Michael Kanellos of CNET wrote a piece in which he analyzed Google.org's ReChargeIT data. Google has a fleet of plug-in converted Priuses and Escapes which employees drive, and the data is summarized and posted here .

His conclusion is that there is an annual savings of 88 gallons of gas for the Prius plug-in versus the standard Prius. Allowing $3 per gallon of gas, and factoring in the cost of electricity, thats $250 per year savings. Kanellos argues that since the cost of the plug-in conversion currently is $15,000, it would take 95 years to recoup the outlay. He goes on in the story to include the Volt as an example of a plug-in hybrid.

He does point out that CO2 emission are reduced close to 50% in the plug-ins over the standard Priuses.

I wondered how the Volt would compare to a Prius.

First, the Volt is not a hybrid, it is an electric car with a gas range extender. If one drives 40 miles or less per day, no fuel will be used.

At a maximum for benefit, take a 40 mile per day driver (14,600 miles/year), and say a Prius gets 40 mpg. For the Volt, thats one gallon of gas per day or 365 gallons/year savings, which equals $1095/year. At $6 gas its $2190/year. The cost of electricity is about 80 cents for 40 miles (8 kWh) on average, some better some less. Thats $292. Thus, at a maximum, the Volt costs $803 per year less then the current Prius to operate.

If a Prius is $24,000 and a Volt is $35,000, it will take 13.7 years to pay off the difference.

Compared to a standard ICE-driven car, in the same setting, roughly double the numbers for Volt savings (for a $24,000 car getting 20 mpg), and the pay off time will decrease to 6.8 years.

The real question is, is cost-savings the only reason for wanting a Volt?