A solar bus or solar-charged bus is a bus which is powered exclusively or mainly by solar energy. A solar-powered bus service is referred to as a solar bus service. The use of the term "solar bus" normally implies that solar energy is used not only for powering electric equipment on the bus, but also for the propulsion of the vehicle.
Existing solar buses are battery electric vehicles or (in the case of hybrid solar buses) hybrid vehicles equipped with batteries which are recharged from solar (or other) power sources; a launch of a solar bus service often goes hand in hand with investments for large-scale installations of stationary solar panels with photovoltaic cells. Similarly as other solar vehicles, many solar buses have photovoltaic cells contained in solar panels on the vehicle's roof which convert the sun's energy directly into electric energy to be used by the engine.
Solar-only and solar-electric bus services
The distinction between a solar-only electric bus and an electro-solar bus is fluid, as the distinction depends on the actual usage: whether the bus is recharged from solar or other power sources.
Solar-only bus services involve recharging the bus from solar energy, usually from solar panel-covered bus station canopies. The concept is similar to that of solar parking plot for cars and bicycles, where vehicles can re-charge while parked. The need for recharging poses constraints on the run and standstill times of the bus. The implementation of a solar bus service benefits from an optimization of over-all requirements for the specific bus service.
Commercially available shuttle buses
Open air low-speed electric shuttle sightseeing buses equipped with a solar panel-covered roof are produced in series and are commercially available. According to the producers, the solar panels save energy and prolong the battery life cycle.
The Tindo solar battery-charged bus ("Tindo", Kaurna word for sun) is an experimental battery electric vehicle that operates in Adelaide, Australia. It is the world's first solar bus, operating since 2007. It uses 100% solar power, is equipped with a regenerative braking system and air conditioning and can carry up to 40 persons, 25 of whom are seated. The bus itself is not equipped with solar panels. It receives electric power from a photovoltaic system on Adelaide's central bus station. Hailed as the world's first bus service powered exclusively by solar power, the bus service connects Adelaide City and North Adelaide as part of Adelaide City's sustainable transport agenda. The Tindo is part of the 98A and 98C bus service (until recently known as the Adelaide Connector) which is offered as free public transport. 
Within the Chinese government's program for clean transport sector, China's first solar hybrid buses were put in operation in July 2012 in the city of Qiqihar. Its engine is powered by lithium-ion batteries which are fed by solar panels installed on the bus roof. It is claimed that each bus consumes 0.6 to 0.7 kilowatt-hours of electricity per kilometer and can transport up to 100 persons, and that the use of solar panels prolongs the batteries' lifetime by 35 percent.
Austria's first solar-powered bus was put in operation in the village of Perchtoldsdorf. Its powertrain, operating strategy, and design specification were specifically optimized in view of its planned regular service routes. It has been in trial operation since autumn 2011.
The tribrid bus is a hybrid electric bus developed by the University of Glamorgan, Wales, for use as student transport between the University’s different campuses. It is powered by hydrogen fuel or solar cells, batteries and ultracapacitors.
Other forms of green bus transport
Non-solar powered engines
Solar panels are also used for powering electronic devices of the bus such as heating and air conditioning, even in buses with non-solar-powered engine. In the US, such buses are advertised to meet anti-idling regulations in several states.
Refitting existing non-existing vehicles with photovoltaic panels that feed the original battery with additional electric power has been shown to have the potential for making a substantial contribution to CO2 emission mitigation and to the reduction of pollution. The thus transformed buses are however not solar buses in the strict sense of the word, as they do not use solar energy for propulsion. The use of buses in public transport implies frequent stops with the opening and closing of doors, which influence the way the energy of the battery is used.
Renewable energy powered
In principle also trolleybuses or other non-autonomous electric buses or alternately powered buses such as fuel cell buses or dual-mode buses could be used for solar bus services, provided the origin of all or most of the energy used for propulsing the bus would be solar energy. In practice however such systems also draw on other sources of energy, at least also other renewable energy sources such as wind energy. An example is that the city of Hamburg, Germany, received the 2011 European Green Capital Award for, among others, its fuel cell bus service that is claimed to be the world's largest hydrogen-powered bus fleet and is intended to use hydrogen generated from solar and wind energy.
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