As photovoltaic energy increasingly penetrates in power systems, transmission system operators have started to request its participation in providing ancillary services. One of the demanded services is the power ramp-rate control (PRRC), which attempts to limit the power ramps produced by intermittent irradiance conditions. In order to achieve the desired objective, solutions based on storage systems or modifying the maximum power point tracking (MPPT) in perturb and observe (P&O) algorithms are commonly adopted. The starting point in PRRC is the determination of the instantaneous power ramp-rate, and different methods have been proposed in the literature for its calculation. However, the accuracy and computational speed of existing procedures can be improved, which may be critical in situations with rapid irradiance fluctuations. In this paper, a decoupled photovoltaic power ramp-rate calculation method is presented, in which the effect of variable irradiance and the P&O algorithm are computed separately. The proposed method has been theoretically demonstrated and tested through simulation and experimental tests. Simulation results show that it can improve the previous methods in terms of accuracy and computation time. Experimental validation with hardware-in-the-loop demonstrates the suitability of the proposed method for real-time applications, even in presence of noisy measurements.