HyMotion has a PHEV battery for your Prius

While If you are worried about getting that extra mileage on battery alone, this may be a good idea for you. Turn your hybrid into a plug-in with one big battery that hooks into a socket. The only problem I see is the price. At $5000, what they hope the price is going to be when it goes on sale to the public next year, that's a steep addition to any budget. Add in the extra cost of buying a hybrid in the first place and most consumers will probably balk at the upgrade.

HyMotion's PHEV battery lets you plug in your hybrid - Engadget

But HyMotion is here to help with their new PHEV Plugin Hybrid Electric Vehicle), which adds an extra battery to you hybrid that can be charged from a regular wall outlet.