NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Historical Trends in Wind 

Energy Operations Costs and 

Project Availability

September 2012—July 2013

DNV KEMA

Seattle, Washington

NREL Technical Monitor: Eric Lantz

Subcontract Report

NREL/SR-6A20-62875

August 2018
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Historical Trends in Wind 

Energy Operations Costs and 

Project Availability

September 2012—July 2013

DNV KEMA

Seattle, Washington

NREL Technical Monitor: Eric Lantz

Prepared under Subcontract No. AFT-2-22435-02

Suggested Citation

DNV KEMA. 2018. Historical Trends in Wind Energy Operations Costs 

and Project Availability. Golden, CO: National Renewable Energy 

Laboratory. NREL/SR-6A20-62875.

https://www.nrel.gov/docs/fy18osti/62875.pdf.

Subcontract Report

NREL/SR-6A20-62875

August 2018
NOTICE

This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable 

Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding 

provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Wind Energy 

Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the 

U.S. Government.

This report is available at no cost from the National Renewable 

Energy Laboratory (NREL) at www.nrel.gov/publications.

U.S. Department of Energy (DOE) reports produced after 1991 

and a growing number of pre-1991 documents are available 

free via www.OSTI.gov.

Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795.

NREL prints on paper that contains recycled content.
iv

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Table of Contents

1 Introduction ...........................................................................................................................................1

2 Project Data and Data Classification ..................................................................................................3

2.1 General Description of Project Data ..............................................................................................3

2.2 Classifying the Data.......................................................................................................................7

2.3 Characterization of the Projects.....................................................................................................9

2.4 Data Summary .............................................................................................................................11

3 Database Design and Implementation..............................................................................................12

3.1 Step 1: Synchronizing Start Date.................................................................................................12

3.2 Step 2: Allocating Annual Data by Year of Operation ................................................................12

4 Data Analysis.......................................................................................................................................15

5 Results .................................................................................................................................................17

5.1 O&M Cost/Availability/Component Replacements Summary....................................................17

5.2 Statistical Analysis Results..........................................................................................................24

5.3 Summary of Responses to Qualitative Questions........................................................................29

6 Conclusions.........................................................................................................................................33

7 Recommendations..............................................................................................................................35

List of Figures

Figure 2-1. Operating data provided to DNV KEMA for each project, synchronized by operating year

..............................................................................................................................................................11

Figure 3-1. OpEx database schema .........................................................................................................13

Figure 5-1. Total component replacements per 100 MW for all projects in the database..................18

Figure 5-2. Total component replacements per 100 turbines (%) for all projects in the database ...18

Figure 5-3. Average turbine rating, all projects......................................................................................19

Figure 5-4. Aggregate replacements per 100 MW for all projects in the database.............................19

Figure 5-5. Aggregate replacements (%) for all projects in the database ...........................................20

Figure 5-6. OpEx per MW for all projects ................................................................................................20

Figure 5-7. OpEx per turbine for all projects ..........................................................................................20

Figure 5-8. Average total OpEx per MW, all projects .............................................................................21

Figure 5-9. Average total OpEx per turbine, all projects .......................................................................21

Figure 5-10. Average turbine O&M cost per replacement, all projects................................................22

Figure 5-11. Average OpEx by COD.........................................................................................................22

Figure 5-12. Annual average project availability, all projects ...............................................................23

Figure 5-13. Annual average production per MW, all projects..............................................................23

Figure 5-14. Blade replacements (%): maximum, mean, minimum, and one-standard deviation (error

bars)......................................................................................................................................................24

Figure 5-15. Generator replacements (%): maximum, mean, minimum, one-standard deviation (error 

bars)......................................................................................................................................................24

Figure 5-16. Gearbox replacements (%): maximum, mean, minimum, one-standard deviation (error

bars)......................................................................................................................................................25

Figure 5-17. Main bearing replacements (%): maximum, mean, minimum, and one-standard deviation 

(error bars)...........................................................................................................................................25

Figure 5-18. Total replacements (%): maximum, mean, minimum, one-standard deviation (error bars)

..............................................................................................................................................................25

Figure 5-19. Number of turbines included in the statistical analysis of component replacements .26

Figure 5-20. Turbine O&M costs per MW: maximum, mean, minimum, one-standard deviation (error 

bars)......................................................................................................................................................26

Figure 5-21. BOP O&M costs per MW: maximum, mean, minimum, one-standard deviation (error bars)

..............................................................................................................................................................27

Figure 5-22. Soft O&M costs per MW: maximum, mean, minimum, one-standard deviation (error bars)

..............................................................................................................................................................27

Figure 5-23. Total O&M costs per MW: maximum, mean, minimum, one-standard deviation (error bars)
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..............................................................................................................................................................27

Figure 5-24. Total megawatts included in the statistical analysis of OpEx.........................................28

Figure 5-25. Turbine availability: maximum, mean, minimum, one-standard deviation (error bars) 28

Figure 5-26. Overall availability: maximum, mean, minimum, one-standard deviation (error bars).28

Figure 5-27. Total number of projects included in the statistical analysis of availability..................29

List of Tables

Table 2-1. Basic Parameters for Wind Turbine Classes ..........................................................................3

Table 2-2. Typical Turbine O&M Cost Categories ....................................................................................5

Table 2-3. Typical Balance-of-Plant O&M Costs.......................................................................................5

Table 2-4. Typical Soft Costs......................................................................................................................6

Table 2-5. Criteria for Selecting Data Sets for O&M Analysis .................................................................7

Table 2-6. Classification Matrix ..................................................................................................................8

Table 2-7. General Summary of Projects...................................................................................................9

Table 2-8. Number of Projects Reporting in Each Classification Bin...................................................10

Table 3-1. Categories for the Master Query Table..................................................................................14

Table 4-1. Additional Derived Parameters for Data Analysis................................................................15
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Acknowledgements

This work was conducted for the National Renewable Energy Laboratory by DNV KEMA under 

subcontract (AFT-2-22435-02). This work was overseen by the Alliance for Sustainable Energy, 

LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. 

Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding for this work 

was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy 

Wind Energy Technologies Office. The views expressed in this report do not necessarily represent 

the views of the Department of Energy or the U.S. government. The U.S. government retains a 

nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce this work, or allow 

others to do so, for U.S. government purposes.
vii 

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Preface

The work detailed herein was completed in 2013 and references both current and recent trends and 

data as of that date. Given the time elapsed between the completion of the work and this 

publication, readers should be aware that these references apply primarily to the period of time

during and prior to 2013 and may no longer be considered either current or recent. 

The work has been published at this delayed date as a reference volume for current and future 

research activities in wind power operations and maintenance. 

Parallel work was undertaken by Garrad Hassan America, Inc. (GL GH) in the same time frame. 

Subsequently, DNV KEMA and GL GH merged to form the DNV GL Group. Certain analysis 

methodologies and assumptions may have changed since the original research was conducted.

Recent thought leadership on related topics from DNV GL may be found at: 

https://www.dnvgl.com/energy/publications. 
1

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1 Introduction

The National Renewable Energy Laboratory (NREL) retained DNV KEMA Renewables, Inc. 

(DNV KEMA) to study the historical trends in wind energy operation and maintenance (O&M) 

costs, component replacement trends, and project availability. This scope of work is governed by 

the NREL Subcontract No. AFT-2-22435-02, dated September 27, 2012, under Prime Contract No. 

