Does size matter?: EVs, batteries, and electric miles

Does size matter?: EVs, batteries, and electric miles

Posted on

Fri, 10/25/2013 - 3:19pm

by Brett Williams

With U.S. and California plug-in electric vehicle (PEV) commercialization underway, the state continues to forge a path forward, incentivizing increasing numbers of PEVs, more all-electric vehicles, and big batteries. As sales increase and serious incentive and other investments are being made, examining supportive policies for their effectiveness over time is increasingly important. This blog provides some background for that effort by: discussing the state’s zero-emission-vehicle (ZEV) policy, introducing the importance of electric miles (e-miles) as a metric linking ZEVs to their impact, and examining the e-mile potential and cost-effectiveness of the current fleet of light-duty PEVs in the U.S.

3.3 is better than 1.5

Yesterday, the California Air Resources Board (CARB) held a hearing to review the latest proposed changes to its landmark regulations for zero-tailpipe-emission vehicles (ZEVs) and to get an update on the extension of that program through 2025 under the auspices of the Advanced Clean Cars program.

As part of an event concerned with changes that can fairly be considered minor relative to a history of major overhauls starting back in 1996, there nevertheless was a significant announcement:  the unveiling of a memorandum of understanding signed by 8 out of the 9 governors of states in the union that have chosen to adopt California’s more aggressive vehicle regulations rather than stay on cruise control with the federal standards. Joining forces in this public way, CARB was able to riff on the meme created by the Governor Brown’s Executive Order calling for 1.5 million ZEVs on California roads by 2025 with a CARB homepage headline that reads: “3.3 Million Zero Emission Vehicles by 2025.”

But it is complicated

Under the surface, the hearing, and the ZEV regulations, create a much more complicated picture. This is due to a complex and evolving ZEV credit scheme that disconnects the number of credits from the number of vehicles. It was originally phrased as a percentage, e.g., 2% of all vehicles produced for sale in California in 1998 shall be ZEVs. It was adapted into a flexible credit banking and trading system that has allowed partial credit for vehicles with combustion engines and has given extra credit for desired vehicle attributes—e.g., long electric range provided by big batteries. It also requires increasing numbers of all-electric vehicles over time. Plug-in hybrids—which can in practice be ZEVs but can also operate having never seen a charge station—are allowed to contribute, but for less credit per vehicle and in a diminishing second class referred to as “transitional.”

Through this complex ZEV credit system—along with a host of related and supportive other policies such as differential purchase rebates and carpool lane stickers, and investments in infrastructure implicitly or explicitly supportive of specific kinds of PEVs and drivers—California effectively incentivizes a complex, confounded web of factors relating to PEVs and electric fuel. Many of these factors—e.g., battery size and number of all-electric vehicles—are good factors to increase over time, and there are a wider variety of justifications for doing so than will be articulated here. However, many of these factors are rough proxies for the real headline, which isn’t the number of cars or even reduced emissions, but avoided damage to human and natural systems. But these factors are separated from impact by several links in a complex chain of causality that introduces uncertainty at each stage (Figure 1).

Figure 1. Complex casual chain of compounding uncertainty

Figure 1. Complex casual chain of compounding uncertainty

Stepping forward: e-miles

To examine the effectiveness of policies supportive of PEVs and electric fuel, it is therefore desirable, when practical, to understand links in the chain leading to impact. An important linkage between ZEVs and their impact is, in a word, “use.” Producing high volumes of big-batteried vehicles for sale could be important (e.g., for reaching critical economies of scale in vehicle and battery-cell production [1]). But it only gets us partway to reduced impact if those vehicles aren’t out on the roads displacing many petroleum-fuel vehicle miles travelled (VMT) with those fueled by electricity (e-VMT). And thus, increasing attention is being paid not just to the size of batteries and the number of vehicles, but to “e-miles.”

Chopping things down to size

Even without data directly measuring e-VMT accumulation by the U.S. fleet, an interesting picture emerges when we consider both market adoption and expected use. Figure 2 illustrates the total e-mile potential of PEVs sold in the U.S. from December 2010 through September 2013 [2]. The first column is the sum of the sales-weighted electric ranges of the U.S. PEV fleet. In essence, it shows how many e-miles U.S. PEVs could provide with each charge. The second column caps the e-range of each vehicle at 30 miles per day, a reasonable estimate of average daily driving. In essence, it shows how many e-miles U.S. PEVs are likely to provide each day (e.g., assuming that each vehicle returns home each night to recharge).

