Existing solar buses are battery electric buses or (in the case of hybrid solar buses) hybrid buses 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.