Wind Power: Capacity Factor, Intermittency, and what happens when the wind doesn’t blow?

Wind turbines convert the kinetic energy in moving air into rotational energy, which in turn is converted to electricity. Since wind speeds vary from month to month and second to second, the amount of electricity wind can make varies constantly. Sometimes a wind turbine will make no power at all. This variability does affect the value of the wind power……

Editor’s Note: This ‘fact sheet’ is, on the whole, a comparatively fair report. The definitions provided for capacity factor, efficiency, reliability, dispatchability, and availability are useful. Its discussion of back-up generation, marginal emissions and Germany & Denmark, however, is disingenuous as is, to a lesser degree, its discussion of capacity factor and availability. IWA's comments (updated October '06) on these issues follow selected extracts from the 'fact sheet' below.

From the ‘Fact Sheet’

Intermittency and the Value of Wind Power

The wind does not always blow; sometimes a wind power plant stands idle. Furthermore, wind power is really not “dispatchable” – you cannot necessarily start it up when you most need it. As wind power is first added to a region’s grid, it does not replace an equivalent amount of existing generating capacity – i.e. the thermal generators that already existed will not immediately be dismantled…….

While wind power does not replace an equal amount of fossil-fuel capacity, it does replace production – for every MWh that is produced by a wind turbine, one MWh is not produced by another generator. The damage done by our existing electricity generation is primarily a function of production, not capacity. Burning less coal has a positive environmental impact, even if the coal plant is not shut down permanently…..

The impact of intermittence on the grid

Intermittency does have an impact on the grid, though it is not the impact that wind power critics usually assume. When the concentration of wind power in a region is low, the impact is negligible. Keep in mind that loads fluctuate constantly, so a small amount of fluctuating generation can be said to act as a “negative load” and have almost no measurable impact on the grid. Many modern wind turbines can supply some grid support as well (referred to as “ancillary services,” e.g. voltage support), just as most power plants do. As the concentration of wind power increases in a region, though, intermittence and the difficulty of forecasting wind power production do have a real cost associated with them…….

Denmark and northern Germany are the examples of large-scale grids with the highest penetration of wind power. Though more densely populated than New England and not particularly more windy, they produce about 20% of their energy from the wind.

Higher penetration levels on big grids are likely in the not so distant future, but for much higher pentration levels, special controls, grid support equipment, and perhaps storage may be needed.

The need for back-up generation

Wind power plants have been installed in the United States for long enough that detailed studies have been completed on the impacts and costs of its intermittency. A recent study concluded that
“...the results to date also lay to rest one of the major concerns often expressed about wind power: that a wind plant would need to be backed up with an equal amount of dispatchable generation. It is now clear that, even at moderate wind penetrations, the need for additional generation to compensate for wind variations is substantially less than one-for-one and is often closer to zero.”

- from Utility Wind Interest Group’s “Wind Power Impacts on Electric-Power-System Operating Costs, Summary and Perspective on Work Done to Date, November 2003”


Editor’s Comments:

Back-Up Generation

The need to dedicate reliable generating capacity to back-up wind energy production is a controversial and complex issue. The following comments attempt to provide a perspective (w/o being too technical) on this issue by addressing how electricity grids operate and the effective capacity of wind energy. It concludes by addressing the related impact on emissions. Technical experts are encouraged to debate the detailed specifics.

Brief Primer on Grid Operations:

The electric power grid’s primary purpose is to efficiently provide reliable electricity on demand to its customers 24/7/365. This means the grid must match reliably aggregate production and consumption instantaneously and continuously.

Following policies developed and enforced by the North American Electric Reliability Council (NERC), each grid assembles a portfolio of generating sources to ensure ‘adequacy’ (the ability of the electric system to supply the aggregate electrical demand and energy requirements of the end-use customers at all times, taking into account scheduled and reasonably expected unscheduled outages) and to accommodate ‘contingencies’ (the unexpected failure or outage of a system component such as a generator, transmission line, circuit breaker, switch or other electrical element). This means, in addition to having ‘contingency reserves’, the grid must routinely have generating sources not only to meet base load demand (the minimum amount of electric power delivered or required over a given period at a constant rate) but also sources to meet peak demand (the highest hourly load within a given period-day, month, season or year) as well as the operating flexibility to respond to everything in between.

