The solar energy source is characterized by variability, intermittency, and unpredictability. The annual average direct normal irradiance and global horizontal irradiance only gives a rough estimation of the amount of energy that can be harvested by a solar photovoltaic plant. It is however also relevant to quantify the variability of the energy production, which is not only the result of alternating daylight and night time but also to accept variability from one day to another due to weather and seasonality. A design that permits a more stable controllable output is preferable because it reduces the need for external energy storage by batteries. Here we quantify the coefficient of variability of existing photovoltaic plants, and compare it with the coefficient of variability of the resource, to validate solar PV plant models.