DE-AC36-08GO28308.

Gaining a greater understanding of the factors that impact cost, major component replacements, and 

availability of utility-scale wind projects is an important step in the effort to improve wind project

operations. Once a project has been installed and the capital investment made, the primary variable

operators have to control costs is in how efficiently and reliably they can run their projects. The 

three factors of operational expenditures (OpEx), replacements of major turbine components, and 

project availability are tightly interrelated. If a component fails, that puts a turbine out of service 

and availability suffers. If a condition monitoring system is implemented to improve preventative

maintenance then operating expenses go up, but availability may also improve and thereby offset 

the cost increase with more energy production. Conversely, if an operator decides to reduce 

operating expenses by cutting corners on maintenance activities, then turbines may be less reliable, 

which can reduce availability and subsequently revenues. With more projects coming online, and 

many coming off warranty within the next several years, operators are increasingly interested in 

optimizing their processes, maximizing energy production, and minimizing downtime and 

equipment failures. One important step to aid this process is to gain insight into the historical trends 

in OpEx, component replacements, and wind project availability.

DNV KEMA’s access to operating, cost, and availability data from a wide range of utility-scale 

wind projects provided the foundation for this study. Specifically, O&M cost, component 

replacement, and availability data for 5 gigawatts (GW) of installed capacity over a period of up to

14 years were extracted and organized. These data, which cover more than 50 North American 

projects were then used to establish a baseline database and analysis tool to begin analyzing trends. 

By definition, analysis of historical trends is backward looking and, as such, data reported include 

turbine types of an array of vintages (commissioned between 1999 and 2009), some of which may 

be more or less comparable to commercial technology being installed at the time of this study. 

Although the aggregate data used in this project, along with the accompanying data analysis tool, 

represent a significant step forward, it is DNV KEMA’s opinion that this initial effort should be 

viewed as a foundation from which future work should continue and that additional data should be 

brought forth and analyzed as it becomes available.

To mitigate the limitations of a relatively small data set, DNV KEMA also reached out to industry

owners/operators both to verify that the preliminary analysis and results were consistent with their

experiences, and to survey them regarding their thinking on issues affecting operational costs.

The raw data used in this study were not made public because of confidentiality restrictions;

however, an array of summary tables and figures are provided in this report. Summary statistics 

across a number of parameters are also presented.
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The overall objectives of this study were to:

• Develop a historical OpEx baseline grounded in a representative sample of empirical

data from North America

• Enhance the understanding of whether and how technological advancements have

impacted project operational expenditures

• Serve as a starting point to inform modeling efforts designed to quantify the impact of

future technological advancements on wind energy operational expenditures

• Provide NREL with a summary of empirical OpEx data from a large data set of operating

wind power plants in the United States and provide significant insights into some of the

variables that impact annual operational expenditures.

This report presents the assumptions, methodology, results, conclusions, and recommendations of

DNV KEMA’s work.
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2 Project Data and Data Classification

2.1 General Description of Project Data

The primary objective of this study was to establish a baseline of historical operational 

expenditures, component replacements, and availability trends for wind projects, with the goal of 

better understanding the variables that impact these metrics. Executing this task required the 

extraction and organization of the relevant project level data from various sources. Project level 

statistics were captured for several parameters including:

• Project Age. Defined as the time elapsed since the commercial operation date (COD), in 

which the COD is the date that all turbines in a given project have been commissioned and 

are available to operate. Project age is computed monthly, and then annualized based on the 

COD. Thus, for a project with a June COD, the first year of operation is from June through 

July of the following year.

• Turbine Size. Turbines are classified by power rating, which gives an indication of a

turbine’s size. Turbine size generally scales with power rating, although rotor diameter can 

be and frequently is scaled independent of power rating. Turbine size is also roughly 

indicative of vintage, because larger turbines tend to be newer. To protect the confidentiality 

of project data, specific turbine models were not identified in this report.

• Turbine Class. Turbines are classified by the wind turbine class of each model as defined in 

International Electrotechnical Commission (IEC) standard 61400-1: Ed. 3. IEC Class 

provides an indication of the operating envelope (average and peak wind speed, turbulence 

intensity) for which a given turbine is designed. In general, turbines designed for lower 

average wind conditions, such as IEC Class III, tend to be newer, more advanced models 

than those designed for stronger winds (IEC Class I). Table 2-1 shows the basic parameters 

for each wind turbine class.

Table 2-1. Basic Parameters for Wind Turbine Classes

Wind Turbine Class I II III S

Vref (m/s) 50 42.5 37.5 Valuesspecified

by the designer Vave (m/s) 10 8.5 7.5

A Iref 0.16

B Iref 0.14

C Iref 0.12
4

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where

Vref is the reference wind speed average over 10 minutes Vave is the 10-minute average wind 

speed

A designates the category for higher turbulence characteristics

B designates the category for average turbulence characteristics 

C designates the category for lower turbulence characteristics

Iref is the expected (average) value of turbulence intensity at 15 m/s.

• OpEx. General OpEx cost accounting does not categorize overhead costs such as central 

control facilities and management in a consistent manner, and balance-of-plant (BOP) 

maintenance costs can be heavily influenced by who has responsibility for substation and 

transmission line maintenance. OpEx are not uniformly defined across all owners/operators, 

and in some cases, not even from year to year for the same owner/operator. Although the 

quantitative data DNV KEMA reviewed generally included itemized breakdowns of costs, 

the subcategories varied from project to project. After examining OpEx data from many 

projects, DNV KEMA determined that the only consistent categorization across all projects 

was the three broad categories: turbine O&M costs, BOP O&M costs, and “soft” costs. The 

items typically included in these broad OpEx categories are:

o Turbine O&M. This category consists of annual costs related specifically to the 

turbines, including all components from the base of the tower and up, as well as the 

transformer. Turbine O&M comprises all aspects of maintenance, both scheduled 

and unscheduled. It may contain turbine warranty costs unless those are already 

included as part of a turbine supply agreement (TSA). TSA warranties typically 

cover parts and labor for unscheduled maintenance. Scheduled maintenance by the 

original equipment manufacturer (OEM) during the warranty period is typically 

covered under a service and maintenance agreement for an annual fee. Table 2-2 

summarizes the cost categories related to turbine O&M, which vary depending on 

whether the turbine is under warranty, under an OEM or third-party service and 

maintenance agreement, or being maintained by the operator.
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Table 2-2. Typical Turbine O&M Cost Categories

Category Notes

Turbine warranty Most projects include a warranty on all turbine-related maintenance for the first 2‒5

years of operation. Scheduled maintenance is typically covered by an annual service

maintenance fee. Unscheduled maintenance is typically covered under the warranty, 

which may be embedded in the original turbine supply contract, depending on specific 

terms and conditions negotiated at the time of sale.

Turbine operations/service and

m aintenance fee/management

operationsfee

Includes turbine-related maintenance, wages and salary, overtime, bonus,

burdening, medical/life insurance, and other costs as well as scheduled and

unscheduled maintenance.

