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You can make a guess at the amount of kWh needed for this plug-in hybrid (PHEV) from info on the EDrive FAQ page at


They say that the conversion was designed for 50 miles of electrically boosted range; they also say that up to 6.4 kWh of the nominal 9kWh capacity of the battery pack may be used. Assuming that these two figures refer to the same set of driving conditions (which is probably conservative, since it takes no account of regenerative braking), in 50 miles of fully boosted driving with a nominal 100 MPG indicated, you'd use half a gallon of gasoline and 6.4 kWh of wall-plug electricity. With an ordinary Prius at 50 MPG, you'd use a whole gallon of gas and no utility electricity (since it can't be plugged in).

Converting these two quantities of energy into equivalent units is a bit tricky. One way is to use a figure from the DOE's Energy Information Administration that says there are 33.7 kWh in a gallon of gasoline; using this figure, the EDrive Prius PHEV uses the equivalent of less than a fifth of a gallon of gasoline to replace the "other" half-gallon of gasoline that would be used by the ordinary Prius. With this conversion, at


the Wrightspeed X1, an insane electric supercar that can skin any production car (short of a million-dollar, 1000 HP Bugatti) in a drag race, is said to have a fuel economy equivalent to 170 MPG in city driving at 0.2 kWh per mile.

However, this comparison is biased in favor of electricity. A gasoline drivetrain (including that in a non-plug-in hybrid) wastes an enormous amount of energy in the car itself, whereas an electric drivetrain doesn't; however, a significant amount of energy is lost "upstream" from the car in generating and transporting the electricity. To be fair to the gasoline car, you should include this as part of the energy use of the plug-in vehicle. At


AC Propulsion (who made the electric motor of the Wrightspeed X1) compares their slightly less insane supercar, the tzero (which can still take out a Ferrari or a Porsche in a drag race, but by a less embarrassing margin than the X1) with a Ferrari Maranello and a Honda Insight. Accounting for the "upstream" losses in electrical generation, they arrive at a conversion factor of 10.2 kWh per gallon of gasoline and thus quote a fuel economy equivalent of 56 MPH (same as the Insight hybrid!) for 0.18 kWh per mile power use in mixed driving. Using this conversion factor, the electricity used by the EDrive Prius is equivalent to a little over half a gallon of gasoline; accounting for the "upstream" energy waste in gasoline refining (about 10%, if I'm not mistaken), I think it's fair to say that the EDrive Prius uses the equivalent of about half a gallon of gasoline in "primary energy" (say, natural gas) to displace half a gallon of gasoline in the ordinary Prius.

So are PHEVs a wash? I would say certainly not, for two main reasons now and one more in the future. First, electrical generation at a powerplant is cleaner than gasoline use in a car, and it gets cleaner over time as utility generating plants are upgraded, while a gasoline car gets dirtier over time as calibrations drift, catalysts age, etc. The California Air Resources Board calculated that a battery EV would be 95% cleaner over its lifetime in terms of smog-forming emissions than even the cleanest 2002 cars, including PZEV hybrids like the Prius; a PHEV, which still uses some gasoline, would also be cleaner than a non-plug-in hybrid, though not twenty times as clean! Second, almost none of that "half gallon" of electricity comes from oil; shifting half our transportation fuel use from oil to resources for which we don't have as much import dependency would be a good thing in its own right, and over time more and more of it could be shifted to renewable energy sources without a hiccup. (Try putting ethanol or biomethane in a non-plug-in Prius and see how it reacts...)

And finally, looking toward the hoped-for future, please remember that the EDrive Prius is a retrofit, done by a "garage" startup. EDrive was limited by design choices that Toyota made for its non-plug-in Prius; if the major automakers can be persuaded (or coerced--I'm _tired_ of waiting, dammit!) to stop bad-mouthing the plug long enough to design factory PHEVs, they can avoid those compromises, put in a larger electric motor to take more of the load off the gasoline engine, put in more batteries for a longer all-electric range and higher all-electric speed, and so on, and so on. The fact that the first prototype of a retrofit kit, like the EDrive, equals the energy efficiency of the highly refined, second generation Prius (even ignoring the advantages of the non-petroleum nature of half that energy) surely bodes well for factory PHEVs in the future. UCS should throw its weight behind PHEVs wholeheartedly!


Forgot to mention--that "half gallon" of electricity will cost you about a quarter if you charge up overnight with a time-of-use rate; it's free if you have solar panels on your roof. Compare that with a buck and half for the half-gallon of gas--and going up...

