Intermittency strongly affects some sources of [[renewable energy]] sources differently, as they are dependent on diverse natural processes that are to some degree unpredictable. The timeframe underby considerationwhich to measure intermittency is significant and affects the degree to which the problem is considered relevant, however. For example, [[biomass]] renewables are dependent on solar energy and weather conditions, but the timeframe is substantially longer and variations other than drought have little effect on the output; in addition, biomass itself represents a form of energy storage, and the decision of when to utilize the stored energy is to some degree controllable.
Two forms of intermittent renewable energy, wind and solar electricity generation, present the most significant challenges due to the timeframe of changes in generation and the limited correlation with demand cycles. BothSolar canwill vary greatly throughout the course of a [[diurnal]] cycle, and both may also be subject to substantiala seasonalwide range of variation profiles based on season and ___location. In addition, day-to-day power generation may vary significantly (due to prevailing winds or cloud cover) with limited predictability estimated by weather services. Similarly, the ability of operators to control output for both is generally limited to [[curtailment]] or storage: power output can be decreased or stored, but generally not increased at will. Curtailment and storage of output is aare common featurefeatures in electrical grids and for wind and solar installations, butthough reducing power sold to the grid may substantially affect project economics. Proponents of high penetrations of variable sources argue that a newspot operatingpricing philosophyor demand response will be required whereby dispatchable sources are curtailedpricing or advanceddemand inis adjusted lineinversely with the variable output of the intermittent sources, and that this has an inherently low cost.
[[Hydropower]] can be both intermittent and/or dispatchable, depending on the configuration of physical plant. Typical [[hydroelectric]] plants in the dam configuration may have substantial storage capacity, and be considered dispatchable. [[Run of the river]] hydroelectric generation will typically have limited or no storage capacity, and be intermittent on a seasonal or annual basis (dependent on rainfall and other factors). Hydroelectric dams have limits to dispatchability, since storage is finite and there are often environmental and regulatory requirements that detail minimum and maximum release into the water system. [[Pumped-storage hydroelectricity|Hydrostorage]] is also used in many locations to manage supply and demand, but true hydrostorage is not an energy source: it is a mechanism used to store excess generating capacity for use during times of greater demand.