Condition monitoring system repairs May include equipment; may or may not be included in O&M service agreement

Manufacturer’sincentive payment Not included in warranty or O&M service agreement

Equipmentrentals Includes cranes and other heavy equipment

Supervisory control and data

acquisition (SCADA) system

maintenance

Materials and supplies

Small tools and equipment

Turbine parts

o Balance-of-Plant O&M. This category consists of costs related to maintaining 

facilities and infrastructure from the base of the towers down (including the turbine 

foundations), and from the grid side of the transformer up to and including the 

substation. For some projects, additional transmission line maintenance may also be 

included. Balance-of-plant O&M costs generally include the electrical collection 

system, maintenance of all equipment and equipment facilities, storage, maintenance 

and office buildings, and waste removal. Table 2-3 shows the typical balance-ofplant O&M costs.

Table 2-3. Typical Balance-of-Plant O&M Costs

Collection system maintenance

Equipment maintenance

Equipment purchase

Equipment rental

Facility expense

O&M building rent

O&M service

Road maintenance

Substation maintenance

T-Line maintenance

Waste management
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o Soft costs. These costs represent other costs that are unrelated to the daily operations 

of a project, such as insurance, land leases and royalties, scheduling fees assessed by 

grid operators, legal fees, and environmental and community expenses and taxes. 

Table 2-4 provides a list of items included in the soft costs category.

Table 2-4. Typical Soft Costs

Audit compliance

Bank fees

Community involvement

Environmental expense

Fiscal management fees

Forecasting fees

Franchise tax

Insurance

Land lease and royalties

Legal fees

Miscellaneous expense

Payment in Lieu of Taxes (PILOT)

Project management fees

Property tax

Schedule imbalance charges

Scheduling fees

Tax compliance

Telecommunications expense

Transmission operator charges

Utility expense

Vehicle expense

• Replacements. Refers to replacement of five major turbine components that require a crane 

for removal and reinstallation. Major components, for the purposes of this study, include 

blades, gearbox, generator, main bearing, and an uptower transformer. Uptower or other 

minor repairs of those five components were not tracked, nor were any other replacements.

• Availability. Wind turbines can only produce electricity when they are able to operate. 

When they are unable to operate, they are considered “unavailable” and the lost electricity 

production associated with the downtime is called an availability loss. Lost production can 

be caused by issues with the turbine itself (such as routine maintenance downtime, fault 

downtime, or downtime caused by component failures) or with issues that go beyond the 

turbine, such as a facility’s electrical infrastructure, the power grid, or weather-induced 

downtime. In this study, “turbine availability” is understood as the percentage of time the 

turbine is available to operate, regardless of the external conditions. Turbine availability is 

also known as OEM availability or contractual availability. Curtailment is not included in 

turbine availability calculations. “Overall availability” is the total percentage of time when
7

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both the turbines and the facility are available to operate, regardless of the wind speed. 

Project downtime that is the result of curtailment will show up in overall availability.

The quality and resolution of the original data sources imposed some limitations on the database. 

For example, none of the OpEx data sets provided a distinction between fixed versus variable costs, 

or scheduled versus unscheduled maintenance; therefore, the database does not include that level of 

resolution. Whenever possible, DNV KEMA included the date (month and year) of warranty 

expiration, although for some projects such data were not available.

2.2 Classifying the Data

Several criteria that projects were required to meet to be included in this study were developed 

jointly by DNV KEMA and NREL. The criteria were selected to maximize the potential for 

achieving the objectives of the work while keeping the volume of data manageable within the scope 

of the study. At a minimum, each project data set in this study was required to include a project size 

of at least 20 turbines, a log of major component replacements and when those occurred, turbine 

and project availability, operational costs, as well as a minimum of 2 years in service. Among the 

projects that met these minimum criteria, DNV KEMA sought out data sets that also included: at 

least 1 year of data beyond the expiration of the turbine equipment warranty, a breakdown of 

downtime attributed to component failures (e.g., how long to remove/replace components, how long 

until the replacement parts were available, and so on), availability by turbine (not just aggregate 

project availability), and operating costs by component. In addition, DNV KEMA sought out highquality data sets that exhibited consistency of data formatting and reporting. This latter criterion in 

particular was quite difficult to enforce, as the breakdown of cost by category for many of the 

operations reports varied from year to year and in some cases from month to month. Table 2-5 

summarizes the criteria for selecting data sets for this study.

Table 2-5. Criteria for Selecting Data Sets for O&M Analysis

Project Criteria Required Preferred

Time in service 2 years 1 year after warranty

Project size 20 turbines

Data Requirements

Major component repair/replacement X Breakdown of downtime

Availability by manufacturer/balance of plant X Availability by turbine

Operation costs X Cost by component

To address the objectives of this study while preserving the confidentiality of the data sets, 

DNV KEMA categorized the data by several metrics, including project size, turbine size, number of 

years the project has been in operation, aspects of turbine technology, and IEC wind class. The 

researchers subsequently classified each project using turbine rating and number of turbines in the 

project as the sorting criteria. Each parameter was divided into three bins, as defined in Table 2-6.
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Table 2-6. Classification Matrix

Metric Bin 1 Bin 2 Bin 3

Project size 20‒50 megawatt (MW) 50‒100 MW > 100 MW

Turbine rating 0.6‒1.25 MW 1.26‒1.8 MW > 1.8 MW

Turbine IEC class I II III

Years of operation 2‒3 3‒6 >6

The goal was to size each bin to ensure that the data remained anonymous yet retained meaningful 

and statistically significant results. Turbine size bins were set to include submegawatt turbines [0.6‒

1.25 megawatts (MW)], turbines in the 1.5-MW range (1.26‒1.8 MW), and multimegawatt turbines 

(greater than 1.8 MW). Project size was binned based on “small” (less than 50 MW), “medium” 

(50‒100 MW), and “large” (greater than 100 MW). Projects were also binned based on years of 

operation. For this case, Bin 1 was set at 2‒3 years, which captures the end of the TSA warranty and 

the transition to a different service and maintenance arrangement. Bin 2 was set to 3‒6 years, which 

is the period after the warranty has expired yet prior to the onset of most failures. The third bin 

under “years of operation” is for projects that have been in operation more than 6 years, when a 

project is considered mature. Final binning was established after evaluating all available data and in 

consultation with NREL.

DNV KEMA extracted the O&M data used for this analysis from over 50 projects representing 

more than 5 GW of installed capacity. All projects considered are located in North America. Of 

those total projects, 30 met the defined criteria laid out in Table 2-5. The data were in a variety of 

forms and formats including spreadsheets, monthly reports, and quarterly reports. Some reports

changed the O&M cost category names and definitions multiple times over the course of a project.

Changes in the categorization of turbine O&M occurred when:

• Owners changed service providers, transitioning from an OEM to a third-party service

provider. In these instances cost categories often changed from a single service and

maintenance fee for a comprehensive contract that covered all turbine O&M activities to

itemized categories for service fees, unscheduled maintenance, and parts and crane costs.

• Projects transitioned from a third-party service provider to self-operated. In these situations,

the O&M cost categories changed from service and maintenance fees and incentive

payments to detailed lists of internal costs including wages and salaries, overtime, bonuses, 

burden, medical/life insurance, as well as a more detailed breakdown including equipment, 

tools, parts, and consumables.

