With the increasing deployment of renewable energy sources and the gradual phase-out of conventional power plants, transmission system operators increasingly source ancillary services from distributed and renewable generators. Market-based procurement mechanisms for reactive power create new revenue opportunities for operators of photovoltaic (PV) power plants and wind power plants. However, reactive power provision induces additional losses along the power conversion chain, directly affecting the economic viability of market participation. This study presents field measurements from a 650 MWp PV power plant directly connected to the 380 kV transmission grid. During the analysed period, 605 MWp of DC capacity and 415 MVA of inverter capacity were operational. Power was measured at four levels within the plant, enabling a detailed analysis of losses from the DC side of the inverters, through the medium- and high-voltage transformers, up to the point of common coupling. The measurements quantify the loss chain and the incremental losses associated with reactive power provision over a wide range of operating points, including reactive power provision at night. At a feed-in of approximately 400 MW, the total losses between the inverter DC terminals and the point of common coupling were approximately 13 MW. Within the observed underexcited operating range, the estimated incremental losses were approximately 0.7 MW per 100 Mvar during normal feed-in and 1.4 MW per 100 Mvar during night-time reactive-power operation. The results are compared with a simulation model of the plant. The results provide a field-data benchmark for validating loss-estimation models for utility-scale PV power plants.