This paper discusses the methodologies to investigate the efficiency of on-board electricity generation in the city bus. The research object, i.e. the Mercedes Conecto was tested under steady conditions. A chassis dynamometer, i.e. the MAHA LPS 3000 LKW provided the same conditions during the entire research. The paper presents the samples of the measurements performed at a single operating point of a given crankshaft speed and torque. This operating point was specified in line with the guidelines for the WHSC test. In our study, an electrical load was changed with an extra receiver, or a light bulb, at a fixed amount of fuel. The electrical loads were 0.4 kW, 1.6 kW and 2.8 kW. The research results enabled us to specify the amount of fuel to generate 1 kWh of electrical energy.