Figure 2.

cumulative electric-mile potential of U.S. retail PEVs sold

As can be seen in the figure, the e-mile potential of the U.S. fleet decreases dramatically for the expected-use case. Further, the vast majority of that decrease comes from all-battery electric vehicles (BEVs). Indeed, on a sales-weighted basis in expected use, plug-in-hybrid EVs (PHEVs) actually contribute more daily e-mile potential, in total. Thus, at this particular moment in time, the “transitional,” less incentivized class of plug-in hybrids with relatively small batteries is probably doing more than their share of cleaning the air.

Figure 3 illustrates this fact further by showing how many daily miles of travel would be required, on average, of the existing fleet of PEVs for the contribution to be equal: 36 miles per day—a significant increase. Indications are that this comparison may be conservative—at the CARB hearing it was mentioned that data from the EV Project indicate that Volts are racking up about 20% more e-mile than LEAFs, and in LA, 40% more.

Figure 3.

Cumulative e-mile potential, break-even

Turning it around: asset utilization

Another way to think about this effect is in terms of cost effectiveness per e-mile. The comparison between vehicle capability and daily driving highlights the “surplus” battery capacity in BEVs, on average. A ~100-mi e-range appears to be necessary for BEVs to penetrate majority markets, but is not, at this particular juncture, obviously more valuable from the perspective of cleaning the air. Indeed, from the perspective of near-term e-mile cost-effectiveness ($/e-mi), battery size is not an inherent “good.” Rather, it is an asset that should be fully utilized each day in order to spread its costs out over the maximum e-mile benefit it is capable of providing. Thus, even if a vehicle averages 30 mi/day, if they only exceed 30 mi on 40% of days—as it appears the 2009 National Transportation Household Survey indicates here [3]—on 60% of days the battery is not fully utilized. Of course, BEVs do not have the luxury of being designed to average needs. And the nominal cost-effectiveness of their batteries suffers as a consequence.

This can be seen in Table 1, which assumes a $500-per-rated-kilowatt-hour, across-the-board battery cost to illustrate the cost per e-mile of EPA-rated electric range. In the first cost column, which examines vehicle capabilities, all vehicles are roughly comparable, with most plug-in hybrids on the high end of the spectrum. The second cost column, however, shows that costs per e-mile of average daily driving vary quite dramatically, with capable BEVs suffering for their “surplus” capacity.

Table 1. Illustrative e-mile cost effectiveness (assuming $500 per rated kWh across the board)

Table 1. Illustrative e-mile cost effectiveness (assuming $500 per rated kWh across the board)


First things first?

Part of the confusion surrounding the debate on whether or not PEV commercialization is going well or not [a matter I argue is more one of Managing EV Expectations] is the fact that media commentators often don’t properly distinguish between the two very different products that are all-battery and plug-in-hybrid EVs. California policy clearly does, however, to the significant disadvantage of the vehicle type that is, at this particular moment in time, probably pulling more than its weight of cleaning the air.

This is not necessarily to say that it shouldn’t. [We’ll save that discussion for a future blog.] Or that the state shouldn’t stay on an aggressive path towards large numbers of the truly zero-tailpipe-emission vehicles that will be needed soon enough to meet the state’s daunting challenges. Personally, I’d like to see all-battery EVs succeed; at one point before moving to LA, I was on the waiting list for two different models. Further, I’m used to imagining the long game, having previously criticized incremental approaches by saying “a chasm can’t be crossed in two jumps,” and having worked for what some would call a “controversial” and “bleeding edge” visionary.

However, as I expressed in The Path of Least Resistance: An Introduction, I believe there is still an underlying market dynamic that is both worth understanding and worth capitalizing on for those truly interested in maximizing the penetration of all electric-fuel vehicles and the benefits of their use. This is not about being incremental or myopic. It is about a path complementary to the inspiring Tesla example and “beyond oil” narrative—a path that may involve lower adoption costs (broadly defined) and greater numbers of consumers. It is about getting going, truly going, with PEVs. It’s about crossing the chasm to majority markets without letting a view of the perfect be the enemy of the good that will come from getting more PEVs on the road and cleaning the air.



1.         Williams, B. D.; Lipman, T. E. Strategies for Transportation Electric Fuel Implementation in California: Overcoming Battery First-Cost Hurdles; CEC-500-2009-091; California Energy Commission Public Interest Energy Research (PIER) Transportation Program: Sacramento, 2009.

2.         Williams, B., "U.S. Plug‐in Electric Vehicle (PEV) Sales Trends & Analysis: Dec 2010 — Sep 2013." UCLA Luskin Center for Innovation. 2013.

3.         Williams, B. D.; Martin, E.; Lipman, T.; Kammen, D. "Plug-in-Hybrid Vehicle Use, Energy Consumption, and Greenhouse Emissions: An Analysis of Household Vehicle Placements in Northern California." Energies 2011, 4, (3), 435-457.




by Brett Williams