Because of the lack of consistency in reporting turbine O&M when transitioning a project from 

OEM-operated to third-party-operated to self-operated, it was not possible to break turbine O&M 

costs into subcategories. Nor was it possible to assign OpEx costs to scheduled or unscheduled 

maintenance. The situation was similar with balance-of-plant OpEx when service and maintenance 

changed from a third-party provider to self-operated.
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Additional challenges presented by the operating reports included:

• Switching from site-reported OpEx to centralized accounting systems. In a centralized 

accounting system, some costs were categorized only as “site maintenance” or “damage 

repairs,” with no indication of whether costs were associated with turbine O&M or balanceof-plant OpEx. In these cases, DNV KEMA contacted the operator to clarify the nature of 

the expense so that they could properly categorize it in the database.

• Some project financials presented expenses in general categories, such as operation and 

maintenance, depreciation, amortization and accretion, general and administrative, and taxes 

other than income, with an accompanying report describing the significant contributors to 

each expense category. For these projects, DNV KEMA filtered the identified expenses into 

the turbine, BOP, and soft categories.

• Component replacements were generally logged on a monthly basis, whereas OpEx was

typically provided annually. For some projects, turbine availability was recorded monthly on 

a per-turbine basis, whereas for others only annual availability for all turbines was provided.

2.3 Characterization of the Projects

DNV KEMA collated the data, which were compiled into a common format for input into a

Microsoft Access custom database. Table 2-7 summarizes some general facts about the 30 projects, 

which amounts to 1,895 turbines with a total installed capacity of 2.5 GW. The turbines range in 

size from 600 kW to 3.0 MW and comprise 16 different turbine models from seven of the major 

turbine manufacturers.

Table 2-7. General Summary of Projects

Total number of projects 30

Total number of turbines 1,895

Total installed capacity 2,489 MW

Number of turbine models 16

Number of turbine manufacturers 7

Range of turbine ratings 600 KW to 3,000 KW

Range of rotor diameter 47 m‒93 m

Earliest COD January 1999

Latest COD March 2009

Number of project years of data 150

Table 2-8 indicates the number of projects that fell into each of the different bins defined above in 

Table 2-6. DNV KEMA’s goal was to have a minimum of five projects in each bin, which was 

achieved for project size, turbine operation, and years of operation, but not for IEC class. None of 

the 30 projects in this study included Class III turbines. In addition, because the latest COD in the 

database is March 2009, recent advanced turbine models that have been deployed since that time are 

not represented.
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Table 2-8. Number of Projects Reporting in Each Classification Bin

Metric Bin 1 Bin 2 Bin 3

Project size 10 13 7

Turbine rating 11 12 7

Turbine IEC class 7 23 0

Years of operation 20 16 7

The projects in this study had CODs dating as far back as 1999 and as recently as 2009, with project 

ages ranging from 2 to 14 years. DNV KEMA’s operating data for these projects cover the period 

from January 2001 to July 2012, and vary from project to project. For 20 projects, DNV KEMA

received operating data beginning from the COD. For the remainder, however, the data received did 

not cover project operations from inception. Most of the data in the database fell within the 10-year 

period from 2001 through 2011, and seven projects provided data through mid-2012.

Figure 2-1 shows the period of operation for each project, synchronized by operating year. This 

representation shows how many projects are reporting in each year of operation in the database. The 

dark yellow portions of the bars indicate the periods for which DNV KEMA received operating, 

OpEx, and availability data. In addition, the figure indicates the end of warranty (EOW) for most 

projects. Half of the projects in the database have 5-year warranties, whereas only four projects 

have 2-year warranties and the rest either have a different arrangement or EOW information was 

not available. Figure 2-1 also provides significant insight into the composition of the OpEx 

database. Notably, the database includes considerably more data for the first 6 years of project life, 

with relatively less data beyond 10 years.
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Figure 2-1. Operating data provided to DNV KEMA for each project, synchronized by operating year

2.4 Data Summary

In this study, it was challenging ensuring that data from different projects and time periods were 

handled in a consistent way. DNV KEMA analyzed the various data sources and applied their best 

judgment in assembling the data tables; however, the qualitative nature of the operating reports and 

the variations in the cost categories introduce some uncertainty into the database. This uncertainty

could be reduced by: 1) expanding the number of projects in the database, 2) instituting uniform

operating, OpEx, and availability data reporting within the wind industry, and 3) automating the

reporting of operational data to facilitate data extraction.

Y Y Y3

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

X

X

X

X

Project in operation

Period for which DNV KEMA

obtained operating data

X EOW

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

EOW01/21

EOW01/21
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3 Database Design and Implementation

The OpEx database was organized around three main factors: major component replacements 

(major interventions), operating expenses, and turbine availability. All data were synchronized by 

year of operation in a two-step process described below.

3.1 Step 1: Synchronizing Start Date 

Each project had a different start date and so the data had to be synchronized in some way, 

organized either by year of operation or by calendar year. Because the COD does not always occur 

conveniently at the start of a new year (January 1), the database was set up to record data by year of 

operation, regardless of what month a project began operations. Examining the data by year of 

operation is useful for determining trends that tied to time in service, such as the impact of going 

out of warranty. DNV KEMA considered comparing project data by calendar year; however, this 

was complicated by the fact that the COD can occur at any time in the year, as a result, the first year 

of data may include fewer than 12 months of records. Furthermore, if partial-year records were 

excluded simply because the COD does not occur on January 1, then important O&M trends that 

occur early in a project’s life (i.e., pre-EOW) would be missed. On the other hand, retaining partialyear records in the database presents its own problems. Most notably, it could lead to biased results 

because partial-year data would be treated the same as full-year data. For these reasons, DNV 

KEMA elected to synchronize all the data by year of operation.

3.2 Step 2: Allocating Annual Data by Year of Operation

To compare projects from different calendar years, time stamps were converted first to the number 

of months from the COD and then aggregated into each year of operation. For example, a project 

with a July COD that is in its fifth year of operation could experience some replacements that occur 

during the 7 months of calendar year five and others that occur during the 5 months of calendar year 

six. Similarly, the operating expenses, turbine output, and availability time stamps were grouped 

according to year of operation. Finally, with all three data sets grouped by operational year, they

were joined by Project ID and year into one master query for further analysis (discussed below).

Figure 3-1 shows a schematic of the tables that comprise the OpEx database, as well as the 

associated interconnections among the tables.
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Figure 3-1. OpEx database schema

The OpEx database was developed to produce a master query table with the categories as defined in

Table 3-1.
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Table 3-1. Categories for the Master Query Table

Master Query from Access Parameters Definitions

Project ID Unique ID assigned to each project reporting

Year of operation Operating year of the project, where year one begins on the COD

Blades Number of blade replacements per year

Gearbox Number of gearbox replacements per year

Generator Number of generator replacements per year

Main bearing Number of main bearing replacements per year

Transformer Number of uptower transformer replacements per year

Production (megawatt hours [MWh]) Total annual net energy production

Average turbine availability Annual average project-wide turbine availability, %; turbine

availability generally includes all turbine-related downtime,

component replacements, and scheduled maintenance

Average site availability Annual average overall project availability, %; overall

availability includes ALL downtime-related events at a project,

such as collection-system maintenance or failures, weatherrelated incidents, and curtailment; in general, overall

availability will be lower than turbine availability

Replacements and costs; number of months Number of months in a given year of operation for which

there is component replacement and OpEx data

Turbine Turbine-related OpEx, $, referenced to year of operation

BOP BOP-related OpEx, $, referenced to year of operation (e.g., 

roads, maintenance facilities, snow removal, electrical

collection system)

