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* '''Intermittency''' can mean the extent to which a power source is unintentionally stopped or unavailable, but intermittency is frequently used as synonym of '''variability''',<ref name=GrahamSinden>Graham Sinden, "[http://www.ukerc.ac.uk/Downloads/PDF/05/050705TPASindenpres.pdf Assessing the Costs of Intermittent Power Generation] {{webarchive|url=https://web.archive.org/web/20090318231439/http://www.ukerc.ac.uk/Downloads/PDF/05/050705TPASindenpres.pdf |date=2009-03-18 }}", UK Energy Research Centre, 5 July 2005</ref><ref name=MEFlowBattery>{{cite web|url=http://www.memagazine.org/backissues/membersonly/oct05/features/rerere/rerere.html |title=renewable. rechargeable. remarkable. |accessdate=2008-10-20 |last=Kuntz |first=Mark T. |author2=Justin Dawe |year=2005 |work=VRB Power Systems |publisher=Mechanical Engineering |deadurl=yes |archiveurl=https://web.archive.org/web/20090115212122/http://www.memagazine.org/backissues/membersonly/oct05/features/rerere/rerere.html |archivedate=2009-01-15 |df= }}</ref> which is the extent to which a power source may exhibit changes in output.<ref name="GrahamSinden"/>
* '''[[Dispatchable generation|Dispatchability]]''' or '''maneuverability''' is the ability of a given power source to increase and decrease output quickly on demand. The concept is distinct from intermittency; dispatchability is one of several ways system operators match supply (generator's output) to system demand (technical loads).<ref name="MEFlowBattery"/>
* '''Penetration''' in this context is generally used to refer to the amount of energy generated as a percentage of annual consumption.<ref name="ieawind.org">[http://www.ieawind.org/AnnexXXV/Publications/Task25/Task%2025%20Design%20and%20Operation%20of%20Power%20Systems%20UWIG.pdf International Energy Agency Wind Task Force, "Design and Operation of Power Systems with Large Amounts of Wind Power"] {{Webarchive|url=https://web.archive.org/web/20071025113737/http://www.ieawind.org/AnnexXXV/Publications/Task25/Task%2025%20Design%20and%20Operation%20of%20Power%20Systems%20UWIG.pdf |date=2007-10-25 }} Oklahoma Conference Presentation, October 2006</ref>
* '''Nominal power''' or '''[[nameplate capacity]]''' refers to the maximum output of a generating plant in normal operating conditions. This is the most common number used and typically expressed in [[Watt]] (including multiples like kW, MW, GW).
* '''[[Capacity factor]]''', '''average capacity factor''', or '''load factor''' is the average expected output of a generator, usually over an annual period. Expressed as a percentage of the nameplate capacity or in decimal form (e.g. 30% or 0.30).
* '''[[Capacity credit]]''': generally the amount of output from a power source that may be statistically relied upon, practically the minimum power within a longer period, usually expressed as a percentage of the nominal power.<ref name='WindCC'>{{cite web |url=http://ejournal.windeng.net/3/01/GGiebel-CapCredLit_WindEngEJournal_2005_right_links.pdf |title=WIND POWER HAS A CAPACITY CREDIT |accessdate=2008-10-16 |last=Giebel |first=Gregor |work=Risø National Laboratory |archive-url=https://web.archive.org/web/20090318231423/http://ejournal.windeng.net/3/01/GGiebel-CapCredLit_WindEngEJournal_2005_right_links.pdf |archive-date=2009-03-18 |dead-url=yes |df= }}</ref>
* '''Firm capacity''' the amount of power that can be guaranteed to be provided as [[base power]]
* '''Non-firm capacity''' the amount of power above the firm capacity that is usually to be sold at higher price on the [[spot market]]
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[[File:ATTParkannualoutput.png|250px|thumb|right|Seasonal variation of the output of the solar panels at AT&T park in San Francisco]]
Intermittency inherently affects solar energy, as the production of renewable electricity from solar sources depends on the amount of sunlight at a given place and time. Solar output varies throughout the day and through the seasons, and is affected by dust, fog, cloud cover, frost or snow. Many of the seasonal factors are fairly predictable, and some solar thermal systems make use of heat storage to produce grid power for a full day.<ref>[http://www.nationalgeographic.com.es/2011/10/25/gemasolar_energia_non_stop.html Gemasolar, energía non stop] {{Webarchive|url=https://web.archive.org/web/20130206152532/http://www.nationalgeographic.com.es/2011/10/25/gemasolar_energia_non_stop.html |date=2013-02-06 }} Spanish 26 October 2011</ref>
* '''Intermittency:''' In the absence of an [[energy storage system]], solar does not produce power at night or in bad weather and varies between summer and winter. When intended to produce electricity only for peak [[air conditioning]] loads in the summer, there is no intermittency; in the winter can be complemented with wind power for peak loads.
