The rapid development of perovskite solar cells has been based on improvements in materials and device architectures, yet further progress towards their theoretical limit will require a detailed study of the main physical processes determining the photovoltaic performance. Luminescence can be a key parameter for this purpose, as it directly assesses radiative recombination. We present steady-state absolute photoluminescence of an operating device at varying voltages as a tool to study the loss mechanisms in perovskite devices. The calibration to absolute photon numbers gives access to the variation of the relative radiative/non-radiative recombination weighted along the measured potentials, and allows identifying charge extraction deficits not only at short-circuit conditions but at each point of the current-voltage curve, which we show on a vacuum-deposited solar cell. These measurements can be crucial to characterize the nature of the charge losses, a step to develop approaches to improve this photovoltaic technology.