An experimental bifacial PV test installation has been setup to evaluate a novel approach for determining the bifacial gain, which applies an internal monofacial reference established by covering bifacial PV modules on the back. The direct comparison of bifacial and ‘monofacial’ operation allows monitoring the bifacial gain of a PV plant in operation and solves the issue of finding an appropriate reference. The bifacial gain observed in the test system was about 7 %, proving the usefulness of bifacial PV modules also on mounting racks not optimized for bifacial PV. Measurements of albedo and backside irradiance enable the detailed evaluation of the bifacial PV performance. However, measurement of the backside irradiation is complex and over- or underestimation has to be carefully avoided. The method of internal monofacial reference shall be developed further for supervision and fault detection in Bifacial PV plants.