June 2013

U.S. Energy Information Administration | Distributed Generation System Characteristics and Costs in the Buildings Sector 

APPENDIX B 
 

EIA Task Order No. DE-DT0000804, Subtask 3 

The Cost and Performance 

of Distributed Wind 

Turbines, 2010-35 

Final Report 

 

June 2010 

Prepared for: 

Office of Integrated Analysis & Forecasting 

U.S. Energy Information Administration Prepared by: 

ICF International 

Contact: 

Robert Kwartin 

T: (703) 934-3586 

E: RKwartin@icfi.com 
 ii 

Table of Contents 

Executive Summary ..................................................................................................................... iv

Introduction ................................................................................................................................... v

1. Technology Overview............................................................................................................1

2. Market Overview ...................................................................................................................2

3. Potential Market Size ............................................................................................................4

3.1 Technical Factors.......................................................................................................4

3.2 Economic Factors - Benefits......................................................................................6

3.3 Economic Factors - Costs..........................................................................................7

3.4 Market Potential - Conclusions ..................................................................................7

4. Performance Objectives........................................................................................................8

5. Interpreting Performance Data for Distributed Wind Technology........................................10

6. Data Issues – Implications for NEMS..................................................................................12

7. Technology Baseline...........................................................................................................13

8. Sources for Improvement....................................................................................................14

8.1 Technological...........................................................................................................14

8.2 Cost..........................................................................................................................15

9. Projection Methodology.......................................................................................................17

Endnotes.....................................................................................................................................19

Bibliography ................................................................................................................................21
 

iii

Tables 

Table 1: The U.S. Distributed Wind Market ..................................................................................3

Table 2: Market Projections of Grid-Connected Domestically Installed Wind Turbines................8

Table 3: Percentage Difference between Manufacturer Literature and NREL Test Data ...........12

Table 4: Dimensional Data for Selected Distributed Wind Turbines ...........................................13

Table 5: Cost and Performance Data for Selected Distributed Wind Turbines, 2009.................14

Table 6: Improvement in the GE 1.5 MW Turbine ......................................................................14

Table 7: Annual Operation and Maintenance Expenses.............................................................16

Table 8: Assumptions for the Base and Advanced Cases..........................................................18

Figures 

Figure 1: Main Components of a Wind Turbine ............................................................................1

Figure 2: Growth of the Small Wind Market in the U.S. ................................................................4

Figure 3: National Wind Resource Map ........................................................................................5

Cover photos courtesy of the American Wind Energy Association. 
 

iv

Executive Summary 

This report and its accompanying data tables provide cost and performance projections 

for distributed wind turbines in the 1-100 kW nominal size range over the 2010-35 

period. These factors were developed by compiling manufacturer-provided cost and 

performance of popular present-day turbines in this size range; adjusting these 2009 

data to reflect independent test results and to conform to new data definitions that will 

apply in 2010 and beyond; and then developing cost and performance trajectories for 

the forecast period. These trajectories are based on interviews with market participants, 

particularly manufacturers, distributed wind project developers, and researchers. 

Projections were developed for both a reference case and an advanced case. The 

advanced case is distinguished from the reference case by an assumption of much 

higher private sector research and development investment, resulting in more rapid and 

more substantial improvements in cost and performance over the projection period. 

Specific parameters include: 

Performance: Turbine productivity measured in kWh generated per year is projected 

to increase by 28% in the base case and 36% in the advanced case over the forecast 

horizon compared with present-day turbine productivity. 

Cost: Distributed wind installed costs, in constant dollars, are projected to fall by 20% 

in the base case and 24% in the advanced case through 2035. 

Economic Viability: The combination of improving performance and falling cost is 

projected to yield a 37% and 44% reduction in installed cost per annual kWh produced 

in the base case and the advanced case respectively over the 2010-35 period. 

Operation and Maintenance (O&M) Expenses: O&M expenses are projected to fall 

by 10% in the base case and by 12% in the advanced case by 2035 compared with 

current-day costs. 

Availability: Turbine availability was assumed to be 98% under both scenarios. 

Equipment Life: Turbines were assumed to have a 25-year lifetime under both 

scenarios. 
 

v

Introduction 

The purpose of this paper is to provide cost and performance projections for residential 

and commercial scale distributed wind turbines over the period 2010-35. These 

projections will be used as inputs for the U.S. Energy Information Administration (EIA) 

National Energy Modeling System (NEMS), the principal modeling platform used by EIA 

to develop its Annual Energy Outlook. The paper will provide an overview of existing 

technology, briefly explain its applicability in the market, and discuss potential changes 

in the cost and performance of the technology over the projection period. 

This paper is not intended to be an exhaustive review of distributed wind technology. 

Rather, it is intended to provide a conceptual framework for analyzing the projected 

evolution of distributed wind technologies, and to provide a credible basis for projecting 

cost and performance characteristics. 
 

1 

1. Technology Overview 

This section will discuss wind technology generally, and explain why residential and 

commercial customers adopt wind turbines at the lower end of the available size range. 

The discussion will continue with a description of the factors that influence turbine 

energy productivity and the components of system cost. 

 

Figure 1: Main Components of a Wind Turbine1

Modern wind turbines capture the kinetic energy of moving air and convert it into shaft 

power to drive an electrical generator/alternator. The turbine is typically comprised of 

three basic parts: the rotor, the nacelle and the tower. 

The rotor includes the turbine’s blades (most often 3 in horizontal wind axis turbines) 

and the nose cone/hub. The nacelle contains the driveshaft, transmissiona, the unit’s 

generator/alternator, the electronic controls to convert the generator’s or alternator’s 

electrical output to quality suitable for use, and the tail vane or yaw drive that keeps the 

turbine oriented to the wind, either upwind or downwind depending on the turbine’s 

design. 