* '''Capacity factor''' Photovoltaic solar in Massachusetts 12–15%.<ref name="RERLWind">{{cite web|url=http://www.ceere.org/rerl/about_wind/RERL_Fact_Sheet_2a_Capacity_Factor.pdf |title=Wind Power: Capacity Factor, Intermittency, and what happens when the wind doesn't blow? |accessdate=2008-10-16 |work=Renewable Energy Research Laboratory, University of Massachusetts Amherst |deadurl=yes |archiveurl=https://web.archive.org/web/20081001205145/http://www.ceere.org/rerl/about_wind/RERL_Fact_Sheet_2a_Capacity_Factor.pdf |archivedate=2008-10-01 |df= }}</ref> Photovoltaic solar in Arizona 19%.<ref name='THSolarVsWind'>{{cite web|url=http://www.treehugger.com/files/2008/03/solar-versus-wind-power.php |title=Solar Versus Wind Power: Which Has The Most Stable Power Output? |accessdate=2008-10-16 |last=Laumer |first=John |date = June 2008|work=Treehugger }}</ref> Thermal [[solar parabolic trough]] with storage 56%.<ref name='NRELSolar'>{{cite web|url=http://www.nrel.gov/docs/fy04osti/35060.pdf |title=Executive Summary: Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts |accessdate=2016-11-07 |date = October 2003|work=National Renewable Energy Laboratory }}</ref> Thermal [[solar power tower]] with storage 73%.<ref name="NRELSolar"/>
The impact of intermittency of solar-generated electricity will depend on the correlation of generation with demand. For example, solar thermal power plants such as [[Nevada Solar One]] are somewhat matched to summer peak loads in areas with significant cooling demands, such as the south-western United States. Thermal energy storage systems like the small Spanish [[Gemasolar Thermosolar Plant]] can improve the match between solar supply and local consumption. The improved capacity factor using thermal storage represents a decrease in maximum capacity, and extends the total time the system generates power.<ref>[http://www.iea.org/impagr/cip/pdf/issue36SolarP.pdf Spain Pioneers Grid-Connected Solar-Tower Thermal Power] p. 3. Retrieved December 19, 2008.</ref><ref>{{cite web|url=http://www.renewableenergyworld.com/rea/magazine/story?id=52693 |title=A solar-powered economy: How solar thermal can replace coal, gas and oil |accessdate=2008-10-17 |last=Mills |first=David |author2=Robert G. Morgan |date = July 2008|work=RenewableEnergyWorld.com }}</ref><ref>{{cite web |url=http://www.farm-energy.ca/IReF/index.php?page=solar-air-cooling-ataglance |title=Solar Air Cooling |accessdate=2008-10-17 |date
===Wind energy===
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Wind-generated power is a variable resource, and the amount of electricity produced at any given point in time by a given plant will depend on wind speeds, air density, and turbine characteristics (among other factors). If wind speed is too low (less than about 2.5 m/s) then the wind turbines will not be able to make electricity, and if it is too high (more than about 25 m/s) the turbines will have to be shut down to avoid damage. While the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas the average power output becomes less variable.<ref name="IEA2005WindVar"/><ref name=connect/><ref name=Archer2007/><ref name = Diesendorf>{{cite journal| author = Diesendorf, Mark |year= 2007| title =Greenhouse Solutions with Sustainable Energy|page =119| quote =Graham Sinden analysed over 30 years of hourly wind speed data from 66 sites spread out over the United Kingdom. He found that the correlation coefficient of wind power fell from 0.6 at 200 km to 0.25 at 600 km separation (a perfect correlation would have a coefficient equal to 1.0). There were no hours in the data set where wind speed was below the cut-in wind speed of a modern wind turbine throughout the United Kingdom, and low wind speed events affecting more than 90 per cent of the United Kingdom had an average recurrent rate of only one hour per year.|title-link= Greenhouse Solutions with Sustainable Energy}}</ref>
