Although building-integrated photovoltaic (BIPV) systems have great potential, investment in this field is not at the desired level. There are two main reasons for this: the lack of technical analysis and economic reasons. Manufacturers and investors utilize datasheets of modules to determine the systems performance, which is determined at standard test conditions (STCs). However, there are apparent differences between STCs and outdoor measurements. Most studies in the field of BIPV system analysis have only focused on long-term outdoor measurements. Besides that, uncertainty of measurements is necessary to achieve scientific results. The aim of this study is to emphasize the importance of measurement uncertainty, to describe how measurement uncertainties are calculated, and to find out the uncertainty of an outdoor BIPV measuring system. In this study, different roof-integrated photovoltaic systems with 15$^{\circ}$, 30$^{\circ}$, and 45$^{\circ}$ inclination angles were tested in the Fraunhofer Institute for Wind Energy and Energy System Technology measurement field. Maximum power point current, voltage, power, and temperature of each system were measured. The uncertainty of current, voltage, and temperature was calculated as 0.29%, 0.05%, and 1.15%, respectively.