City buses are equipped with an increasing number of electrical devices that are designed to improve the driver work and increase passengers' comfort. The alternator must therefore meet the requirements of increased levels of electric energy demand. In this paper, the focus is set on the amount of chemical energy in the fuel transformed into electric energy by the alternator, especially in the case of urban transport.The article presents the results of operating states of the alternator in the city bus while driving. During the study waveforms of rotational speed and load current of power generation system were recorded. The results of measurements made it possible to draw histograms showing the share of working time in different points of the alternator work. Only about 43% of the total working time corresponds to the bus stop and the operation of the idle speed of the alternator 2250 rpm. The remaining time corresponds to the average speed about 4200 rpm and does not exceed 6600 rpm.To determine the average efficiency of the bus alternator, the bench tests were performed. Test bench consists of a DC engine and alternator that were connected via a belt transmission. The results of the study were the characteristics of the alternator operating under different loads. The maximum alternator efficiency amounts to 65%. Assuming an average bus engine efficiency of 20 %, the efficiency of generating electricity on board of a bus is 10.2%.