Because wind speed generally increases and turbulence decreases with height, a tower 

helps the system increase its energy production and reduces turbulence-induced 

mechanical stresses, thus enhancing its economic benefit. 

The ability of a turbine to produce energy from the wind fundamentally depends on the 

wind resource and the swept area of the turbine. Simplifying somewhat, the power 

output of a turbine is proportional to the cube of the wind velocity and the square of the 

blade length. A doubling of the wind speed thus yields an eight-fold increase in wind 

power while a doubling of a turbine’s blade length yields a four-fold increase in energy 

capture (all other things kept constant). 

Larger turbines with longer blades not only produce more energy for a given wind 

resource, they are also more capital cost-effective as well, as a result of inherent 

 

a

 Many small turbines are direct drive. Larger turbines more often use gearboxes to step up the rotor’s 

rotational speed to a rotational speed suitable for the generator/alternator. 
 

2 

economies of scale as well as inefficiencies in the market for smaller turbines. As 

shown in Table 5, the installed cost/kW for a small turbine is twice that for a mid-scale 

turbine and can be several times as expensive as that for a utility-scale turbine. 

While these factors argue for choosing the windiest sites and installing the largest 

turbines on the highest towers that are cost-effective for the site – an argument 

understood by wind farm developers – residential and commercial site hosts cannot 

follow this logic to its conclusion. It is a rare home or business owner that is going to 

move their establishment simply to take advantage of a windier site. And several 

practical constraints prevent home and business owners from using the giant turbines 

typically found in utility-scale wind farms. Neighbors might object to the presence of a 

turbine hundreds of feet tall because of safety, noise and visual concerns. The turbine’s 

capital costs are an additional consideration: even if a site host has the space for a giant 

turbine, the multi-million dollar capital cost can be difficult to finance for someone not in 

the wind industry. As a result of these constraints, most distributed wind turbine 

installations are sized roughly equivalent to the site host’s electrical load and use 

turbines much smaller than those found in current-day wind farms. This analysis 

therefore assumes that residential customers will install turbines with nominal capacity 

ratings of 1-9 kW, while commercial customers will install turbines with nominal capacity 

ratings of 10-100 kW. 

2. Market Overview 

Distributed wind technology is used to power homes, small businesses, farms and 

ranches, schools and colleges, county and state facilities, and many other site hosts. 

Buyers are motivated by a variety of factors, typically a blend of the following: 

 a distributed wind turbine may simply be a good investment; 

 buyers may be seeking to moderate the volatility in the prices they pay for 

electricity; 

 buyers may want to reduce their environmental impact by generating 

electricity without fossil fuel combustion; 

 some buyers may be motivated by economic development concerns: a 

distributed wind turbine creates employment during installation and for 

ongoing operation, and onsite electricity generation can keep funds in the 

community; and 

 particularly for public sector and educational institutions, there is a 

corollary goal of demonstrating a new technology and educating citizens 

and students about renewable energy-generation possibilities. 

Whatever the balance between these motivations, more turbines will be purchased as 

project economics improve; few buyers can afford to ignore cost and economic return 
 

3 

entirely, no matter how strongly they might otherwise be motivated.b Project economics 

are discussed in greater detail below. 

The small wind market grew rapidly in the U.S. in 2008. The American Wind Energy 

Association’s survey2 indicates that over 10,000 small (100 kW and smaller) turbines 

were sold in the U.S. in 2008 with an aggregate nameplate capacity of 17.3 MW. This 

represented an increase of 14% in unit sales and 78% in nameplate capacity sales 

compared with 2007. The distribution of sales by size is shown in Table 1 and the 

growth in sales is shown in Figure 2 below: 

 Table 1: The U.S. Distributed Wind Marketc

0-0.9 kW 1-10 kW 11-20 kW 21-100 kW Total 

Units Sold 6,706 3,521 72 87 10,386 

Capacity (kW) 2,784 7,599 1,331 5,660 17,374 

 

b

 The author is not aware of public-domain literature providing a rigorous analysis of buyer motivation for 

installing distributed wind turbines. (Informal pre-purchase surveys have been conducted by Home 

Power magazine, for example, and other informal surveys have assessed barriers to purchase.) 

Anecdotally, it is clear that many buyers are motivated by non-economic factors, but the extent of this 

motivation and its relative importance in different buying segments is not clear. Such a study would 

increase the realism of market penetration studies by public and private analysts. c

 The majority of turbines with nameplate capacity of 1 kW and below are purchased for off-grid 

applications, such as powering remote loads, off-grid cabins, boats, etc. AWEA estimates that 7,402 

turbines with a nameplate capacity of 3,764 kW were sold for off-grid uses in 2008. These applications 

are not represented in NEMS and are not further treated in this paper. 
 

4 

Figure 2: Growth of the Small Wind Market in the U.S.3

3. Potential Market Size 

The potential market for distributed wind is constrained by technical and economic 

factors. 

3.1 Technical Factors 

A proposed distributed wind project can be impaired by a number of different technical 

considerations. The most important is the availability of wind. A site with poor wind 

resources cannot support an economically viable wind project. Figure 2 below provides 

a coarse-scale representation of the country’s wind resources at a 50-meter hub height; 

the map displays Wind Power Classes, which are based on wind power density (Watts 

of wind power per square meter of rotor cross section). 

Although it is not an ironclad rule, in general, if a site has a wind resource below Wind 

Power Class 3, it is unlikely to be economically successful. Almost 70% of the land 

surface in the Lower 48 states is in Wind Power Classes 1 and 2.4 At the lower hub 

heights used for the small to mid-scale turbines evaluated in this paper, the percentage 

of low-wind surface area is even higher. 
 

