However, a significant barrier to the increased penetration of renewable energy arises from the “intermittent” nature of the electricity produced. While some sources are “dispatchable” (e.g. hydro, geothermal, and biomass), wind, solar and wave/tide power are subject to varying weather and ecological conditions. 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, and as the penetration of intermittent renewable sources increases the need for such capacity increases.