New Flyer Xcelsior in the context of "New Flyer"

An electric bus is a bus that is propelled using electric motors, as opposed to a conventional internal combustion engine. Electric buses can store the needed electrical energy on board, or be fed mains electricity continuously from an external source such as overhead lines. The majority of buses using on-board energy storage are battery electric buses (which is what this article mostly deals with), where the electric motor obtains energy from an onboard battery pack, although examples of other storage modes do exist, such as the gyrobus that uses flywheel energy storage. When electricity is not stored on board, it is supplied by contact with outside power supplies, for example, via a current collector (like the overhead conduction poles in trolleybuses), or with a ground-level power supply, or through inductive charging.

As of 2017, 99% of all battery electric buses in the world have been deployed in Mainland China, with more than 421,000 buses on the road, which is 17% of China's total bus fleet. For comparison, the United States had 300, and Europe had 2,250. By 2021, China's share of electric buses remained at 98% while Europe had reached 8,500 electric buses, with the largest fleet in Europe being Moscow.

A hybrid electric vehicle (HEV) is a type of hybrid vehicle that couples a conventional internal combustion engine (ICE) with one or more electric engines into a combined propulsion system. The presence of the electric powertrain, which has inherently better energy conversion efficiency, is intended to achieve either better fuel economy or better acceleration performance than a conventional vehicle. There is a variety of HEV types and the degree to which each functions as an electric vehicle (EV) also varies. The most common form of HEV is hybrid electric passenger cars, although hybrid electric trucks (pickups, tow trucks and tractors), buses, motorboats, and aircraft also exist.

Modern HEVs use energy recovery technologies such as motor–generator units and regenerative braking to recycle the vehicle's kinetic energy to electric energy via an alternator, which is stored in a battery pack or a supercapacitor. Some varieties of HEV use an internal combustion engine to directly drive an electrical generator, which either recharges the vehicle's batteries or directly powers the electric traction motors; this combination is known as a range extender. Many HEVs reduce idle emissions by temporarily shutting down the combustion engine at idle (such as when waiting at the traffic light) and restarting it when needed; this is known as a start-stop system. A hybrid-electric system produces less tailpipe emissions than a comparably sized petrol engine vehicle since the hybrid's petrol engine usually has smaller displacement and thus lower fuel consumption than that of a conventional petrol-powered vehicle. If the engine is not used to drive the car directly, it can be geared to run at maximum efficiency, further improving fuel economy.