5 

 

Figure 3: National Wind Resource Map

Land availability and usability is the second most important technical factor. The land 

parcel on which the turbine will be sited needs to be of sufficient size to satisfy any local 

zoning codes related to set-back as well as safety, noise and visual considerations. In 

addition, the parcel must have sufficient room so that the turbine will not be in close 

proximity to trees, structures or other features that can slow the wind or create 

turbulence. Smaller residential turbines mounted on towers of appropriate height 

typically require a parcel of half an acre or a full acre5

. Northern Power Systems 

recommends 500 feet of clearance around one of its 100 kW turbines, equivalent to an 

18 acre parcel.6 Thus, it is difficult to implement distributed wind turbines in cities and 

heavily developed suburbsd

. 

Several other technical issues constrain the implementation of distributed wind: steep 

terrain; high elevations; zoning restrictions on tower heights; availability of 3 phase 

power (generally for machines > 30 kW), concerns about bird and bat kills; etc. 

 

d

 Vertical axis wind turbines (VAWT) have some potential to fit into smaller land parcels, as they can be 

installed on lower towers or even mounted on buildings, and thus require less set-back. Doing so, 

however, reduces the available wind resource and increases mechanical stress arising from turbulence. 

VAWTs have generally suffered from lower energy productivity compared with horizontal axis turbines 

and have struggled to demonstrate their commercial viability. 
 

6 

3.2 Economic Factors - Benefits 

The financial analysis of a wind project includes the following factors: 

 Revenue from electricity generation. In most cases, the majority of the 

“revenue” from a distributed wind turbine’s electricity generation is actually the 

displacement of electric power deliveries by the electric utility. This displacement 

of electricity sales at relatively high retail rates is usually the largest single 

revenue source for a distributed wind turbine.e

 Any excess electricity above the 

site’s consumption can be sold the local electric utility, but the value of these 

sales varies dramatically. At a minimum, utilities are obliged to pay at least some 

proxy for wholesale electricity prices for electricity purchased from a 

customer/generator. In many states, excess generation of electricity above the 

site’s consumption can be sold back to the distribution utility at the full retail rate 

and excess generation from one month may be carried over to net against 

electricity purchases in subsequent months, often for up to a year. 7 

 Benefits from policy support. A variety of public policies provide additional 

benefits to distributed wind project owners: 

o Federal tax benefits. At the Federal level, tax benefits are available to 

distributed wind turbine owners. The most important is the recentlyenhanced Investment Tax Credit (ITC). This credit is valued at 30% of the 

project’s installed cost, without any upper limit on the credit amount, and is 

available through December 31, 2016. Under Section 1603 of the 

American Recovery and Reinvestment Act, this tax credit can also be 

converted into an outright cash grant from the Treasury, which is 

particularly favorable for taxable entities that do not anticipate sufficient 

taxable income to take full advantage of the ITC and for entities that prefer 

the certainty of a cash grant in the near term to a tax credit taken during 

one or more subsequent tax years. This conversion option is available 

only if significant project efforts (5% of project costs) are made by 

December 31, 2010. Alternatively, a project may take advantage of the 

Production Tax Credit (PTC), worth approximately 2 cents per kWh when 

output is sold to an unrelated third party over a 10-year period. (For the 

majority of distributed wind projects, the ITC is more valuable than the 

PTC.) In addition, wind turbines are eligible for accelerated depreciation 

under the Modified Accelerated Cost Recovery System.8

o Direct spending benefits. Federal and State agencies provide 

incentives for distributed wind projects through a variety of programs. For 

example, the U.S. Department of Agriculture’s Rural Energy for America 

 

e

 The valuation of displaced electricity requires some analysis. For residential customers with simple kWh 

meters, displaced electricity will be worth the full retail rate (less any fixed customer charge). For 

commercial/industrial customers whose tariffs include both capacity (kW) and energy (kWh) based 

charges, consideration needs to be given to the uncertainty as to whether the wind turbine will reliably 

reduce the kW-based charges, for example, by comparing the facility’s load profile against the likely 

power production profile of the turbine. Unlike photovoltaic technology in hot climates, distributed wind 

generation cannot be assumed to be peak-coincident. In reality, a distributed wind turbine may not 

reliably avoid the peak capacity charge at all, in which case its displacement value is limited to the energy 

component of the tariff. 
 

7 

Program offers grants for feasibility studies and renewable energy 

installations. Many states offer direct grants for distributed wind projects 

or production-based incentives. 

o Renewable Energy Certificates (RECs). RECs can be understood to 

represent the positive environmental and fuel diversity attributes arising 

from the generation of each MWh of renewable electricity. RECs can be 

marketed separately from the electric commodity and are purchased by 

entities subject to state Renewable Portfolio Standards to satisfy their 

obligations under those programs (“mandatory RECs”). In addition, many 

electricity customers purchase RECs voluntarily to “green up” their 

electricity supply (“voluntary RECs”). A distributed wind project owner can 

choose to retain the RECs created by their project (to keep their own 

electricity supply “green”), or sell the RECs created by their project, or 

some combination of the two. REC sales can be a significant additional 

revenue stream for a distributed wind project. 

o Other policy support. A variety of other policy tools are used to enhance 

the financial viability of distributed wind projects: government-sponsored 

low-interest loans, sales tax abatements, property tax abatements, state 

income tax credits and deductions, preferential feed-in/buy-back tariffs, 

etc. These policy tools evolve rapidly; refer to the Database of State 

Incentives for Renewables and Efficiency (DSIRE) for additional details. 

