If the driver pushes the button, the car reverts to electric vehicle mode, meaning the Prius is powered completely by its two batteries. Each charge, which takes about four hours, uses the equivalent of 2.7 kilowatt hours of electricity. In electric mode, the Prius gets 99.9 miles a gallon, according to a gauge on a screen in the middle of the dashboard. But it cannot go very far: the plug-in hybrid’s two batteries hold enough power for only seven miles (11 km). This makes Prius also an green electric car.

By contrast, G.M. claims that the Volt will be able to hold a charge equal to 40 miles (60 km), after a six-hour charge. Still, the electric mode of the Toyota plug-in is enough to start the car and run it until the engine reaches the point where it needs to tap the gasoline engine. The plug-in Prius can stay in electric mode until 62 miles per hour (90km/h), versus around 30 miles per hour (50 km/h) for the conventional Prius.

Inside the car, there is a button with the letters “EV” inside an outline of a car. Despite its decision to step up its plug-in hybrid development, Toyota is not sure how much more consumers will want to pay for it, Mr. Lentz said. The Prius starts at $21,100. Some after-market companies are charging nearly that much to convert Prius models into plug-ins, he said.