One of the constraints of solar power is that it is not always available: it is dependent on daylight hours and clear skies. In order to fill these gaps, a storage solution or a backup infrastructure of fossil fuel power plants is required -- a factor that is often ignored when scientists investigate the sustainability of PV systems.
Whether or not to include storage is no longer just an academic question. Driven by better battery technology and the disincentivization of grid-connected solar panels, off-grid solar is about to make a comeback. How sustainable is a solar PV system if energy storage is taken into account?