3.3 Economic Factors - Costs 

The most important cost component for a distributed wind project (60-80%) is the cost 

of the hardware: rotor, nacelle and tower. Transportation and installation costs 

(including labor, equipment rental, concrete, wiring, metering, interconnecting with the 

distribution utility, etc.) can be considerable (10-35%), particularly with taller towers, 

remoter sites and more difficult terrain. Pre-construction costs – feasibility analyses, 

project design, permitting, zoning, etc. -- may be modest for a rural residential project 

confronting limited zoning and permitting challenges, but run to tens of thousands of 

dollars for a commercial-scale project with more complex design and engineering 

requirements, and can account for 5-15% of initial project costs. In addition, turbine 

owners need to plan for annual operation and maintenance (O&M) costs, warranty 

expenses, as well as costs related to insurance, incremental property taxes (if any), and 

eventual decommissioning of the turbine when it reaches the end of its useful life. 

Example capital costs for present-day distributed wind turbines are shown in Table 5; 

O&M costs are show in Table 7. 

3.4 Market Potential - Conclusions 

A recent analysis evaluated the technical and economic potential of existing mid-scale 

distributed wind projects in the 10-5000 kW size rangef. The analysis found that 

 

f

 The study evaluated individual turbines up to 1000 kW as well as small community wind projects 

consisting of five 1000 kW turbines. 
 

8 

commercial, industrial and institutional buyers motivated purely by economics would 

purchase over 2700 10 kW turbines, about 10,000 50 kW turbines, approximately 200 

100 kW turbines, and about 3500 250 kW turbines if 2008 incentive levels were 

assumed to remain unchanged for 10 years. 9 

This study had some important limitations. It did not evaluate residential buyers in the 

1-10 kW range. It also did not attempt to quantify market penetration driven by noneconomic factors, such as the desire to reduce greenhouse gas emissions, even though 

anecdotal information suggests that non-economic factors are important drivers of 

distributed wind projects. In addition, the study was completed prior to the 

implementation of the uncapped 30% ITC, which can be expected to dramatically 

increase the number of viable projects. 

This study also evaluated the impact of improving today’s mid-scale distributed wind 

turbines. Longer blades, taller towers, greater productivity (particularly at low wind 

speeds), and lower costs combined to increase the potential market for 250 and 500 kW 

turbines by a factor of 25 or more.10 

A second study provides market potential estimates based on technology application: 

Table 2: Market Projections of Grid-Connected Domestically Installed Wind Turbines11

Cumulative Units 

Installed 

Residential (1-25 kW) Farm, Business, 

Industrial (10-400 kW) 

2005 1,800 20 

2010 6,250 1,270 

2015 14,000 4,270 

2020 36,500 7,395 

4. Performance Objectives 

Ideally, a wind turbine would extract the maximum amount of kinetic energy from the 

wind at the lowest possible cost. However, wind turbines, like all other goods, represent 

a set of compromises to satisfy multiple goals. These include: 

1. Energy productivity: Turbines vary in their ability to extract energy from the 

wind, and all other things being equal, a turbine that produces more energy from 

a given wind resource is more valuable than a turbine that produces less. This 

can be achieved by using longer blades, more efficient blade design, more 

efficient transmission (or direct drive), a more efficient generator/alternator, better 

yaw control, etc. The turbine’s behavior over a dynamic range is also a critical 

factor: turbines can be designed to start spinning at lower speeds, to produce 

more energy at the most frequent wind speeds, or to continue producing at 

higher wind speeds, but it is difficult to design a turbine that can do all three. 
 

9 

Because wind is variable at a given site and even more variable across many 

sites, no one turbine model is optimal for every site. 

2. Project Cost: Turbines are expensive, and manufacturers seek out ways to 

reduce costs. This can include using less-expensive materials, improving 

manufacturing techniques, sourcing less-expensive components, and improving 

the efficiency of distribution and installation. Volume is an important determinant: 

many turbine models in the 1-250 kW range are produced in limited quantities, 

which drives up unit costs. Larger production runs give the manufacturer more 

leverage in negotiating with upstream suppliers, and permits more investment in 

production tooling. 

3. Overspeed Control: Once the turbine is producing at its maximum output, any 

further increase in wind speed is essentially “wasted” from a power generation 

perspective and at very high speed can cause structural damage to the turbine. 

Turbines use a variety of methods to regulate turbine loading and rotational 

speed: by furling the rotor towards the tail vane (thereby reducing the rotor crosssection presented to the wind), deploying various types of blade-mounted brakes, 

changing the blade pitch, stalling, and/or by electrical braking. 

4. Tower Height and Cost: In most terrains, wind speed increases with height, at 

least for tens of meters. Tall towers improve cost-effectiveness in most areas. 

Some turbines are available on tilt-up towers, while others can only be mounted 

on fixed towers. For fixed towers, erection costs increase as tower height 

increases. Taller towers also make maintenance visits more hazardous and 

time-consuming, driving up O&M costs. 

5. Reliability/Durability: Utility-scale wind farms can afford to have on-site 

technicians to provide regular maintenance and repair services. A single 

distributed wind turbine at a home or business cannot be expected to receive the 

same level of attention, and therefore should be designed and built to minimize 

maintenance requirements. (Some level of maintenance will always be 

required.) Although wind turbines have few moving parts, they are subjected to 

significant stresses and vibration from winds that vary in speed and direction. 

The rotor and drivetrain is expected to spin hundreds of times per minute with 

blade tip speeds of over 100 mph, and this performance is expected to last for 

decades. To reduce maintenance costs, production degradation and downtime, 

the turbine should be built with well-engineered components fabricated from 

long-lasting materials. However, this drives up costs. Lower rotor speeds can 

increase longevity, but at the cost of reduced energy production. 

