In general, a Hybrid Electric Vehicle (HEV) combines one of the energy conversion technologies, one of the energy storage systems, a power unit, and a vehicle propulsion system. Many combinations are possible although batteries are by far the most popular choice for energy storage.

The combination of conventional and electric propulsion systems is believed to offer the possibility of reducing emissions and fuel consumption, and giving consumers the extended range and convenient refueling.

In parallel configurations, HEV is configured with two power paths, so the hybrid power unit engine and/or the electric propulsion system can be used to produce power to turn the wheels.

The electric-only mode can be used for short trips. For longer trips, the engine would provide primary power to the vehicle, with the electric motor provide addition power for hill climbs, fast acceleration, or other periods of high power demand.

In a series HEV, a small fuel-burning engine is built to generate electricity by driving an alternator directly. The electricity is then stored in the batteries or sent to the electric motor to power the wheels.

A series vehicle can operate in zero-emissions mode. In addition, the engine turns on to recharge the batteries when they are drained. Since it is less dependent the changing demands of the power of the vehicle, the engine can operate within a narrower but more efficient range of speeds.