As cost, after reliability, is a critical component in source selection, large reliable power plants with low fuel costs are generally dedicated to providing base load power, e.g. coal, nuclear power and (at least seasonally) conventional hydro power (the variable/fuel cost of hydro and nuclear power is essentially zero). On a daily/seasonal basis to meet routine increases in demand and, of course, peak demand, additional capacity is brought on-line. Routine, fairly predictable increases in demand can be accommodated by coal, biomass, and hydro (if available). Sometimes natural gas and oil are used but, in recent years, they have become unattractive cost alternatives. To meet short duration peak demand, the key requirements are fast startup and low investment cost. As such, natural gas generators have become increasingly popular.

The grid’s need to match reliably aggregate production and consumption instantaneously and continuously can, conceptually, be met by controlling generation, demand/load, or both. Historically, grid operators have tended to control generation almost exclusively. In doing so, they maintain controllable reserves to achieve the required generation/load balance, e.g. regulating reserves for continuous random minute-to-minute fluctuations in load and uncontrolled generation, frequency-responsive reserves for frequency deviations , etc. Sudden failures of generation and transmission are addressed, as a general rule, by three additional reserves: 10-minute spinning reserve, 10-minute non-synchronized reserve, and 30-minute operating reserve.

Consistent with the grid’s primary purpose, the common attribute of the aforementioned sources is reliability, i.e. they are controllable and predictable.

Wind Energy’s Effective Capacity

The grid’s need to meet demand reliably means that of the three ‘capacities’ associated with wind power- nameplate/rated capacity, capacity factor and effective capacity- the latter is the most critical. Effective capacity (a.k.a. capacity credit) is a measure of a generating source’s contribution to system reliability and is tied to meeting peak demand/load.

Because wind is intermittent, variable, uncontrollable and largely unpredictable (except for the very short term), a recent study determined that it is difficult to generalize the capacity credit for wind as ‘it is a highly site-specific quantity determined by the correlation between wind resource and load”….with values ranging “from 26 % to 0% of rated capacity” (

It is noteworthy that this conclusion is based, in part, on a 2003 study by the California Energy Commission that estimated that three wind farm aggregates- Altamont, San Gorgonio and Tehachpi, which collectively represented ( 75% of California’s deployed wind capacity- had relative capacity credits of 26.0%, 23.9% and 22.0% respectively. While we do not know how these three facilities performed during California’s summer ’06 energy crunch, these effective capacities for California’s wind power appear high. As has been widely publicized in the press, California wind power produced at 254.6 MW (10.2% of wind’s rated capacity of 2,500MW) at the time of peak demand (on July 24th, 2006) and over the preceding seven days (July 17-23) produced at 89.4 to 113.0 MW, averaging only 99.1 MW at the time of peak demand or just 4% of rated capacity.

The following excerpt from the Electricity Reliability Council of Texas’ (ERCOT) 2005 study suggests a more conservative assessment of wind’s effective capacity (emphasis added) (

In addition to meeting the state’s energy needs (MWh), the electric system must also meet expected peak demand (MW). Generation resources other than wind will be needed to meet most of the projected growth in peak demand, as maximum output from wind resources does not correspond to system peak demand. ERCOT currently assigns 10% of the installed capacity of wind turbines to its calculation of the ERCOT peak capacity reserve margin. Based on a review of historical data of actual wind turbine generation during ERCOT system peaks (from 4 p.m. to 6 p.m. in July and August), the average output for wind turbines was 16.8% of capacity. However, the data also showed that for any hour during these months, the output of the wind turbines could range from 0% of installed capacity to 49% of installed capacity. Stakeholders comprising the ERCOT Generation Adequacy Task Group have expressed concern that use of an average number (i.e., 16.8%) was too optimistic because it fails to adequately recognize the intermittency of wind generation. Accordingly, the group is working to assign a peak capacity value for wind using an appropriate “confidence factor.” While the group has not yet formally made a recommendation to the ERCOT Technical Advisory Committee, it is currently considering recommending a wind capacity value of 2%. In summary, in order to reliably meet system peak demand, dispatchable resources (such as gas, coal, biomass) would be required to replace the wind resources when wind is not blowing."