6. Noise: Several design choices that increase energy productivity (longer blades, 

higher rotational speed, and certain overspeed controls, such as furling) increase 

the sound pressure produced by the turbine. This tradeoff can be particularly 

objectionable for distributed wind turbines as they are ordinarily sited close to 

homes and businesses. 

NEMS “builds” distributed generation (DG) in future years based on how cost-effective 

the technologies are. NEMS incorporates a 30-year discounted cash flow model that 

assesses the internal rate of return (IRR) of various DG technologies; NEMS then uses 

the IRR and a learning function model to forecast the technology’s penetration in the 
 

10 

marketplace. Other thing being equal, if a DG technology’s IRR increases in a future 

year, it will penetrate the market further than if its IRR remains unchanged or falls.12 

Of the key performance objectives listed above, only the turbine’s cost and performance 

figure into the IRR calculation. Overspeed control, reliability/durability and noise are not 

considered. In the real world, however, these parameters are part of the turbine design 

process, and manufacturers make necessary compromises to ensure that turbines 

perform safely and reliably over the long run. The cost and performance projections 

presented later in this paper were developed under the assumption that manufacturers 

would continue to balance multiple objectives in the future as they have done in the 

past. 

It is also important to note that NEMS only considers decisions made on economic 

grounds; the model does not endogenously account for turbines installed for reasons 

other than economics. 

5. Interpreting Performance Data for Distributed Wind 

Technology 

Distributed wind technology performance data needs to be interpreted with care. The 

challenge arises partly from a lack of industry standardization, partly from the highly 

site-specific performance of wind turbines, and partly because some manufacturers 

provide inaccurate or incomplete data to their buyers. 

Industry standardization. Until very recently, the distributed wind industry lacked 

standardized terminology, test methods or product certification processes. Rated 

capacity, for example, is a less meaningful metric than it may seem. Manufacturers 

choose the wind speed at which they rate their turbines’ capacity, and as noted in Table 

4, the values can vary considerably. This makes it difficult to make an “apples to 

apples” comparison of two turbines even of the same nominal rated capacity; one may 

be rated based on an 11 m/s wind, for example, while another is rated at 14 m/s. 

Lack of industry standardization should begin to recede as an issue in the near future. 

The American Wind Energy Association (AWEA) published a performance and safety 

standard for small wind turbinesg

 in late 2009 that established specific test methods for 

various performance parameters, including rated capacity, annual energy production, 

and noise.13 For example, the standard established that a turbine’s capacity should be 

rated at a wind speed of 11 m/s. 

In addition, 2010 will see the Small Wind Certification Council commence operations. 

The SWCC is an independent organization that will certify turbine testing conducted in 

accordance with the AWEA small wind turbine standard, provide an SWCC label to 

certified turbines, and provide test results on its web site.14 The combination of 

standardized test methods and independent certification of testing results will make a 

material contribution to improving the usefulness of turbine performance data. 

 

g

 The standard applies to turbines with swept areas of 200 square meters, or a rotor diameter of about 

16m. This translates to a nominal capacity of about 65 kW. 
 

11 

Site-specific data. Although a turbine’s rated capacity is often the first point of 

reference, in fact, the most important metric for a wind project is the turbine’s estimated 

annual energy production (AEP) for the project site. This metric drives the project’s 

potential revenue much more directly than rated capacity does. To estimate AEP, it is 

necessary to know the following information: 

 The project’s hub height; 

 The wind resource distribution: how many hours per year the wind blows 

within specific speed ranges (bins) at the project’s hub height; 

 Turbulence at the project’s hub height due to nearby trees, buildings and 

other obstructions; 

 The turbine’s production curve: how many kWh the turbine produces for 

each specific wind speed bin. 

 The turbine’s expected availability and losses (e.g., line losses) 

For large, utility scale projects, it is typical to measure the wind resource at hub height 

at multiple points across the development area for at least a year-long period, and then 

to use sophisticated software to estimate the effects of terrain and of the turbine array 

itself on AEP for each turbine. Distributed wind projects in the 1-100 kW range rarely 

utilize such a data-intensive approach, usually only at the upper end of the range. 

Instead, homeowners, turbine dealers and project developers rely on a combination of 

coarse-scale wind maps (themselves derived from extensive modeling) and 

manufacturer-supplied production curve data. 

It is not uncommon for this approach to result in significant mis-estimation of AEP. To 

start with, the typical state-level coarse-scale wind map is merely a starting point for 

wind estimation. Actual wind conditions within each of the map’s raster cells can vary 

dramatically. Second, even assuming that the map provides an accurate estimate of 

the wind resource at a specific site, the map may be estimating wind resources at one 

height (e.g., a 50-meter hub height), but the turbine may be mounted at a different 

height (e.g., 30 meters, where wind power is substantially less). Or turbulence caused 

by site conditions (e.g., nearby buildings or large trees) may be ignored, although it can 

have a powerful effect on turbine performance. 

A third factor is incorrect or incomplete information supplied by manufacturers, 

particularly the turbine’s power curve. (See Table 3 below.) For example, one widelyused distributed wind turbine (Turbine A) has recently undergone independent testing at 

NREL. Comparing NREL’s measured data with manufacturer literature indicates that 

the manufacturer overstates AEP by 71% at the AWEA reference speed of 5 m/s. The 

differences between NREL’s measured data and manufacturer literature data were 

somewhat lower at higher wind speeds, falling to a 25% difference at an 8 m/s annual 

average wind speed for this turbine. Note, however, that the literature associated with 

Turbine B goes to some lengths to qualify potential turbulence factors at various wind 

speeds (e.g., hedges, windbreaks, buildings) and appears to have adjusted its AEP 

values accordingly.15 
 

12 

Table 3: Percentage Difference between Manufacturer Literature and NREL Test Datah

Average Annual 

Wind Speed Wind Turbine A Wind Turbine B Wind Turbine C 

4 m/s NA NA 46% 

5 m/s 71% -23% 26% 

6 m/s 43% -24% 9% 

7 m/s 31% NA NA 

8 m/s 25% NA NA 

Manufacturers and dealers may also deemphasize noise considerations or fail to 

educate the buyer on the desirability of (or need for) a tall tower. The advent of the 

AWEA small wind standard and the SWCC will make it easier for buyers to obtain 

accurate and meaningful information, but only education will ensure that buyers get the 

most value from their significant investment in distributed wind. 