Soft "Soft" OpEx, $, referenced to Year of Operation (e.g., 

insurance, taxes, lease payments)

Operational year costs; number of months Number of months in a given year of operation for which

there is OpEx data

Project name Commercial name of the project

Number of turbines Total number of turbines installed at each project

Capacity (kilowatts [kW]) Total capacity of each project

Average machine size kW Average turbine rating at each project; where there is more than

one turbine model, the weighted average turbine rating is used

Calendar year of operation Calendar year of project operation (always begins on January 1)

Project size (MW) Category defining the project size classification (see Table

2-6)

Turbine size (MW) Category defining the turbine rating classification (see

Table 2-6)

Start date O&M reporting Month/year of first O&M report

End date O&M reporting Month/year of last O&M report
15

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4 Data Analysis

DNV KEMA completed an initial analysis of the data to address issues and ensure an “apples to 

apples” comparison of data from different projects, as described previously. The 

MasterQueryFromAccess table derived from the OpEx database described earlier and shown in 

Figure 3-1 was imported into a Microsoft Excel workbook and further processed to derive the 

additional parameters listed in Table 4-1.

Table 4-1. Additional Derived Parameters for Data Analysis

Additional Derived Parameters Definitions

Average turbine availability Average turbine availability, in which overall (site) availability is used 

to fill missing records, where possible

Average site availability Average overall (site) availability, in which turbine availability is used 

to fill missing records, where possible

Project size for availability (megawatts 

[MW])

Total MW of installed projects

# turbines for availability Number of turbines for which the database has availability

Production per MW Annual production divided by total project size (MWh/MW)

Installed capacity (MW) Installed capacity

IEC class Category defining the IEC class for which the turbine model was

designed

Total operating cost per MW Annual average total OpEx (sum of turbine+BOP+soft costs)

divided by the project capacity

Total operating cost per turbine Annual average total OpEx (sum of turbine+BOP+soft costs)

divided by the number of turbines at a project

Total # of component replacements Total annual major component replacements

(blades+gearbox+generator+main bearing+transformer)

Cost per replacement Total annual OpEx divided by total # of component replacements

Blades per 100 turbines Number of blade replacements per 100 turbines

Gearboxes per 100 turbines Number of gearbox replacements per 100 turbines

Generator per 100 turbines Number of generator replacements per 100 turbines

Main bearing per 100 turbines Number of main bearing replacements per 100 turbines

Total replacements per 100 turbines Number of blade replacements per 100 turbines

Turbine O&M per MW Annual turbine O&M divided by project rating

Turbine O&M per turbine Annual turbine O&M divided by number of turbines in a project

BOP per MW Annual BOP OpEx divided by project rating

BOP per turbine Annual BOP OpEx divided by number of turbines in a project

Soft OpEx per MW Annual Soft OpEx divided by project rating

Soft OpEx per turbine Annual Soft OpEx divided by number of turbines in a project

From the master table and additional parameters, we developed a series of interactive tables and

graphs showing trends in operations-related metrics throughout the lives of wind projects. The user

can elect to bin the data according to project size, turbine rating, or IEC class.
16

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Prior to deriving the results tables, the MasterQueryFromAccess table was filtered to remove partial 

years of data. Care was taken to ensure that in cases where OpEx or availability data were missing, 

or in other instances of partial records, the empty cells were ignored when calculating statistics.

The final results tables and charts were organized into the OpEx-Replacements-Availability-Trends

(ORAT) workbook,1 which consists of eight worksheets as follows:

• Notes. General notes about the ORAT workbook and how to use it.

• MasterQueryTable-Definitions. Tables listing definitions of all of the parameters used in 

the MasterQueryFromAccess table, as well as the additional derived parameters. Also 

includes a table defining the bin sizes for classifying the data.

• Three worksheets include tables and charts showing component replacements, OpEx, 

availability, and production data over a project’s life:

o QueryA-ProjSize–interactive tables, filtered by project size

o QueryB-Rating–interactive tables, filtered by turbine rating

o QueryC-IEC–interactive tables, filtered by IEC Class

• Three worksheets provide summary statistics (max, min, mean, and one-standard deviation

error bars on the mean):

o Stats-OpEx–statistics on turbine, BOP, soft and total OpEx

o Stats-Replacements–statistics on blade, gearbox, generator, and main bearing

replacements

o Stats-Availability–statistics on turbine and overall availability.

 1 This workbook is available from NREL upon request. 
17

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5 Results

As described in Section 4, the ORAT workbook that accompanies this report (upon request) and is 

an additional deliverable for this project enables the user to interactively view trends in the data 

over time, as well as the statistical variability of the data set. The database, though somewhat 

limited in volume, is sufficient for making preliminary observations about the historical trends in 

project operations, costs, and availability. It provides tables and charts of aggregated OpEx, 

component replacement, and availability data that allow the user to examine trends over time based 

on project size, turbine size, IEC class, and project age as defined in the classification matrix shown 

in Table 2-6. The following sections present some initial results.

5.1 O&M Cost/Availability/Component Replacements Summary

Figure 5-1 and Figure 5-2 show the number of component replacements that occur each year per 

100 MW and as a percentage of total units, respectively, for all projects in the database. Five major 

components were tracked in the database, and four are shown in the charts: blades, gearbox, 

generator, and main bearing. The fifth, uptower transformers, is not shown in the charts because the 

data set contained a negligible number of uptower transformer replacements. For each component, 

total replacements in each year of operation were tallied. The blue vertical bars in Figure 5-1 

indicate the total installed megawatts for each year of operation. Data for years one through three

are pulled from an annual average of 2 GW of installed capacity, which represents approximately 

5% to 10% of the installed capacity in the United States over the data recording period from 2005 to 

2010. In years 6-10, the data are sourced from an average installed capacity of 550 MW, also 

representing approximately 5% to 10% of the installed capacity in the United States for projects of 

that vintage. 

Beyond year 10 the total installed capacity drops to 100 MW, which represents approximately 5% 

of the total installed capacity of projects with more than 10 years of operation. Although on a 

relative basis the percent of projects represented in this database is similar over time, the small 

absolute sample size beyond year 10 is insufficient to draw meaningful conclusions at this time. The 

green vertical lines in Figure 5-2 show the corresponding number of turbines represented in each 

year of project life, which ranges from approximately 1,600 turbines during operating years one

through three down to 80 in year 13, again emphasizing that results from later years of operation 

(i.e., older projects) have a high degree of uncertainty. This uncertainty can only be mitigated by 

incorporating more long-term project data into the database. Each of the graphs presented below 

includes an indication of how much data was available for each operating year.

