This paper examines the possible electric energy collected by the photovoltaic (PV) modules installed on the roof of the bus. The tests were carried out for Mercedes Conecto LF12 buses equipped with CI engine meeting the standards of Euro 5. The research was conducted in the urban contexts/operation. Detailed results of the measurements such as fuel consumption, electric energy consumption, engine speed and vehicle speed are shown. Additional measurements included: the quantity of solar energy coming from the roof of the moving vehicle and electric energy generated from the photovoltaic modules installed. The study was conducted on a city bus taking different routes in the city of Lublin in Eastern Poland. The research bus was supplied with photovoltaic modules and a measurement system. The test results enabled to determine the ratio of the amount of electric energy acquired through the photovoltaic modules mounted on a bus to equivalent amount of energy acquired by a non-shaded photovoltaic system arranged horizontally on the roof of the building. The engine idle speed percentage distribution within the urban operation was specified to the value of 45%.The Photovoltaic system mounted on a bus on average produces 17.6% less energy compared to an identical stationary system. The PV system could provide 26% of the annual demand for electric energy of a city bus. Thus it may save reduce fuel-associated operation costs by 3% per year, as a result of generating up to 26% electric energy from free solar power rather than depending on fuel combustion.