BMW i3 achieves new Green Score high (but where’s Tesla?)

April 27, 2015

The 2015 BMW i3 has earned the highest-ever score of 65 in’s annual environmental ratings. The i3 missed out on our February 6th release, and a spot on the Greenest list, due to its belated appearance in agency databases. It now appears in the vehicle ratings, along with two other BMW models: the i8 and the i3 REX (range extender). The i8 earns a score of 44 and the range extended i3 (a plug-in hybrid version of the all-electric i3, which has an additional 87 miles of range due to gasoline operation) has a Green Score of 56. The 2015 i3 is by far the most fuel-efficient vehicle in the market, with city and highway fuel economies of 5.8 miles per kilowatt-hour (kWh) and 4.7 miles per kWh, respectively. The i3 is the first vehicle offered for sale in the United States with a frame entirely composed of carbon fiber reinforced plastic, which results in a lighter, and hence more fuel-efficient, vehicle.  

The arrival of a brand new electric winner is a good time to address some recurring reader inquiries about our plug-in vehicle scoring methodology. We often receive questions about why certain plug-in vehicles—more often than not the Tesla Model S or the Chevrolet Volt—don’t show up on the Greenest list, despite operating partially or fully on electricity. The answer requires a deeper dive into our methodology.

ACEEE’s ratings are based on a lifecycle analysis of emissions, which means that we include emissions from vehicle and battery manufacturing in calculating the Green Score. Vehicle and battery weights and battery composition are the basis for our estimates of manufacturing impacts. The Tesla Model S 65kW model (which scores in the 90th percentile but doesn’t appear on the Greenest list) weighs 5,000 pounds (test weight) and is heavier than most other electric vehicles on the market. To a lesser extent, its relatively large battery (more than 1,000 lbs.) also keeps the score down, though this feature is what gives the Model S its impressive range. Likewise, the plug-in hybrid Chevrolet Volt is a relatively heavy vehicle at 4000 lbs., although the Volt’s unimpressive fuel economy for gasoline operation is also a drag on its Green Score.  

The other reason, and perhaps the most important reason, that electric vehicles don’t automatically land on the Greenest list is that our ratings reflect emissions from all stages of the fuel and vehicle cycles, including electricity generation. While an all-electric vehicle has no tailpipe emissions, emissions do result from the generation of the electricity used to charge the battery. To rate electric vehicles, we use the national average grid mix (approximately 30% coal, 25% natural gas, and 40% nuclear, as estimated in Argonne National Laboratory’s GREET model).

Use of a plug-in vehicle will generate fewer emissions when charged on cleaner electricity. For example, charging a plug-in in the state of Washington typically results in fewer emissions than charging one in the Midwest, due in large part to the fact that Washington’s grid uses a significant share of hydroelectric power. For this reason, as part of this year’s release, we have added a regional Electric Vehicle Calculator to our resources to show how Green Scores would vary if they took into account the region of operation.

This year’s Greenest list continues to showcase the impressive range of plug-in vehicles available in the American market this year. While a handful of battery electrics didn’t make the cut due to vehicle and battery weights, they nevertheless scored quite well. All the plug-ins in this year’s data set scored amongst the top 10% of all vehicle offerings.