The graphs below include all the data in the ORAT (all projects). This report focuses on what the 

database is revealing as a whole in terms of general trends, and leaves further investigation into 

trends with project size or turbine rating for future reporting.
18

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Figure 5-1. Total component replacements per 100 MW for all projects in the database

Figure 5-2. Total component replacements per 100 turbines (%) for all projects in the database

Because of the uncertainties resulting from the limited number of records beyond year 10, 

observations regarding trends in O&M costs, component replacements, and availability are limited 

to the first decade of operation. Figure 5-1 and Figure 5-2 show that gearbox replacements peak in 

years five and eight, after which they appear to decrease gradually over time (on a per-turbine 

basis). The peak in year five coincides with the EOW for many of the projects in the database, 

suggesting that peak gearbox replacement and the EOW are related. Generator replacements 

increase through year seven and then begin to decrease. When examined on a per-turbine basis, 

however, generator replacements are more or less constant from year eight. Figure 5-3 shows the 

average turbine rating in the database over time. The downward trend indicates that older projects in 

the database are equipped with smaller turbines, and because the average rating of the older turbines 

is less than 1 MW, the component replacement rate per turbine is lower than the replacement rate 

per MW. Over time, if more data from land-based wind projects are added to this database, the 

overall average turbine rating over the entire project life will increase, because turbine sizes have 

been increasing. Thus, relative trends per turbine would also shift.

Blade replacements shown in Figure 5-1 and Figure 5-2 indicate some initial replacement activity 

very early in life, then an increasing trend through year five to an average of four blade 

replacements per 100 turbines, then decreasing over the next 5 years to one replacement per 100 

turbines. Blade replacements increase again in years 11 and 12. Main bearing replacements did not 
19

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show up until year four, after which they gradually increased to two per 100 turbines per year by 

year 13.

Figure 5-3. Average turbine rating, all projects

Looking at the component replacement data in aggregate in Figure 5-4 and Figure 5-5 we see 

similar trends. With the exception of blades, the percentage of major component replacements is 

relatively low in the first 2 years of project life (see Figure 5-4). Current owners/operators 

expressed some surprise at the low failure rates in the first 2 years of operation. The low 

replacement rates may be the result of new equipment, or it may reflect a lack of reporting by 

OEMs when turbines are under warranty. After year two replacements increase and peak to 

approximately 16% in year five, mainly driven by gearbox failures. Year five coincides also with 

the EOW for the majority of projects in the database. Project owners often hire an independent third 

party to conduct EOW inspections of the turbines 6 to 9 months prior to expiration. It is speculated

that the high number of replacements near the EOW may be the result of EOW inspection findings 

resulting in warranty claims. Beyond year 10 there are insufficient data to identify a trend with any 

certainty.

Figure 5-4. Aggregate replacements per 100 MW for all projects in the database
20

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Figure 5-5. Aggregate replacements (%) for all projects in the database

The OpEx for each project is plotted in Figure 5-6 and Figure 5-7. The turbine and soft costs are 

generally of similar magnitude, and show a gradual increase over time when looked at on a per-MW

basis, but remain constant or decrease slightly when looked at on a per-turbine basis because the 

average turbine rating decreases to below 1 MW after year 10. BOP costs are significantly below 

the turbine and soft costs, and show a decreasing trend over project life; however, given the lack of 

OpEx data beyond year nine, it is premature to conclude whether these lower costs are typical, or 

simply a function of the particular long-term data used in this study.

Figure 5-6. OpEx per MW for all projects

Figure 5-7. OpEx per turbine for all projects
21

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The total OpEx per turbine is shown in Figure 5-8 and Figure 5-9. In Figure 5-8, total OpEx per 

MW is fairly constant at $40,000 per MW during the first 4 years of project life, increasing to 

$50,000 per MW by year six and remaining fairly constant through year 13, which is the oldest date 

for which we had data. When looked at on a per-turbine basis (Figure 5-9), total average OpEx 

starts at $60,000 per turbine during the first few years (which also corresponds to the warranty 

period) and appears to decrease over the turbine life. This apparent reduction does not suggest that 

for a given turbine model OpEx will decrease over the project life, but rather that in this database 

the projects that are the oldest also have considerably smaller turbine models with correspondingly 

lower OpEx per turbine. Taking both Figure 5-8 and Figure 5-9 together would suggest that OpEx 

costs for a specific turbine model are likely to rise somewhat over the life of the project.

Figure 5-8. Average total OpEx per MW, all projects

Figure 5-9. Average total OpEx per turbine, all projects

The OpEx data in the database do not specify explicit component replacement costs; however, by 

dividing turbine O&M per year by the total number of component replacements per year—for all 

five component types—we gained some insight into average (total) component replacement costs. 

The results are plotted in Figure 5-10. In years one and two, the turbine O&M per replacement 

averages $600,000, which is significantly more than in subsequent years where it drops to 

approximately $200,000 to $300,000 per replacement. The high average costs per event in years 

one and two evidently reflect the fact that new machines tend not to experience very many failures, 

with the exception of a serial failure or other “teething pains.” In addition, many turbines are under 

warranty for at least the first 2 years of the project life, so the warranty fee or service maintenance 

fee is paid whether or not failures occur. Note that the average cost per replacement does not 
22

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represent the actual cost to replace a particular component, but simply gives the average turbine 

O&M per replacement.

Figure 5-10. Average turbine O&M cost per replacement, all projects

In addition to examining OpEx as a function of operating year, we also looked at OpEx as a 

function of installation date to learn, for example, whether it is more or less expensive to maintain a 

project installed in 2007 compared to one installed in 2000. Figure 5-11 shows average OpEx per 

MW as a function of COD. The vertical bars show the number of project years of data reporting for 

each COD year. BOP costs have not changed significantly for more recent projects, and soft costs 

appear to have fallen in the early 2000s, but have risen since 2005. Turbine O&M, however, seems 

to have declined nearly every year from 1999 to 2008. Overall, average total OpEx has been more 

or less steady since 2003, but the distribution of cost seems to be shifting away from turbine O&M 

and toward soft costs. Figure 5-11 shows no data for 2002 because none of the projects in the 

database had a COD in that year.

Figure 5-11. Average OpEx by COD

The trends in availability and production are shown in Figure 5-12 and Figure 5-13. Availability 

data in this study were limited to the 30 projects, for which we also had OpEx and component 

replacement data, and indicate somewhat lower turbine and overall availability during the first 11 

years of operation than we have observed from wider availability studies. Beyond year 11 

availability appears to jump considerably; however, as mentioned previously the small sample size 

is not representative. Some projects reported only turbine availability, and other projects reported 

only overall (site) availability. In those cases, the reported availability was used for both turbine and 
23

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site availability. Consequently, turbine availability in Figure 5-12 is lower than the actual, whereas 

overall availability is higher than the actual. Adding more availability data to the database, where 

both turbine and overall availability are provided for each project, will improve the accuracy and 

decrease uncertainty in availability trends over project operating life.

Figure 5-12. Annual average project availability, all projects

Figure 5-12. Annual average project availability, all projects

Figure 5-13. Annual average production per MW, all projects

Figure 5-13 shows annual average production per megawatt, where the data have been split out by 

project size. There is not a clear impact of project size on production per MW, but after year nine all 

project sizes evidently are trending less productive. This is generally in line with the decrease in 

availability through year 11 (shown in Figure 5-12).

Several trends can be gleaned from the results shown above. First, the component replacement data 

(Figure 5-4 through Figure 5-5) show two periods of increased activity, the first occurring around 

year five (corresponding to the EOW), driven by gearbox replacements, and the second occurring 

around year 11, driven by both gearbox and blade replacements. Second, the gradual downward 

trend in availability over the first decade of operation (Figure 5-12) corresponds approximately with 

the gradual decrease in average production over time, as seen in Figure 5-13. This trend also 

corresponds to a gradual increase in OpEx per MW over project life (shown in Figure 5-8). Taken 

together, these trends suggest that the cost of energy on average increases over the life of a project.
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Looking at the total OpEx per turbine shown in Figure 5-9, along with turbine rating in Figure 5-3,

we see that total OpEx costs correlate with average turbine rating.

