OEMtek can turn your Prius hybrid into a plug-in hybrid. Concretely, this means adding a battery pack in the spare tire trunk of the car (without removing it) to extend the battery capacity* of the vehicle. The 9 kWh BREEZ battery can be recharged from the grid and is composed of standard-size cylindrical lithium-phosphate cells and can give the Prius an all-electric range of 30-35 miles (48-56 kilometers) per full charge. This increases fuel economy to 90 miles per gallon on the first 47 miles on the highway, and more than 150 mpg in low-speed urban driving for about the first 30-35 miles. OEMtek has been getting 125 mpg in combined driving during their own test (~7000 miles in 90 days).Electricity from the grid in most of the U.S. might be dirty, but even with that factor taken into consideration, it would be better to plug in than to burn gasoline according to Tesla's white paper (pdf). And as the grid gets cleaner, the cars that get energy from it get cleaner.
OEMTEK’s core pack technology includes multiple safety systems, both passive and active, that monitor the level of charge and current of the battery cells. [...]
Initially the OEMtek product will only be available in California, where more than 500,000 Toyota Prius vehicles have been sold. [...] OEMtek plans to offer plug-in conversions of Ford’s Escape hybrid SUV in late 2008.
The company already has 300 battery packs from Valence Technology, and they have announced that they would get 600 more modules. They are planning to do about 100 conversions per month by the end of 2008.
It's a small number, but you have to start somewhere, and it's a good educational tool to familiarize people with plug-in hybrids; people are always afraid of what they don't know, just like they were at first with hybrids.
Prices for the BREEZ pack are still high, though. Expect between 12,000 and 15,000 for a conversion.
*For those who are curious, the un-modified Prius has 1.2 kWh of battery capacity.