6. Data Issues – Implications for NEMS 

NEMS “builds” distributed wind turbines in Census divisions, which are then overlaid 

with an NREL wind map. NEMS refers to the geographical overlay of a Census division 

and wind map polygon as a “niche”. For each niche, NEMS estimates the distribution of 

wind speeds using a Weibull k of 2 and a wind shear exponent of 0.2. 

The model then estimates AEP by applying a turbine’s rated capacity across a cubic 

power equation. 

This method is vulnerable to the following sources of error: 

 The wind resource map may not be appropriate for the turbine’s hub 

height; 

 The wind speed distribution may not be appropriate in light of the 

topography of the niche; 

 The varying altitude across the niches may not be appropriately 

incorporated into the computations; 

 Turbulence may not be appropriately represented; 

 The nominal power curve may not be appropriate for other specific 

turbines; 

 Scaling the AEP based on ratios of rated capacity may not be appropriate 

in view of the inconsistency of capacity rating methods. 

 

h

 Values represent the difference between the manufacturer’s data and NREL’s data, divided by the 

NREL data. Positive differences indicate that manufacturer AEP data are higher than NREL AEP data for 

the same average annual wind speed. Manufacturer data for Turbines B and C were not available for unit 

values of m/s and were interpolated from values expressed in half meters per second (e.g., 4.5 m/s, 5.5 

m/s, etc.) using a simple average. 
 

13 

Going forward, the author recommends that EIA take the following steps to increase the 

realism of its modeling efforts: 

1. Verify that the national wind map corresponds to the hub heights of the typical 

distributed wind turbine, for example 30-40 meters. 

2. Consider whether to vary the Weibull k and wind shear exponent based on the 

land surface and altitude within the niche. 

3. Altitude and turbulence factors could also be more directly represented (e.g. the 

decrease in air pressure at higher altitudes will decrease the energy output for a 

wind turbine at a lower altitude, all other things being equal). 

4. As soon as it becomes available in quantity, use AWEA-standard AEP data that 

have been certified by the SWCC for the base year, and use these data to 

interpolate the AEP for other turbine sizes that have not yet been certified by 

SWCC. 

5. Until SWCC data become available: 

a. use manufacturer-supplied AEP data for the base year, but derate these 

data to reflect the discrepancies described in Table 3 above. 

b. apply the adjusted AEP data to specific sites, and further modify the data 

to reflect local wind resources, altitude, Weibull k, and wind shear 

exponent. 

7. Technology Baseline 

Cost and dimensional data for baseline distributed wind technology are represented in 

Tables 4 and 5 below. The turbines chosen for this table were selected to represent a 

range of project capacities and are popular models within their respective size classes. 

Note that each turbine’s rated capacity is derived from a different reference wind speed. 

Table 4: Dimensional Data for Selected Distributed Wind Turbines

Typical

Rated Rated Wind Rotor Tower

Capacity Speed Diameter Height

Manufacturer Model (kW) (m/s) (m) (m)

Southwest Windpower Skystream 3.7 2.4 13 3.7 26

Bergey BWC XL-S 10 14 7 37

Entegrity EW50 50 11.3 15 37

Northern Power Systems Northwind 100 100 14.5 21 37
 

14 

Table 5: Cost and Performance Data for Selected Distributed Wind Turbines, 200916

AEP @ 

Typical 5 m/s

Rated Rated Wind Rotor Tower Installed Installed average annual

Capacity Speed Diameter Height Cost Cost/kW wind speed

Model (kW) (m/s) (m) (m) ($) ($) (kWh)

Skystream 3.7 2.4 13 3.7 26 19,000 $ 7,917 $ 3,600 

BWC XL-S 10 14 7 37 62,000 $ 6,200 $ 13,200 

EW50 50 11.3 15 37 230,000 $ 4,600 $ 72,000 

Northwind 100 100 14.5 21 37 435,000 $ 4,350 $ 145,000 

8. Sources for Improvement 

8.1 Technological 

Advances in modeling, materials science, fabrication techniques, blade design and 

electronics have allowed utility-scale turbines to grow steadily and to harvest the scale 

economies of that growth for almost 3 decades. Technological improvement has 

translated into lower installed cost per kW, greater energy production, and higher 

reliability. 

However, as discussed above, it is not practical for the typical residential or business 

customer to enjoy these benefits by following the modern turbine’s increase in size and 

hub height. The distributed wind turbine purchased by these customers has seen less 

rapid improvement, but particularly in recent years, significant improvement has been 

achieved. In some instances, small turbines have been earlier adopters of advanced 

technologies compared with larger turbines. 