5.2 Statistical Analysis Results

In addition to the summary graphs shown above, we also conducted a statistical analysis of several 

parameters (on a per project basis) to shed light on the variability and scatter in the data. Each plot 

below includes the mean (simple average), maximum, and minimum as well as error bars indicating 

±1- standard deviation. Because it is not possible for any of the parameters to be less than zero, all 

minima and error bounds are truncated at zero.

Figure 5-14. Blade replacements (%): maximum, mean, minimum, and one-standard deviation (error

bars)2

Figure 5-15. Generator replacements (%): maximum, mean, minimum, one-standard deviation (error 

bars)

 2 The max blade replacement percentage of 32% is reported for year one of operation; this value exceeds the 10% 

maximum set value for the y-axis to maintain visibility of blade replacement trends over the years of operation.
25

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Figure 5-16. Gearbox replacements (%): maximum, mean, minimum, one-standard deviation (error

bars)

Figure 5-17. Main bearing replacements (%): maximum, mean, minimum, and one-standard deviation

(error bars)

Figure 5-18. Total replacements (%): maximum, mean, minimum, one-standard deviation (error bars)

The statistics on blade replacements (Figure 5-14), generator replacements (Figure 5-15), gearbox 

replacements (Figure 5-16), and main bearing replacements (Figure 5-17) indicate considerable 

variability in failure rates from project to project. The standard deviation provides a measure of how 

anomalous the maximum/minimum values are. For example, the project with the maximum blade 

failures in year one replaced 32% of blades in just that single year, whereas in the average project 

2% of blades were replaced in year one, with a standard deviation of about three times the average. 
26

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Figure 5-18 shows that variability of total component replacements appears to decrease with project 

life, but Figure 5-19 shows that the pool of data as project age increases was very small.

Figure 5-19. Number of turbines included in the statistical analysis of component replacements

Summary statistics of the operational expenses, including maximum, minimum, mean and error 

bars showing the one-standard deviation are presented in Figure 5-20 through Figure 5-23. Total 

OpEx costs are dominated by the turbine and soft costs. Figure 5-20 and Figure 5-23 show a bump 

in turbine O&M costs in years six, eight, and 10. The increase in year six may be a result of the high 

level of component replacement that occurs in year five, because costs may not accrue precisely at 

the time of replacement.

Figure 5-20. Turbine O&M costs per MW: maximum, mean, minimum, one-standard deviation (error 

bars)
27

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Figure 5-21. BOP O&M costs per MW: maximum, mean, minimum, one-standard deviation (error bars)

Figure 5-22. Soft O&M costs per MW: maximum, mean, minimum, one-standard deviation (error bars)

Figure 5-23. Total O&M costs per MW: maximum, mean, minimum, one-standard deviation (error bars)
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Figure 5-24. Total megawatts included in the statistical analysis of OpEx

Figure 5-25 and Figure 5-26 show summary statistics for turbine and overall availability. In this

case, availability cannot exceed 100%, but of course performance can fall well below the mean. 

Interestingly, the biggest decreases in availability occur in years five and 11, which also 

correspond to the years with the greatest number of component replacements.

Figure 5-25. Turbine availability: maximum, mean, minimum, one-standard deviation (error bars)

Figure 5-26. Overall availability: maximum, mean, minimum, one-standard deviation (error bars)
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Figure 5-27. Total number of projects included in the statistical analysis of availability

In summary, although the database has known uncertainties and limitations, the statistical analysis 

reveals the potential for trends and correlations among the OpEx, replacement, and availability data 

for these projects.

5.3 Summary of Responses to Qualitative Questions

To gain insight into other factors that have or will influence OpEx in the future, DNV KEMA asked 

wind project owners/operators a series of questions regarding their experience with O&M and 

related costs. Surveys were distributed to operators controlling over 21 GW of installed wind power 

in North America, representing 30%‒35% of the U.S. wind market share. Operators included both 

utilities and developers/owners/operators. Only 50% of those polled responded to the survey. 

Although this response was less than anticipated, DNV KEMA believes that the results from this 

survey provided some useful insights that helped evaluate the trends seen in the historical data, and 

whether these trends are consistent with the survey respondents’ observations. Below is an 

aggregated summary of the responses.

1. How has condition monitoring (CM) impacted OpEx? How is CM expected to affect OpEx in 

the future? What does the initial evidence suggest with respect to new low wind speed 

technology?

o The general consensus by all who took the survey was that CM is generally seen as 

beneficial, whether it actually reduces OpEx or not.

o Some estimated a 2-year payback. Savings are primarily the result of doing uptower 

repairs versus having to do a complete retrofit or replacement that requires the use of 

a crane.

o Other operators are skeptical that CM saves on repairs, and see the early detection of 

problems as leading to premature repairs/replacement. Also, many problems are 

detected by means of a good preventive maintenance (PM) program and so do not 

need to rely on CM monitoring.

o CM facilitates better planned maintenance, thus avoiding unplanned maintenance.

o Others say OpEx costs are driven more by logistics than CM—particularly the 

availability of parts.
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o Some operators have retrofitted their fleet, or a portion of their fleet, with CM 

systems and are seeing sufficient benefits that they may install more.

o Average cost to retrofit has dropped to $5,000‒$6,000 per turbine.

o Vibration and temperature (e.g., main bearing) are monitored.

o One suggestion was to pose this question again in 5 years, asking each project 

operator what they would have done differently without CM.

2. How have more advanced turbine designs impacted OpEx? For example, does initial evidence

suggest that new low wind speed technology impacts OpEx, and if so, how?

o Some operators see lower OpEx as a result of the more advanced (newer) turbine

designs.

o Newer turbines tend to be easier to maintain and have higher reliability.

o The new technologies appear to have fewer gearbox failures and blade problems.

o Operators are definitely seeing the results of improved design—different heat 

treatment of major components (such as the main shaft), the use of off-line filtration, 

and better lubrication.

o On the other hand, machines are getting more complicated, particularly the control 

systems. Complexity leads to additional training requirements for technicians, which 

can increase OpEx costs. Also, increased complexity can result in reduced reliability.

o For low-wind speeds, the turbine designs are improved but the overall impact on 

OpEx is not yet clear.

o For low-wind turbines, operators adjust OpEx estimates for blades.

3. What has been the primary cause of missed OpEx estimates or forecasts? Why (or why not) 

have initial industry estimates changed over time?

o For some owner/operators, budget forecasts are not an issue because their projects 

are either still under warranty or they have service maintenance contracts in place, so 

their costs are known.

o Major cause of missed estimates is unexpected major maintenance, such as 

premature component failures.

o Other causes include lack of useful data when developing the budget, or lack of 

meaningful failure data from OEMs when turbines are transitioning off of warranty.

o For better budgeting, some operators use Weibull curves to establish failure rates.
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4. In the post-warranty period, can discretionary OpEx be used to reduce downtime? (i.e., does 

higher OpEx yield less downtime and result in a cost of energy benefit)?

o Some operators implement improvements that either expand the life of components 

or reduce downtime.

o There is an inflection point, but some operators only offer one level of service, so 

they are not developing customized schemes to achieve certain outcomes.

o Some operators have developed simple tools (Microsoft Excel) to assess issues and 

adjust criteria for when to call out a maintenance action, such as increasing blade 

maintenance and repairs; however, there is a point of diminishing returns on using 

OpEx to reduce downtime.

o Setting up service agreements with incentives that drive maintenance strategy (e.g., 

timing of service actions) to improve energy production is very effective.

