However, a significant additional barrier to the increased penetration of renewable energy arises from the “intermittent” nature of the electricity produced. The extent to which the grid as a whole can accommodate such variations is a function of its capacity to adjust to supply and demand shocks (power system flexibility), and as the penetration of intermittent renewable sources increases the need for such capacity rises as well. Moreover, since those renewables which are the most promising sources of future increases in capacity (e.g. offshore wind, tidal/stream, wave energy and solar photovoltaics) are intermittent in nature, the need to be able to adjust output levels and sources on short notice is likely to rise (see Infield and Watson, 2007, Sinden, 2007).