The Chevrolet Volt, a plug-in hybrid that can be charged using residential electrical outlets, has a battery range of 64 kilometers. Like the Fisker Karma, it is a series hybrid, meaning that an electric motor is required to turn the car’s wheels; when the internal-combustion engine is in use, it drives a generator connected to the motor. In the Volt’s case, the engine can also provide supplemental mechanical torque. Other hybrids have parallel designs, in which either the motor or the engine can propel the vehicle alone.