5. How do these trends vary regionally, and what are the primary drivers of regional variability 

(e.g., workforce, wind resource, surface cover)?

o The labor force varies regionally. For example, it can be difficult to find workers in 

the southwestern United States.

o Operators with projects in many regions notice higher OpEx in areas with high 

incidence of lightning, or coastal areas where corrosion is an issue. Mountainous 

regions have higher OpEx because of extreme wind shear and turbulence. Operators 

now include such regional factors in their O&M models.

o Other operators do not see any significant regional impacts other than weatherrelated variables (e.g., snow).

o Operators do not see regional impacts on OpEx resulting from crane or spare part

availability.

6. What strategies are currently being used to minimize operating, maintenance, and replacement

costs and/or maximize availability? How have OpEx strategies evolved over time (i.e., 

owner/operator services, third-party contractor services, OEM services, insurance providers)?

o Strategies have evolved from run-to-failure (RTF) mode to preventative maintenance

mode.

o A key step has been to align goals between the project owner and the service 

provider.

o One trend has been the move from time-based to energy/yield-based availability.

o Now, all maintenance is proactive condition-based monitoring, including 

vibration/oil analysis, data-oriented system monitoring, and regular borescoping and 

other inspections.

o CM program, performance monitoring, 24/7 monitoring of turbine status, remote 

reset of turbines—all these efforts to automate and monitor the turbines reduces 

operating costs.
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o Advantages of self-operating are not completely obvious. During the transition from 

warranty to post-warranty there is a knowledge gap that takes time and training to 

fill.

o Lowering OpEx is not always the driving factor. For some operators, well-trained 

technicians who are familiar with the technology are preferable to a lower-cost thirdparty provider that may deliver fewer services.

7. Have perceptions of technology reliability or lack thereof been reflected in the weighted

average cost of capital or the cost of debt and equity for more recent projects?

o There is a cost premium for top-of-the-line turbine technology, but cost of capital is 

definitely reduced.

o For less-reliable turbine models, lenders require a letter of credit and may impose 

higher costs for debt or equity financing.

o Proven technology is easier to finance than unproven technology. For large 

owner/operators who have access to their own capital, this may be less of an issue.

8. Other observations?

o Since 2008, independent service providers have been getting squeezed on cost,

mainly because of increased competition.

o OEMs are also experiencing more competition, which is driving them to lower their 

costs and provide longer terms for warranties and service contracts.

o Service is becoming more of a commodity, where the lowest price wins. This can be 

good for an owner/operator, although in some cases you get what you pay for.

o Taken together, these trends suggest that OpEx cost per kilowatt may be decreasing 

for newer projects.
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This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

6 Conclusions

DNV KEMA developed a database of operational expenditures, component replacements, and 

availability for a representative sample of empirical wind project data from 30 North American 

wind projects totaling 2.5 GW of installed power and ranging from 2 to 14 years old. A total of 16 

different turbine models were represented, with rated power ranging from 600 kW to 3.0 MW. In 

all, the database includes over 1,800 months of O&M, cost, and availability data spanning 150 

project years. In addition, DNV KEMA created ORAT, a database analysis tool, which can be used 

to explore the trends and correlations among OpEx, component replacements, and availability. The 

ORAT workbook contains no proprietary operating data.

The workbook has been designed to enhance the understanding of whether factors such as project 

size or technological advancements have impacted project OpEx. It has been set up to filter data by 

project size, turbine rating, and IEC class. In addition, the results show trends of evolving turbine 

technology over project life. Following the analysis of the data set used for this study DNV KEMA

concluded the following:

• The database and ORAT workbook can serve as a useful starting point to inform modeling 

efforts designed to quantify the impact of future technological advancements on wind 

energy OpEx. The data set implemented during the course of this study does have 

limitations, however, they are a result of the lack of data for the second decade of 

operations.

• The ORAT workbook provides a means of exploring empirical OpEx data from a large data 

set of operating wind power plants in the United States and provides significant insights into 

some of the variables that impact annual OpEx. Some of those insights include:

o In the first decade of operation, component replacements peak in operating year five,

which coincides with the EOW for the majority of the projects in the database. The

OpEx per-turbine data show a corresponding increase in OpEx in year five.

o Availability as well as overall production display a gradual decline starting in year 

three and extending past the first decade of operation. Availability also dips in year 

five, which may be a consequence of the increased component replacements. Beyond 

year 10 the trend is uncertain because of limited data.

o The summary statistics indicate considerable variability in OpEx among the different

projects.

• Additional and more detailed empirical OpEx data tied to specific operating and 

maintenance events such as scheduled and unscheduled actions, fixed versus variable costs, 

and cost of major component replacement/repair, would help improve the accuracy and 

utility of the ORAT workbook. Such detailed OpEx data was either not consistently reported 

or not available for this project.

• The proprietary database is designed to receive and analyze additional data as it becomes 

available. More data beyond operating year 10 is needed to evaluate trends over the entire 

20-year project life.
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This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

• The survey of current owners/operators revealed that they all find condition monitoring

beneficial for improving their O&M monitoring, and many are seeing positive impacts on

reducing OpEx.

• Most operators also see improvements in component reliability and lubrication schemes 

with new, more advanced turbine technology, which is reducing OpEx; however, increased 

complexity of control systems, which requires more highly-trained technicians, tends to 

drive OpEx up. Consequently, the impact of advanced turbine technology is not entirely 

clear.

• Regarding the OpEx database, operators were somewhat skeptical of the high component

replacement rates seen in year five, and also of the low component replacement rates in

years one and two.

• Those surveyed agreed that turbine O&M and soft costs are of similar magnitude, and both

tend to increase over the project life.

• Survey respondents did see the gradual decrease in availability during the first decade of

operation as reflective of their experience.
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This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

7 Recommendations

In the course of developing the OpEx, component replacement, and availability database and the 

ORAT workbook, we encountered the following issues that we recommend addressing in any 

future expansion of this work.

As mentioned several times in this report, DNV KEMA believes very strongly that the database 

and subsequent analysis capabilities and utility would be greatly improved by incorporating 

additional OpEx, component replacement, and availability data into the database. The data used 

in this pilot study covered a wide range of project sizes, turbine sizes, turbine technologies, and 

length of service of typical wind projects in North America; however, expanding the database 

would reduce uncertainty by adding many more data records to each data bin and provide 

insights specific to newer technology installed in more recent years.

DNV KEMA recommends incorporating higher-resolution data into the database, such as more 

detailed OpEx categories that clearly define scheduled versus unscheduled maintenance and tie 

costs to specific events like major component replacements.

Culling OpEx and component replacement data from monthly data reports can be an arduous 

task. Although some automated operations reporting is happening today, DNV KEMA suggests

that the wind industry establish mandatory standards, primarily electronic reporting guidelines to 

facilitate further analysis of operating data.