The German grid operator, Eon Netz,- one of the world’s largest managers of wind energy, addresses wind’s effective capacity in its 2005 Annual Report as follows: (emphasis added) (

Wind energy is only able to replace traditional power stations to a limited extent. Their dependence on the prevailing wind conditions means that wind power has a limited load factor even when technically available. It is not possible to guarantee its use for the continual cover of electricity consumption. Consequently, traditional power stations with capacities equal to 90% of the installed wind power capacity must be permanently online in order to guarantee power supply at all times.

As site-specific data relevant to peak demand is not available for all U.S. industrial wind plants, a national average ‘effective capacity’ is not available. That said, it is well known that summer winds are generally ‘light’ particularly during extremely warm ‘air conditioning’ days. Given the 2005 national average capacity factor (the ratio of the actual energy produced in a given period to the hypothetical maximum possible, i.e. running full time at rated power) for U.S. wind projects was 29%, it is not unreasonable to assume a national average effective capacity to meet peak demand in the low single digits.

Clearly, wind energy, as an intermittent, uncontrollable, largely unpredictable and variable energy source, provides limited, if not negligible, benefits to a grid operator’s need for reliable, controllable and predictable electricity generating capacity to meet demand on a 24/7/365 basis regardless of meteorological conditions.

The Need for Dedicated Back-up and Implications for Emissions

Clearly, as seen above, The Fact Sheet’s acknowledgement that “wind power does not replace an equal amount of fossil-fuel capacity” treats a critical issue cursorily. Its claim that wind energy “does replace production - for every MWh that is produced by a wind turbine, one MWh is not produced by another generator”- is simplistic. As discussed below, the displacement is not 1:1 and the level of emissions saved (vs. its implied 1:1) will depend on which generators are actually impacted by wind’s production. In some instances the emissions saved are negligible.

Theoretically- given the grid’s matching of production and consumption instantaneously and continuously, the 1:1 statement (putting the emissions issue aside) is correct. However, in reality- precisely because wind energy provides limited effective capacity, it does not replace other production on a 1:1 basis. To ensure grid stability and reliability, responsive backup generating sources must be available (though not necessarily on a 1:1 basis) to accommodate wind’s intermittency, volatility, uncontrollability and limited predictability. .

How much production from these responsive backup sources of electricity is actually displaced by wind energy and the impact of this displacement on emissions is a function of the level of wind energy’s penetration of the grid, the actual generating units impacted by wind production, the availability of wind energy relative to the demand for electricity and the energy consumed by the wind plant itself.

At very modest levels of wind penetration, it is generally agreed that the normal generating reserves maintained by the grid for balancing supply and demand would probably be adequate to accommodate wind’s intermittency and volatility. As penetration increases, however, as noted above by Eon Netz, there is an approximately commensurate need for dedicated/reliable back-up generating units. The net ‘emissions saved’ will be determined by the types of generators comprising these back-up units. For example, if the back-up unit(s) is hydro, there are no emissions savings- the grid is using wind or hydro to meet demand. If the unit(s) is a ‘slow responsive’ coal fired generator, the emissions calculation must include the emissions generated by the back-up unit while operating in a reserve capacity. In some cases, the emissions generated by dedicated thermal units can completely offset the ‘emissions saved’ accorded to wind production.( Also, as noted above, responsive natural gas units- that generate modest levels of emissions- are often used for balancing needs. Even in these instances, however, a net emissions saved calculation is required.

The availability of wind energy relative to demand is an often overlooked issue in the emissions issue. Wind generally blows harder at night when electricity demand is ‘off peak’ and lighter during the day when demand is ‘on-peak’. The large generating units that supply base load energy (e.g. coal and nuclear power) are designed to be ‘on’, i.e. they do not lend themselves –operationally, structurally or financially- to being ramped up/down. Nuclear power is, of course, emissions free. Should coal fired units be providing base load energy, evening wind production may have, in fact, a negligible impact on emissions. Furthermore, as wind’s capacity value in meeting summer peak demand is limited, reliable generating capacity will need to be built to meet our ever increasing demand for electricity regardless of the amount of wind energy developed. Thus, our choice among the sources of reliable generators will determine future emissions levels, not wind energy.