Dave Kennington

I think you have greatly underestimated the "upstream energy waste" in the gasoline, refining is only part of it. much of the oil is transported thousands of miles before it even gets to the refinery, then the fuel is pumped through pipelines, trucked, ect. Also the production and transport of oil products uses huge amounts of electricity inself, so to be fair you must include that ineffiency. The much lower price of energy from the grid is a clue!

Bob Welch

Also include energy costs and pollution costs associated with methods to enhance production from margnal and depleted oil fields. The military costs of defending access to oil supplies in the mid east and other subsidies are also significant and not reflected in the market price of oil.


Yes, I guess I was trying not to "run up the score." :) I did a bit of checking, and I think I got that 10% number from figure 2 of the recent paper in Science that analyzed energy inputs and greenhouse gas production for ethanol:


10% is the amount of extra petroleum energy used to produce the unit of energy in gasoline; there's also about another 10% from natural gas, coal, and other sources. And you're right, the numbers in that paper appear to be "FOB" at the refinery, so they include energy needed to get the oil to the refinery but they don't include energy spent to transport the gasoline to the end user; by contrast, the AC Propulsion numbers explicitly include that cost for electricity. (Now that I think of it, since ethanol can't be moved via the current pipeline network, this is a disadvantage for ethanol relative to the figures in that paper since moving it by truck or train to the end user uses more energy than pushing gasoline through a pipeline.)

As long as I'm following up on my earlier post, I'd like to note that I didn't mean to accuse Wrightspeed of "cooking the books" in saying their X1 used energy at a rate equivalent to 170 MPG! They were talking about the (insane) efficiency of the vehicle itself, and so the appropriate comparison starts at the fuel pump (or wall plug), which is what they did. AC Propulsion, and the discussions on this page, focus on "system" advantages of electric transportation itself, and thus the "well to wheels" figure is the one to use here.


Thanks all for you comments.

A key question is the efficiency of the battery charge and discharge cycle in real-world use. The 6.4 kWh of energy used from the battery will require more than 6.4 kWh from the wall outlet. The question is, how much more?

What we would really like to see are some "pump to wheels" numbers for the PHEV Prius, tracking miles driven versus the amount of electricity drawn from the wall outlet and the gallons of gasoline added to the tank. Such data are critical to evaluating the practical environmental performance of a PHEV.


Based on the real-world experience of EVs, both the RAV4-EVs still on the road and the GM and Honda cars that have gone to the crusher, I think you'll find that the energy wasted in the recharging process (the difference between electricity into the charger and electricity into the battery) is going to be significantly less than the tens of percent that I neglected above in estimating the energy consumed in getting gasoline from the refinery to the end user, so it's not going to tip the balance in favor of gasoline.

While, as a scientist, I applaud efforts to obtain more data, I do not think that we are talking here about a "key question" or "critical data" in evaluating whether the energetics of PHEVs make sense. The numbers I pulled off the EDrive website, as I noted, are likely to give a conservative estimate, and I think that this estimate of the "practical environmental performance" of the EDrive PHEV is good enough to make the case for PHEVs pretty convincingly. This doesn't need to be "studied to death," any more than global warming does...

I might also note that the AC Propulsion calculation includes charging loss, and other losses, implicitly, while the gasoline energy value they used counts only the energy in the gasoline itself and thus _excludes_ all such losses. Thus EVs (and PHEVs) come out ahead using their conversion factor between electrical and gasoline energy, even though that conversion is biased in _favor_ of gasoline. Gasoline vehicles, including non-plug-in hybrids, can't keep up with plug-in vehicles environmentally, even if given a head start!


I have to say you have a great blog, i really enjoy reading it, i have bookmarked it so i can find it back

Keep up the good work


Nathan - 4 Easy Ways To Prevent Global Warming

I'm glad to hear the conversion for this car works well. I'd be very interested to know how much electricity it takes to charge the battery when you plug it in.

I live in an ecovillage and we get all our electricity from solar and wind, so we would significantly decrease our ecological impact with a plug-in hybrid, but I'm not sure if we could actually afford the solar panels and wind turbines needed to charge a plug-in.

Dennis vand der Broek

Great post i am doing a study hybrid cars and your post gives some insight that i haven't fund on the net (yet :) )



michael jones

Really interesting post! I love driving. I feel free when i am on the road! I an so glad that i passed my driving test.

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