More Efficient Than Its Big BrotherThe EPA has released its official MPG-equivalent and range numbers for the 60kWh version of the Tesla Model S electric sedan. The top-of-the-line 85kWh version had been tested earlier, and now all that's left to be officially rated is the entry-level 40kWh version.
You might expect all these models to be the same except for the range, since the main difference between them is the size of the battery pack, but the variance isn't quite linear. This is because a lighter battery pack makes the overall car lighter, and thus allows it to move using less energy. As you can see in the table below, the MPG-equivalent numbers for the 60kWh are 95, which is 6 more MPGe than the 85kWh version.
Range for the 60kWh Model S is 208 miles, compared to 265 miles for the 85kWh model. So the 85kWh contains 41% more energy than the 60kWh, but its range is only 27.4% more (at least in EPA testing -- real-world driving is different, as we've recently seen). But the 85kWh is about 17% more expensive than the 60kWh (69k vs 59k), so at least the price doesn't scale linearly!
Logically, we should expect the 40kWh version to be the most efficient Model S, even though the total range will be the shortest.
Be Careful About MPG-equivalent RatingsWhat does this mean? Well, MPG-equivalent doesn't mean too much for purely electric cars (and it is also very problematic for plug-in hybrids) because it doesn't really allow you to compare with the MPG of gasoline vehicles. From an environmental point of view, the source of electricity matters most. Even on dirty grids, electric vehicles tend to do better than gasoline and diesels because power stations + electric motors are just much more efficient than internal combustion engines (if you don't waste 70% of the energy as heat with an ICE, you're doing well). But if you can recharge with a relatively clean electricity source, the difference becomes huge, and hopefully the power grid will get cleaner over time, so electric cars should also get cleaner.
In short, even if there was a gasoline car on the market that got 95 MPG, a 95 MPGe electric car would still pollute less in almost all scenarios.