* '''Intermittence:''' Regions smaller than [[Synoptic scale meteorology|synoptic scale]] (the size of an average country) have mostly the same weather and thus around the same wind power, unless local conditions favor special winds. Some studies show that wind farms spread over a geographically diverse area will as a whole rarely stop producing power altogether.<ref name=connect/><ref name=Archer2007/> However this is rarely the case for smaller areas with uniform geography such as Ireland,<ref>{{cite web|url= http://www.inference.phy.cam.ac.uk/withouthotair/c26/page_187.shtml |title=Sustainable Energy - without the hot air. Fluctuations and storage|author=David JC MacKay}}</ref><ref>{{cite web |url=http://www.atom.edu.pl/index.php?option=com_content&task=view&id=92&Itemid=73 |title=Czy w Polsce wiatr wystarczy zamiast elektrowni atomowych? |trans-title=Can the wind suffice instead of nuclear power in Poland? |language=Polish |publisher=atom.edu.pl |author=Andrzej Strupczewski |access-date=2009-11-26 |archive-url=https://web.archive.org/web/20110904155414/http://www.atom.edu.pl/index.php?option=com_content&task=view&id=92&Itemid=73 |archive-date=2011-09-04 |dead-url=yes |df= }}</ref><ref name='BaseFallacy'>{{cite web |url=http://www.sustainabilitycentre.com.au/BaseloadFallacy.pdf |title=The Base-Load Fallacy |accessdate=2008-10-18 |last=Diesendorf |first=Mark |date
* '''Capacity Factor:''' Wind power typically has a capacity factor of 20-40%.<ref name="RERLWind"/><ref name='BWEAMyth'>{{cite web|url=http://www.bwea.com/pdf/ref_three.pdf |title=Blowing Away the Myths |accessdate=2008-10-16 |date=February 2005 |work=The British Wind Energy Association |deadurl=yes |archiveurl=https://web.archive.org/web/20070710024744/http://www.bwea.com/pdf/ref_three.pdf |archivedate=2007-07-10 |df= }}</ref>
* '''Dispatchability:''' Wind power is "highly non-dispatchable".<ref name='NordelNonDispatch'>{{cite web |url=http://cwec.ucdavis.edu/rpsintegration/library/Nordel%20non-dispatchable%20production%20May00.pdf |title=Non-dispatchable Production in the Nordel System |accessdate=2008-10-18 |date
* '''Capacity Credit:''' At low levels of penetration, the capacity credit of wind is about the same as the capacity factor. As the concentration of wind power on the grid rises, the capacity credit percentage drops.<ref name="BWEAMyth"/><ref name='TyndallCenter'>{{cite web|url= http://www.tyndall.ac.uk/research/theme2/final_reports/t2_24.pdf |title=Security assessment of future UK electricity scenarios |accessdate=2008-10-20 |last=Nedic |first=Dusko |author2=Anser Shakoor |author3=Goran Strbac |author4=Mary Black |author5=Jim Watson |author6=Catherine Mitchell |date = July 2005|work=Tyndall Centre for Climate Change Research |archiveurl = https://web.archive.org/web/20070111234301/http://www.tyndall.ac.uk/research/theme2/final_reports/t2_24.pdf |archivedate = January 11, 2007}}</ref>
* '''Variability:''' Site dependent.<ref>name="Junling">{{cite journal |author1=Junling Huang |author2=Xi Lu |author3=Michael B. McElroy |url= http://dash.harvard.edu/bitstream/handle/1/10981611/Meteorologically%20defined%20limits%20to%20reduction%20in%20the%20variability%20of%20outputs%20from%20a%20coupled%20wind%20farm%20system%20in%20the%20Central%20US_1.pdf?sequence=6 |title=Meteorologically defined limits to reduction in the variability of outputs from a coupled wind farm system in the Central US|journal=Renewable Energy|volume=62 |pages=331–340 |year=2014 |doi=10.1016/j.renene.2013.07.022}}</ref> Sea breezes are much more constant than land breezes.<ref name="IEA2005WindVar"/> Seasonal variability may reduce output by 50%.<ref>https://pdfs.semanticscholar.org/1709/4a682549e8e853be7b393e916f4cab91487a.pdf Graham Sinden (1 December 2005). "Characteristics of the UK wind resource" pg4</ref>
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====European super grid====
A series of detailed modelling studies by Dr. Gregor Czisch, which looked at the European wide adoption of renewable energy and interlinking power grids the [[European super grid]] using [[HVDC]] cables, indicates that the entire European power usage could come from renewables, with 70% total energy from wind at the same sort of costs or lower than at present.<ref name="Czisch100Renew"/> This proposed large European power grid has been called a "[[super grid]]."<ref name='DWWorldGregor'>
{{cite web |url=http://www.dw-world.de/dw/article/0,2144,2105061,00.html |title=Dr. Gregor Czisch's green European "Super Grid" |last=MacDonnel
</ref><ref name='IndependentGregor'>{{cite news|url=https://www.independent.co.uk/environment/climate-change/windfuelled-supergrid-offers-clean-power-to-europe-760431.html |title=Wind-fuelled 'supergrid' offers clean power to Europe
|last=Rodgers| first=Paul |accessdate=2008-10-16 |work=The Independent | ___location=London | date=2007-11-25}}</ref>
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</ref><ref>'''(German)'''Gregor Czisch: Szenarien zur zukünftigen Stromversorgung, kostenoptimierte Variationen zur Versorgung Europas und seiner Nachbarn mit Strom aus erneuerbaren Energien. https://kobra.bibliothek.uni-kassel.de/handle/urn:nbn:de:hebis:34-200604119596</ref>
