Medium and large-scale photovoltaic power plants (PVPPs) have the potential to provide frequency support to the power system. In particular, response to under-frequencies requires the existence of a power headroom or energy storage support. In this paper, dedicated reference inverters to estimate the maximum available power are used and a control structure that accounts for irradiance conditions across the PVPP is introduced. Such control allows the sustainment of a specified power reserve in the plant. Usually, non-uniform irradiance conditions in the area of the power plant are not considered in spite of their relevance, reason why the simulations are conducted with real irradiance data, having a measurement available for each control area of the PVPP.