Yesterday, I wrote about the cost of charging your PHEVs compared to gasing it up in a study commissioned by the state of California. I contacted one of the writers of that study and asked him about some concerns our readers raised in comments. He was kind enough to take my questions by e-mail and even send me a copy of the study (under review at Environmental Research Letters) which I will give you a little peak.
Daniel Kammen is Professor in the Energy and Resources Group Energy and Resources Group, Professor of Public Policy in the Goldman School of Public Policy and is Professor of Nuclear Engineering in the Department of Nuclear Engineering (Commenter Mike Z. must be happy to read that) at the University of California, Berkeley.

Lets get to it. Our readers wondered what assumptions were made about the car. Here are more details directly from the study.

"We adopt performance parameters for PHEVs with an all-electric range of 20 miles from EPRI (2002). The all-electric efficiency includes losses from charging (EPRI, 2001). A charging rate of 1 kWh/hr can be obtained by using ordinary 120 Volt technology with a charger efficiency of 82% and a charger size of 1.2 kW, and higher charging rates may be obtained by investing in infrastructure such as 240 Volt chargers. Each compact car PHEV will use 4.1 kWh of stored energy if it drives its entire all-electric range and will require 4.1 hours to recharge, and each full-size SUV PHEV will use 7.1 kWh of stored energy if it drives its entire all-electric range and will require 7.1 hours to recharge.2 The design of PHEVs is an area of great uncertainty; if PHEVs have all-electric ranges that are less than 20 miles so as to reduce initial costs, then each PHEV would require less electricity to fully charge but may charge more often."

Go below the fold for another quote from the study and the responses to your questions. Commenter Kballs will be happy to read the study does consider things like timers and is not just going for the worst case.

"The analysis above suggests that more than 5 million PHEVs might economically charge in some hours, so we examine the effects of 1, 5, and 10 million PHEVs under three plausible charging pattern scenarios. Note that we do not evaluate the worst case situation in which PHEVs would be charging at the peak electric load. Because PHEVs represent new demand in the electric power system, this case would obviously result in higher peak loads and would begin to immediately create the need for more generation and transmission capacity. The cases described above seem more likely than peak charging because they match typical commute patterns. However, PHEVs are not yet available so we do not know how consumers will behave if they obtain PHEVs. Avoiding peak charging may require technologies that discourage or prevent such behavior, but these could be as simple as timers on chargers."

Now, the interview. I tried to ask things raised in your comments.

Q: Do you know Alexander Farrell, the guy behind the California study?
A: I hired Alex into our department. Alex and I wrote the biofuel study together, and more importantly, the two low-carbon fuel studies that put this work into practice in California.

Q: Are you the guys working on the Toyota Prius plug-in project?
A: The Prius project is directed by Dr. Tim Lipman (my former post-doc) and Dr. Susan Shaheen. It is described online at:

Q: Who funded this study and does your school have a policy on funding?
A: As a state university, UC Berkeley accepts funding from both the public sector and the private sector, but accepts no restrictions on publication or other use of the data from those studies. Our work on plug in hybrid vehicles was funded by the non-profit energy foundation of San Francisco and by the U. S. National Science Foundation, and by the Karsten Family Foundation that has endowed my laboratory.

Q: To the study, our readers seemed concerned with the assumptions you made. Can you talk about that a little?
A: The conclusions of the study are that in states with clean electricity grids, such as California, plug-ins can decrease peak load (increasing human health by putting less fossil fuel power plants online during hot, smoggy days), reduce electricity costs, and develop a more competitive electricity system. Second, developing hybird vehicles will require research on better batteries as at present they are not cost-effective (gas would have to be ~ $5.50 gallon for current batteries to make this a good deal).

Q: Have you taken a look on how that might look in states beyond California?
A: It depends on how clean the electricity grid is - which varies utility by utility and state by state. As more states adopt clean energy standards (renewable energy portfolio standards), PHEVS become better and better for the environment.

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