Ironically, it is not well understood that wind turbines cause an increase in aggregate and peak demand as they actually use power from the grid like an ordinary customer. This occurs when their waveform is not exactly synchronized with the grid’s waveform, i.e. they are out-of-phase. Proper design can reduce out-of-phase behavior but it cannot eliminate it because the turbines are deriving power from an unstable source, the wind. For example, if a wind turbine is generating a waveform that is ahead of the grid waveform, perhaps due to a wind gust or a poor sensing system, the grid will have to waste power to ‘pull’ the wind turbine’s waveform back into phase by slowing it down. Similarly, if the wind changes direction or there is a lull in the wind, the wind turbine’s waveform may lag the grid waveform and be a net burden as the grid dissipates power to speed up the wind turbine’s rotation. Wind turbines will be out-of-phase a significant period of time due to changes in wind speed and direction. It is the ‘net’ in-phase’ power generated that matters. Importantly, this means that extensive installation of wind turbines will actually require greater power generation capacity from conventional sources than if the turbines had not been built.

In sum, wind developer capacity and emissions claims should be scrutinized carefully. These claims are often exaggerated as they ignore the effective capacity issue and assume inflated capacity factors, the actual use of all the wind that is available and the displacement of 'dirty' coal. This is not always easy as these projections are typically based upon proprietary meteorological analyses that are not disclosed to either regulatory agencies or the public and upon the ‘type’ of generator displaced (e.g. coal or natural gas) rather than a systematic analysis of the impact of wind production of the specific grid in question.

Finally, it is also worth noting that grid managers/utilities’ ‘acceptance' of wind energy should not be confused with endorsement of it. Assuming wind energy’s penetration is small and, consequently, its impact on the operating efficiency is modest, many will ‘welcome’ wind energy to evidence their support for renewables and deflect criticism for their continued dependence on fossil fuels. Furthermore, in states with mandated renewable portfolio standards (RPS) most of which are tantamount to ‘wind energy’ standards, many are compelled to buy from wind energy producers either directly or via Renewable Energy Credits (RECs) to meet their RPS requirements.

Marginal Emissions

The Fact Sheet is correct to look at marginal emission rates. However, its focus on the ‘average’ marginal emissions rate has little merit analytically. As noted above, wind energy is often not available when demand is greatest. As such, it is more appropriate, given wind’s intermittency and volatility and lack of responsiveness to demand, to look at marginal rates on an hourly basis, determine the offsets (that may be positive or negative) based on actual plant(s)’ production during each hour to meet demand, and then sum up the savings over a 24 hour period. Even more analytically correct would be to use each plant's operational state (ramping up or down) and associated emissions at each moment. Use of the annual average marginal rate is like putting one hand on the stove and one in the freezer—on ‘average’ your temperature is just right.

Germany & Denmark

Denmark and Germany are often accorded ‘role model’ status by wind proponents. Their experience suggests otherwise.

Denmark has indeed built thousands of wind turbines, allowing wind energy to approach 20 percent of its installed electricity capacity. However, because the nation's grid is not adept at integrating wind production, wind contributes only approximately 4 percent of the actual energy on the Danish grid. The requisite backup of this wind production by fossil-fueled plants largely offsets any ‘savings’ in CO2 emissions. It is noteworthy (and not well known) that due to the high cost of wind energy the Danish Government has eliminated subsidies for the wind industry to discourage the construction of more wind plants. This piece is a worthwhile read -

Germany is experiencing many of the same problems. With about 16,000 wind turbines, wind energy now contributes approximately 5%-6% of the nation's installed capacity. However, because of problems associated with intermittency and the need for backup generation, Germany's grids have requested billions of dollars to upgrade transmission systems. This piece is a worthwhile read -

Capacity Factor

In the Fact Sheet’s discussion of capacity factor, please note that some fossil fuel plants (peakers) have ‘low’ capacity factors by choice, i.e. they are run less for economic reasons, not because they are not ‘available’. Wind plants have relatively low capacity factors because of wind ‘s intermittency and volatility. Please also note that we are unaware of any wind plant in the U.S. that has a capacity factor approaching 40%. It is well known that the average capacity factor for all wind plants in the U.S. approximates 29% with capacity factors in the low 30% range considered quite good. Capacity factors over 35% are rare. The aforementioned document also discusses this in greater detail.


Please note that the Fact Sheet’s claim of 95% availability for modern wind plants is for hardware only. Because the wind is not always blowing within required parameters, the percentage of time a wind turbine is actually producing is lower than its ‘availability’.

Rerl Fact Sheet 2a Capacity Factor

Download file (281 KB) pdf

JAN 1 1970
back to top