Charging Behavior Impacts on Electric VMT: Evidence from a 2013 California Drivers Survey

The growing plug-in electric vehicle (PEVs) market features new models of battery electric vehicles (BEVs) and Plug-in hybrid vehicles (PHEVs) with varying battery sizes and electric driving range. How are these different models being used in the real world? A common assumption in PEV impact analysis is that PEV owners will maximize their vehicle utility by appropriately sizing their battery to their driving needs and by charging their vehicles as much as possible to recover the cost of the vehicle purchase. Based on these assumptions we expect PHEV owners who drive more to plug-in more and drivers of PHEVs with small battery to plug in more than owners of vehicles with a larger battery and similar driving patterns. This paper examines the assumptions presented using a survey of more than 3,500 PEV owners conducted in California in May and June 2013. The online survey includes extensive data on driving and charging behavior using web-map questions and includes owners of all PEV models in the market including more than 600 Volts and 800 Plug-in Priuses. The results show that small battery PHEV electric vehicle miles traveled (eVMT) are lower than larger range PHEV or BEVs not only because of the battery size but also as a result of the public charging availability and charging behavior. Higher electric range PHEV drivers and BEV drivers charge more often and report more charging opportunities in the same areas that smaller battery PHEVs could not find chargers. INTRODUCTION Is a plug-in hybrid more of an electric car in terms of driving and charging or is it more similar to an internal combustion engine (ICE) car? The answer to this question lies less with the technology of the vehicle, and more with the behavior of its user. Driving and charging behavior are the main factors in determining the ratio of gasoline to electric miles driven. In the case of BEVs, the travel and charging behavior will impact the vehicle choice on a specific day and in the case of PHEVs, also the driving mode of the car. Current policies are based on the assumptions that users will charge daily while OEMs argue that users will maximize the electric miles driven (eVMT) with their cars by using home and public infrastructure when available. Public charging infrastructure availability may play a major role in impacting whether or not people are even able to charge. An additionally layer shaping behavior may reflect other monetary and non-monetary costs and benefits of plugging in, even if charging is available. In this paper, we explore the charging and driving behavior of 3,500 PEV owners in California using a self-reported web-map survey conducted in May and June 2013 by the California center for sustainable energy (CCSE) and the PH&EV center at the University of California Davis. The survey allow us to compare the usage of the main PHEVs in the market using a sample size of more than 800 Plug-in Prius and 600 Chevy Volts and the most common BEV, the Nissan Leaf with more than 2,100 households. LITERATURE REVIEW: CHARGING BEHAVIOR The on-road performance and environmental impact of plug-in vehicles is the focus of many studies trying to model BEVs and PHEV usage. Only in the last two years the first reports on actual performance started to add to the knowledge base. A wide use type of PHEV and BEV modeling is based on travel data of ICE vehicles simulating plug-in vehicle. This type of modeling uses actual travel patterns but required a set of assumptions about charging behavior including frequency and location. Some studies model vehicle idling time as charging events [1], or modeled only home charging once a day [2] [3] [4]. A more refined modeling work used home dwelling time as charging event and specific locations as potential fast charging locations[5, 6]. Richardson, 2013, did a comprehensive review on the electric vehicles and the electric grid focusing on modeling approaches, Impacts, and renewable energy integration. His review does not include a section about charging behavior and the only assumptions used in modeling the impact on the electric grid are related to smart or simple charging at home[7]. Studies done earlier than 2010 forced to use on modeling or small demonstration project as presented by Davies and Kurani[8]. This work presents the variety of charging frequencies both at home and in public using a small sample of converted plug-in Priuses. Data reported by the EV Project also present a wide variety of charging and driving behavior but does not correlate the two together [9, 10]. Overall, current research illuminates a TRB 2014 Annual Meeting Paper revised from original submittal. 3 Tal, Nicholas, Davies and Woodjack gap of knowledge on charging behavior and its impact on eVMT, and understanding the factors that influence this behavior. SURVEY TOOL AND SAMPLE The overall target population of this survey is new PEV owners in California who applied for the California rebate for plug-in owners between February and August 2012 and have more than 6 month experience with the car. This population includes most of the PEV buyers in this time frame and includes mostly owners of the Nissan LEAF, Chevrolet Volt and the Plug-in Prius. The sample includes PEV owners that were eligible for the state’s Clean Vehicle Rebate Project[18]. This survey was conducted with the California Center for Sustainable Energy (CCSE), in coordination with the California Air Resources Board (ARB). The total number of started surveys was 3,881 with 3,201 usable surveys, reflecting response rates of about 30.6%. The high response rate could be attributed to the willingness of the PEV owners to share their experience and possible feeling of gratitude after receiving state incentives. Only 10% of the sample owned the car for less than 9 months, while 86% owned it less than a year. Leaf owners had the longest time with the vehicles with an average of more than 10 month the survey represents about 13.6% of the CVRP population and about 10% of the PEVs sold in California between January 2010 and June 2013 and have a good representation of the three main vehicle models in use the Nissan LEAF, Chevrolet Volt and Plug in Prius and all five major metropolitan areas. . Table 1: Days Vehicle Owned by Vehicle Model Vehicle Model Days own the car N Mean Std Dev Std Err Mean Lower 95% Upper 95% Nissan Leaf 2205 616 135 3 610 622 Toyota Plug-In Prius 851 362 52 2 359 366 Chevrolet Volt 661 329 54 2 325 333 Tesla Roadster 48 778 203 29 719 837 Ford Focus Electric 36 300 24 4 292 308 Mitsubishi i-MiEV 35 428 62 10 407 450 Other 45 323 45 7 310 336 To compare the survey household characteristics to the general population we used the Californian sample of the 2009 National household travel survey (NHTS), which includes 21,225 households that own 44,526 vehicles. The NHTS survey was conducted about 3 years before plug-in vehicles were available in California and includes income and location weights [19]. TRB 2014 Annual Meeting Paper revised from original submittal. 4 Tal, Nicholas, Davies and Woodjack FIGURE 1 Web map question The survey is based on self-reported travel patterns using web map questions (Figure 1). The web-map questions allow us to collect data on a large sample of habitual travel pattern such as commute trips and charging patterns without using costly travel diaries or GPS loggers. The survey also allows us to inquire about the charging availability, pricing, wiliness to pay and subjective need. The next sections will present PEV owner travel and charging patterns from the survey. To reduce survey burden and improve response and completion rates we allow users to skip on questions that were not relevant or difficult to answer. The actual response rate for each question may vary and reported in every table.