A utility-scale example offers the first demonstration that technological improvement is 

possible without an increase in size. As shown in Table 6 below, the GE 1.5 MW 

turbine has improved steadily in just the past seven years – without a capacity uprating 

– through incremental application of an improved generator and main bearing design, a 

better blade pitch mechanism, longer blades, and an improved gearbox. 17 

Table 6: Improvement in the GE 1.5 MW Turbine 

 2002 2009 

Rotor Diameter (m) 70 82.5 

Capacity Factor (%) 39 52 

Reliability (%) 85 98 

A second example falls in the distributed wind size range. First produced in 1983, the 

Bergey Excel has undergone significant development over the years. The original airfoil 

has been succeeded by two new generations, as has the inverter, most recently in 

2008. 2008 also saw the introduction of a new neodymium-based alternator. The 

cumulative effect of these changes is a 30% increase in energy production, a reduction 

in noise, and no increase in price.18
 

15 

Distributed wind turbines still have substantial performance improvement potential; the 

representative of one distributed wind manufacturer believes that a 10-20% 

improvement in cost and productivity for this category over the next 5 year period will be 

“easy”.19 Improvement may be found in several areas: 

 Blades and rotor: Improved blade designs, lighter-weight and stronger 

materials, and improved manufacturing techniques may allow for lower 

cut-in speeds, greater low-speed energy production, lower-noise 

overspeed control, and greater AEP per 

square meter of rotor cross-section. 

Current day blades are estimated to be 

about 32% efficient; an industry 

workshop set a goal of 42-45% 

efficiency. 

 Generators: Many distributed wind 

turbines are now equipped with rareearth permanent magnet generators, 

which are smaller, lighter, and more 

efficient than ferrite or wound-rotor generators. 

 Inverters: Many small distributed wind turbines use inverters optimized for 

photovoltaic systems. Inverters optimized for small wind turbines would 

have a larger voltage range, and, potentially, greater efficiency. 

 Drivetrain: Most distributed wind turbines use direct drive whereby the 

rotor directly drives the generator, without the use of a gearbox to step up 

the rotational speed. Direct drive increases efficiency by eliminating gear 

losses, and also eliminates a frequent point of failure. 

 Control electronics: At the larger end of the distributed turbine range, it 

may be possible to incorporate more advanced sensors and pitch controls 

to mitigate blade and tower loading, and thus enable the use of longer 

blades with greater energy capture. 

8.2 Cost 

Anecdotal evidence suggests that initial cost and long-term investment rate of return are 

the two most important factors in whether a distributed wind turbine is purchased and 

installed. There are several opportunities to significantly reduce the first cost of the 

turbine, the cost of installation, and ongoing cost of operation and maintenance: 

 Volume: Many distributed wind turbine models have limited production 

volumes. As the market matures, higher volumes can drive down unit 

costs through more efficient operation of manufacturing plant, lower input 

costs, and better amortization of fixed costs. In addition, higher volumes 

(and revenue) can justify greater investment in more advanced tooling and 

Physical Performance Limits 

A turbine cannot extract 100% of the power 

available in a stream of wind. If it did, the wind 

would stop, and so would the turbine. The upper 

limit in practice is 59%, known as the Betz limit 

after its discoverer, Albert Betz. Modern utility 

scale turbines extract about 50% of the wind 

energy at wind speeds below their rated wind 

speed.a
 

16 

manufacturing capacity. In 2008, over $160 million was invested in small 

wind manufacturers worldwide, with about half the funds invested in the 

United States.20

 Greater competition: In certain market segments, only one or a few 

manufacturers offer a product and have the dealer network available to 

support a project in a specific region. Some turbines are in short supply or 

only built-to-order. As more companies enter the market, customers will 

enjoy a greater choice of technology, shorter lead times, and a more 

competitive service environment. 

 Industry consolidation: While industry diversity will benefit certain 

segments compared with today’s baseline, other segments may benefit 

from some consolidation, which would allow greater scale economies in 

manufacturing, distribution and after-market service. 

 Outsourcing: The U.S. is currently a leading area of distributed wind 

turbine manufacturing; U.S. manufacturers account for about half of global 

small wind sales, and about 95% of the U.S. market.21 However, other 

countries, particularly China, are clearly bidding to enter the renewable 

energy market generally and the distributed wind market specifically. 

Imports from regions with lower manufacturing costs may put pressure on 

U.S. distributed wind turbine prices. 

 Component reduction: Some of the performance improvements 

discussed above may increase costs, but others, such as eliminating the 

gearbox using direct drive, can serve to reduce turbine costs. 

 Tower: Tower costs and crane rental can be a substantial fraction of total 

installed costs for a distributed wind turbine. Greater use of tilt-up and 

self-erecting towers, as well as lighter weight towers could reduce project 

costs.22

 Operation and maintenance costs: O&M costs could potentially be 

reduced through hardware and software improvements. Hardware 

improvements include the elimination of the gearbox, better lubrication, 

and more durable blade materials which are also more resistant to fouling. 

Software improvements include design strategies that reduce rotor and 

tower loading, better yaw and overspeed controls, and better monitoring 

technology to minimize the need for site visits and to provide early warning 

of emerging problems. The baseline (2010) O&M costs are shown in 

Table 7 below. 

Table 7: Annual Operation and Maintenance Expenses23

Assumed Annual Expenses Unit Expense 

Operations & Maintenance $/kWh $0.0100/kWh 

Operations & Maintenance 

Contingency Fund $/kWh $0.0030/kWh 

Insurance $/kW $6.70/kW 

Property Tax $/kW $4.70/kW 
 

17 

Admin/Financial/Legal Management $/kW $0.30/kW 

Warranty Expense $/kW $7.70/kW 

Decomm. Fund Post-Warranty 

Expiration $/kW $1.00/kW 

Other Expense $/kW $1.30/kW 

9. Projection Methodology 

For the purposes of the NEMS projections, it was assumed that the 2010 baseline was 

represented by the turbines listed in Tables 4 and 5 above. Essentially, the four 

turbines were assumed to become the prototypical turbines for the next 25 years. For 

the base year, each turbine’s AEP was derated to some degree to reflect the findings 

shown in Table 3. The derating was not constant across turbines. Maintenance costs 

were taken from Table 7. 

