What is the relative cost of charging plug-in hybrid vs. non-plug-in hybrid electric vehicles?

The current crop of hybrid electric vehicles depends on the gasoline engine (and braking) to recharge the batteries. The coming generation of plug-in hybrid electric vehicles can also recharge the batteries by plugging into a normal electric receptacle. My question is what the cost differential is between paying for gas to do the charging on the go versus the hit to your electric bill to recharge at home overnight.

In the first case, you have the cost of gasoline and how much charging is accomplished per gallon of gasoline.

In the second case you have the cost per kilowatt-hour of your residential electricity as well as the charging efficiency. How much of each kilowatt-hour actually end up in the batteries of your vehicle?

I am not enough of an engineer to know the answer.

My hunch is that at least overnight, charging from "the grid" will be cheaper, as well as having no local carbon-based emmissions.

Any engineers out there?

-- Jack Krupansky