The payoff for plug-in hybrids: 95 years?

Plug-in hybrids get far better mileage than standard cars or regular hybrids--and emit far less pollution.

But they are also tough to justify economically at the moment with existing technology, according to the first several months of data from RechargeIT.org, which is studying how well plug-in hybrids work in real-world circumstances.

Plug-ins, in fact, only cut gas consumption by about 88 gallons a year over regular Priuses in urban driving. That comes to an annual savings of $158 to $250 (when you factor in the cost of electricity too). With the conversion running around $15,000, the payoff would take decades.

RechargeIT, part of Google.org, has converted four Priuses and two Ford Escapes so they can be charged from a wall socket. These cars are then used by Google employees as fleet vehicles. RechargeIT monitors the performance of the cars and then compares the data with standard Priuses in the corporate fleet as well as government data on standard cars. Data has been collected since May. There's enough data to bolster arguments for both supporters and critics of plug-ins.

First, the good news. Plug-ins pollute far less than regular cars or hybrids, noted Alec Proudfoot, an engineering product manager at Google working on RechargeIT. The plug-ins emit on average 4,623 pounds of carbon dioxide for 12,000 miles of driving, which is close to 50 percent better than standard Priuses, which might emit 6,924 pounds of carbon dioxide, according to the organization.

Both plug-ins and standard hybrids also get far better mileage than standard cars. RechargeIT's plug-ins are getting 66.2 miles per gallon, about 50 percent better than the 44.6 miles per gallon the group gets with its regular Priuses. The U.S. fleet average is 19.8 miles per gallon, so the plug-ins are more than three times better than that. The plug-ins will use 425 fewer gallons of gas than a standard car in a year while a standard Prius will use 337 fewer.

The 66.2 miles per gallon is actually somewhat low because Google's cars typically go only short distances, Proudfoot said. In the first five minutes of driving a Prius, plug-in or otherwise, the gas engine comes on to power certain internal systems.

If the cars get driven for 40- to 50-mile trips, the mileage goes into the 70 miles to 100 miles per gallon range.

"We're sensitive to the fact that the mileage looks low and we believe it is lower than what consumers will experience," Proudfoot said, adding that he gets about 80 miles to 85 miles per gallon when he takes the car out for longer trips.

When do you break even?
Now the more difficult issue--cost. A plug-in costs a driver $1,168 to operate for 12,000 miles (with gas at $3 a gallon plus electricity at 8 cents a kilowatt hour) while a regular Prius costs $1,010 for the same distance, according to RechargeIT. Because a retrofit costs about $15,000, that means drivers don't break even for 95 years, assuming gas hovers at $3.

In a rosier scenario (gas at $5 a gallon and a $10,000 retrofit with no inflation in electricity prices), the payoff drops to just under 30 years. That's shorter, but about 20 years longer than most people own their cars, making plug-ins a tough sell.

"They (owners) are never going to pay off the price" of a retrofit, Proudfoot acknowledged. "But the big focus for RechargeIT is on the CO2 savings, not the cost savings."

Like in the electric car market, the hitch is the cost of the batteries. Southern California's AC Propulsion, for instance, offers a service for converting a gas-burning Scion into an all-electric car. The Scion costs $13,000 to $18,000. The conversion costs $55,000 plus tax. Oops.

Put the plug-in numbers another way. Let's say you had $20,000 and a Prius. If gas costs $4, you could drive it 223,000 miles (5,000 gallons x 44.6 miles per gallon). If you converted the Prius to a plug-in for $10,000, you'd go 165,500 miles. (2,500 gallons x 66.2 miles per gallon). That doesn't include the price of electricity, so you'd have to reduce the number of miles the plug-in would go for that amount.

The good news is that the costs of plug-ins are expected to decline, particularly when manufacturers make cars as plug-ins at the factory. If the premium drops to $5,000 or so, a plug-in fares better in terms of dollars. "You could pay it off in seven or eight years," Proudfoot said.

General Motors has said it wants to sell the Chevy Volt, a type of plug-in hybrid, for around $30,000. For a standard sedan, that's not outrageous. (A serial hybrid like the Volt will also get better mileage than a retrofit hybrid.) The plug-in nature of the Volt could be seen as an interesting, affordable luxury. Carbon taxes will also shorten any payoff time.

Batteries, meanwhile, continue to improve. Argonne National Labs is working on batteries that could let plug-ins hit the fabled and touted 100 miles per gallon standard, according to Ron Gremban at CalCars.

When you put all of these factors together, plug-in hybrids, along with cleaner diesel cars that can get 60 miles to 100 miles per gallon, remain the best ideas for cutting transportation fuel and emissions.

The individual plug-ins, by the way, range in mileage from 69.9 (Comoe) to 65.1 (Galapagos). One car, Macchu Picchu, is rated at 54.4 miles per gallon, but RechargeIT wants to study what's going on with that one in a little more depth.

On May 7 of last year, Great Barrier, one of the plug-ins, hit 124 miles per gallon on an 18.9-mile jaunt. (One day a car got only 32.7 miles per gallon because the battery switched off.)