Dr. Czisch described the concept in an interview, saying "For example, if we look at wind energy in Europe. We have a winter wind region where the maximum production is in winter and in the Sahara region in northern Africa the highest wind production is in the summer and if you combine both, you come quite close to the needs of the people living in the whole area - let's say from northern Russia down to the southern part of the Sahara."<ref name='InterviewCzisch'>
{{cite web |url=http://www.dw-world.de/dw/article/0,2144,3636864,00.html |title=Interview with Dr Gregor Czisch, a physicist and energy expert at the University of Kassel. |last=DeLisle |first=Heather |accessdate=2008-10-16 |work=DW-World.de }}{{Dead link|date=April 2019 |bot=InternetArchiveBot |fix-attempted=yes }} </ref>
====Grid study in Ireland====
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====Canada====
A study published in October 2006, by the Ontario Independent Electric System Operator (IESO) found that "there would be minimal system operation impacts for levels of wind capacity up to 5,000 MW," which corresponds to a peak penetration of 17%<ref>{{cite web| url=http://www.ieso.ca/imoweb/pubs/marketreports/OPA-Report-200610-1.pdf| title=Ontario Wind Integration Study| year=2006| publisher=Ontario Independent Electric System Operator| accessdate=2006-10-30| archive-url=https://web.archive.org/web/20070225212928/http://www.ieso.ca/imoweb/pubs/marketreports/OPA-Report-200610-1.pdf| archive-date=2007-02-25| dead-url=yes| df=}}</ref>
====Denmark====
A November 2006 analysis, found that "wind power may be able to cover more than 50% of the Danish electricity consumption in 2025" under conditions of high oil prices and higher costs for CO<sub>2</sub> allowances.<ref name='Danish_Wind_Env_Full'>{{cite web |url=http://www.ens.dk/graphics/Publikationer/Havvindmoeller/havvindmoellebog_nov_2006_skrm.pdf |title=Danish Offshore Wind - Key Environmental Issues |accessdate=2008-10-15 |date
|title=dynamic Flash map of Danish grids and flows on interconnectors
|url=http://www.energinet.dk/Integrationer/ElOest/ElsystemetLigeNu/energinet1.swf
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Studies have been performed to determine the costs of variability. [[RenewableUK]] states:
{{cquote|A review of integration studies, worldwide, suggests that the additional costs of integrating wind are around £2/MWh with 10% wind, rising to £3/MWh with 20% wind.<ref name='BWEAIntegrating'>{{cite web |url=http://www.bwea.com/pdf/RAEIntegrationfinal.pdf |title=Integrating renewables |accessdate=2008-10-20 |last=Ford |first=Richard |author2=David Milborrow |date
====Colorado – separate reports by Xcel and UCS====
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====UK studies====
A detailed study for [[UK National Grid]] (a private power company) states "We have estimated that for the case with 8,000 MW of wind needed to meet the 10% renewables target for 2010, balancing costs can be expected to increase by around £2 per MWh of wind production. This would represent an additional £40million per annum, just over 10% of existing annual balancing costs."<ref>[http://www.eurotrib.com/story/2007/1/28/183633/609 No technical limitation to wind power penetration]</ref><ref name='NG_7YR_Summary'>{{cite web |url=http://www.nationalgrid.com/uk/sys_06/print.asp?chap=all |title=GB Seven Year Statement - Executive Summary |accessdate=2008-10-16 |year=2006 |work=National Grid |archive-url=https://web.archive.org/web/20071011214628/http://www.nationalgrid.com/uk/sys_06/print.asp?chap=all |archive-date=2007-10-11 |dead-url=yes |df= }}</ref>
In evidence to the UK House of Lords Economic Affairs Select Committee, National Grid have quoted estimates of balancing costs for 40% wind and these lie in the range £500-1000M per annum. "These balancing costs represent an additional £6 to £12 per annum on average consumer electricity bill of around £390."<ref name='GridToLords'>{{cite web|url=http://www.parliament.uk/documents/upload/EA273%20National%20Grid%20Response%20on%20Economics%20of%20Renewable%20Energy.pdf |title=National Grid's response to the House of Lords Economic Affairs Select Committee investigating the economics of renewable energy |accessdate=2008-10-15 |work=National Grid |date=June 2008 |deadurl=yes |archiveurl=https://web.archive.org/web/20080910115928/http://www.parliament.uk/documents/upload/EA273%20National%20Grid%20Response%20on%20Economics%20of%20Renewable%20Energy.pdf |archivedate=September 10, 2008 }}</ref>
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