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Plug-in hybrids: 100 miles per gallon or 59?

Plug-ins are energy efficient, but there are different ways to calculate that.

Plug-in hybrids have emerged as the favorite form of transportation for reducing greenhouse gases in the near term, but calculating their energy efficiency can be a little complicated.

Plug-ins won't conk out after 130 to 200 miles, like electric cars, and they don't require major technological breakthroughs, like hydrogen cars. Converting a Prius to a plug-in hybrid costs about $10,000 to $15,000 now--that's part of the reason only about 50 exist--but if car manufacturers decide to make these at the factory, the additional cost may only run $6,000 or so, say proponents. That's a lower premium than what electric cars will run, by the way.

Plug-ins have more batteries than regular hybrids and can get charged from a wall socket.

A plug-in hybrid Tom Krazit/CNET

But there's some disagreement over how much energy these cars actually consume. Owners report their cars going 70 to over 100 miles on a gallon of gas. But that doesn't include the electricity used the charge the batteries. The cars do get 100 MPG, but what is the total energy consumption?

When you add in the electricity, plug-ins get the equivalent of 59 miles per gallon, according to John Shore, who is organizing the X Prize for automobiles. He gets this number by examining data from Google's experiments with plug-ins.

"The most reliable information available today about PHEVs is from the Google project RechargeIT, in which they report actual data from real driving conditions for a Prius modified to be a PHEV. Google currently reports that, on average, the vehicle gets 73.6 MPG (gasoline) and uses 118.1 watt-hours/mile of electrical energy--these equate to 59 MPGe, considerably less than the AXP goal of 100 MPGe," he wrote in an e-mail.

Shore, by the way, is a big fan of plug-ins. However, those numbers mean that current plug-ins won't be able to waltz away with the automotive X Prize, which will likely give more than $10 million to inventors who can come up with a car that can get 100 miles a gallon and win a cross-country race.

Not so fast, say Felix Kramer and Ron Gremban of CalCars, one of the chief organizations promoting plug-ins. Argonne National Labs has a plug-in that reportedly gets 150 miles a gallon. If you go through the equivalency calculations, you end up with 103 miles per gallon equivalent.

Regardless of the actual mileage, Gremban points out that the important thing is that plug-ins reduce greenhouse gas emissions and dependence on foreign oil, and there is not a lot of debate about that.

If you live in a state where coal is the main source of electricity, like Ohio, the emissions from a plug-in are about the same as a regular hybrid. But in California, the emissions are much lower because of hydroelectric power, wind and other renewables. (Coal accounts for approximately 52 percent of U.S. electricity, according to the Department of Energy.)