicles. At least that’s one possible outcome.

The other outcome which seems likely over the next 10 years is that the rate of battery improvement levels off. Batteries may become so good that they become commodities. EV buyers may just stop paying attention to vehicle range and efficiency once EVs can drive 800+km on one charge. Just like how batteries aren’t a key consideration in cellphone buying decisions today, the same may become true for cars.

But we’re still 5-10 years away from that possibility, and even further away in regions that experience colder winter temperatures.

That leaves a long runway for innovative automakers to establish battery cost advantages, outsized market share, and favourable public opinion due to superior battery performance. And at such an early stage in the development of the global EV market, no automaker wants a reputation for poor efficiency and battery performance.

As the old saying goes, you never get a second chance to make a good first impression.

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