The validity of solar resource assessment significantly impacts the economics and operational optimality throughout the entire lifecycle of photovoltaic (PV) power plants. Traditional resource assessment terminates at the AC output power estimates, which can be simulated with long-term weather data and plant design parameters. However, owing to the inherent variability of PV power, there is a hidden cost component in the economics of PV, that is, the additional cost incurred to firm up PV power such that its generation profile completely aligns with that of the net load. In this regard, this paper proposes the novel idea of firm PV resource assessment, which quantifies the hidden cost through a metric known as the firm kWh premium. Most certainly, the firm kWh premium depends upon the data used to simulate PV power. Therefore, the second mission of this work is to investigate whether ground-based radiometric observations are necessary for firm PV resource assessment or could it be replaced with satellite-based irradiance retrievals, of which the quality has been strongly enhanced by the advancements in remote sensing technology and inversion algorithms. The empirical part of this work considers ground-based observations from the sole Baseline Surface Radiation Network station in China, in conjunction with radiation data from the Fengyun-4B satellite. Case study reveals that, under cold climatic conditions, satellite-retrieved data significantly underestimates the solar resource and thus the PV power output. The inaccuracy in satellite-retrieved data also leads to an elevated firm kWh premium, substantially lowering the confidence in decisions pertaining to PV investments.