UC Davis Institute of Transportation Studies

This report summarizes findings of household interviews and focus groups conducted with plug-in electric vehicle (PEV) drivers on the effects of demand response management (DRM) strategies on the time of day of PEV charging. The research was conducted in the spring and fall of 2012 with PEV drivers in San Diego County, CA. The DRM strategies were of three basic types. First, electricity pricing included intentional and carefully designed time-of-use (TOU) price signals as part of a household PEV customer rate experiment by San Diego Gas & Electric (SDG&E), as well as uncontrolled implicit TOU signals resulting from differences between home and away-from-home prices of electricity. Home and away-from-home charging had independent price structures—including an initial period lasting several months during which much of the away-from-home charging was free. Second, technology, in this case timers onboard PEVs and in the electric vehicle service equipment (EVSE), i.e., the “chargers” were available to support PEV drivers adherence to a chosen time to start vehicle charging. Third was exhortation—often implicit or indirectly tied to PEVs—about the private and social benefits of shifting electricity demand to off-peak periods. It is not possible to entirely disentangle the effects of the individual DRM strategies. The aggregate effect is a widely told story by our respondents of their goal to maximize the amount of their PEV charging done during the “super off-peak” period from midnight to 5am. Given the qualitative research reported here, this stated behavior should be corroborated with quantitative measures of vehicle charging when they are reported at the end of SDG&E’s TOU rate experiment. Our PEV drivers reveal different comparative standards for whether any electricity price is perceived to be “high” or “low.” Their behavior, in aggregate, suggests the effect of TOU prices may be more like an off-on switch than the continuous change implied by the most common measure of such prices, i.e., own-price elasticity of electricity. This may be because TOU prices convey both a private price signal and a public exhortation to heed a social narrative about the susceptibility of the electrical grid to service interruption during periods of peak demand. PEV drivers with home photovoltaic systems are the most likely to say they charge their PEV at their convenience throughout the day—despite also being on a TOU rate—because of the mistaken belief that their PV system insulates the grid from their vehicle charging. A series of questions is posed about the generlizability and longevity of these findings, in particular as additional PEVs and PEV buyers enter the market.