"I probably get five to seven emails a day and at least three or four phone calls a day from individuals wanting to know how soon they can get their hands on one of these conversion units," says Pat Cadam.
Cadam is one of the owners of Green Gears, a San Francisco company that has been converting Prius hybrids for Google and other companies.
The lithium battery units have been in limited supply, but will be available to individuals next month for $10,400.
"It costs you about 80 cents for a full charge, dead empty all the way to the top, only 80 cents to fill it up. So, that's not much compared to $5 gasoline," says Cadam.
A full charge will get you about 40 miles -- enough for most commutes.
So how long 'til you recoup your investment? It could be five years or 100,000 miles.
There is also a savings in maintenance costs.
"Now, a hybrid car you've got regenerative brakes, well, you just got rid of a few brake jobs over the life of the vehicle. That right there is $1500. You've got no timing belt. That's $1000. You add up all those maintenance procedures that you no longer need, and it pays for the hybrid system," says Nick Rothman from Green Gears.
There is also movement toward converting ordinary gas cars into plug-in's.
"With 800 million existing vehicles on the world's roads, it seems to make sense to convert some of those," says plug-in researcher Fraser Smith, Ph.D.
Fraser smith has been doing research on this. He calculates conversion will cost about $26,000. After cutting fuel costs by 90 percent, the payback would come in 10 years.
"You can essentially have whatever vehicle you like. So you can just start with the vehicle you like best. You don't have to buy the vehicle the automakers are producing for you," says Smith.
By some estimates, there are only about 100 plug-in cars on the road today, but by the end of next year, that figure is projected to jump to 5,000.