The future is represented by two scenarios: a base, or reference case; and an 

advanced case. 

Under the base case, it is assumed that present-day policies will continue in force until 

their legislated expiration (if any); that present-day research and development 

investment flows will continue; and that the trend of technology and cost improvement 

will continue in the future much as it has in the recent past. 

The advanced case is similar to the base case, except that it assumes a much higher 

level of private sector R&D investment, and thus more rapid and more extensive 

improvements in technology performance and more rapid and deeper reductions in 

cost. The advanced case does not assume any changes to the policy environment 

compared with the base case.i

 

For the period 2010-35, three improvement trajectories were developed: 

 Cumulative AEP Improvement: This trajectory describes the increase in 

kWh produced by a turbine compared with its 2010 baseline. 

 Cost Improvement Factor: This trajectory describes the reduction in a 

turbine’s installed costs, in constant dollars, compared with its 2010 

baseline. 

 

i

 The implementation of the uncapped 30% ITC in February 2009 is perhaps the most important policy 

initiative in favor of distributed wind in several decades. This policy will only begin to have full impact in 

2010 and beyond; in effect, the base case does not fully reflect this new policy. This policy could lead to 

larger market volumes, greater private sector investment, etc., producing a scenario more consistent with 

the advanced case. 
 

18 

 O&M Factor: This trajectory describes the reduction in annual O&M costs 

compared with the 2010 baseline. 

Table 8: Assumptions for the Base and Advanced Cases 

Cumulative AEP 

Improvement 

Factor vs. 2010 

Cost 

Improvement 

Factor vs. 2010 

O&M Factor vs. 

2010 

Base Advanced Base Advanced Base Advanced 

2015 10% 12% -8% -10% 

 

0.98 0.97 

2020 18% 21% -13% -14% 

 

0.96 0.94 

2025 23% 28% -16% -18% 

 

0.94 0.92 

2030 26% 33% -18% -21% 

 

0.92 0.90 

2035 28% 36% -20% -24% 

 

0.90 0.88 

Other assumptions include: 

 Capacity factor: As noted above, the authors recommend using AEP as 

the key metric of energy performance. However, the accompanying data 

table provides calculated capacity factors for different turbines over the 

projection horizon by turbine size, year and scenario. 

 Equipment life: A 25 year life was assumed for both scenarios. 

 Availability: We assume 98% availability in both scenarios. 

 O&M Costs: Summing the values in Table 7 yields an annual O&M factor 

based partly on capacity ($21.70/kW-year) and partly on energy 

production ($0.013/kWh).
 

19 

Endnotes 

 

1 http://www.industry.nsw.gov.au/energy/sustainable/renewable/wind

2

 AWEA, Small Wind Turbine Global Market Study: Year Ending 2008. 

3 Ibid, p. 5.

4

 Kwartin et al., An Analysis of the Technical and Economic Potential for Mid-Scale 

Distributed Wind, NREL, December 2008, p. 41. 

5

 Southwest Windpower, “Skystream 3.7: 2.4 kW Residential Power Appliance”. 

6 http://www.northernpower.com/wind-powerbasics/faq.php#WhatMakesAGoodWindSite

7 http://dsireusa.org/summarytables/rrpre.cfm 

8 Ibid. 

9

 Kwartin et al., p. 54 

10 Kwartin et al., p. 61 

11 Forsyth, T., and Baring-Gould, I., “Distributed Wind Market Applications,” NREL, 

November 2007, p. 5. 

12 DOE, Commercial Sector Demand Module of the National Energy Modeling System: 

Model Documentation 2009, DOE/EIA-M066(2009), May 2009, pp 40-44. 

13 American Wind Energy Association, AWEA Small Wind Turbine Performance and 

Safety Standard, (AWEA Standard 9.1 – 2009) © The American Wind Energy 

Association, 2009.

14 http://www.smallwindcertification.org/index.html 

15 Smith, Joe, “NREL Small Wind Technology Update,” AWEA Small & Community 

Wind Conference & Exhibition, November 5, 2009, Detroit, MI, and manufacturer 

literature. 

16 Sagrillo, Mick, “Size Matters!” Windletter, 28(3); AEP data: 

a. Bergey Skystream 3.7, 3-CMLT-1338-01 REV F 1-09 

b. “Power Your Dream With the Wind”, Bergey Windpower, accessed May 2010. 
 

20 

 

c. “Entegrity Wind Systems: Commercial Wind Energy Provider and 

Manufacturer of the EW50”, 2009. 

d. “Northwind 100: Community Scale Wind Turbine”, Northern Power Systems, 

2009. 

17 Frick, Bob, untitled presentation, AWEA Small & Community Wind Conference & 

Exhibition, November 5, 2009, Detroit, MI. 

18 Wilke, Steve, “The Evolution of the Bergey Excel wind turbine,” AWEA Small & 

Community Wind Conference & Exhibition, November 5, 2009, Detroit, MI. 

19 Kruse, Andy, personal communication, AWEA Small & Community Wind Conference 

& Exhibition, November 5, 2009, Detroit, MI. 

20 AWEA, Small Wind Turbine Global Market Study: Year Ending 2008, p. 10. 

21 Ibid, p. 15. 

22 U.S. Department of Energy, 20% Wind Energy by 2030: Meeting the Challenges, 

Proceedings of the Workshop, October 6-7, 2008, pp. 13-25. 

23 Kwartin et al., pp. 33-34, for commercial/industrial turbines and unpublished data for 

residential turbines. Values in Table 7 represent a 2/3 weighting for residential scale 

turbines and a 1/3 weighting for commercial/industrial scale turbines. 
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