Technical Report

NREL/TP-550-46916

December 2009

Preliminary Assessment of the 

Energy-Saving Potential of 

Electrochromic Windows in 

Residential Buildings 

David R. Roberts
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

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

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-550-46916

December 2009

Preliminary Assessment of the 

Energy-Saving Potential of 

Electrochromic Windows in 

Residential Buildings 

David R. Roberts

Prepared under Task No. BET98001
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iii

Executive Summary

Electrochromic (EC) windows provide variable tinting that can help control glare and 

solar heat gain. We used BEopt software to evaluate the performance of EC windows in 

prototypical energy models of a single-family home in Atlanta. The windows were 

assumed to operate automatically, tinting in response to incident solar during the cooling 

season. The models predict the EC windows will produce whole-house source energy 

savings of 9.1% and whole-house electricity demand savings of 13.5% in a 2006 IECCcompliant home, and source energy savings of 3.2% and demand savings of 10.3% in a 

50%-level Building America home. 
iv

Contents

Executive Summary ........................................................................................................... iii

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

Approach ............................................................................................................................. 1

Prototype Model Definitions........................................................................................... 1

DOE-2 Modeling Capabilities ........................................................................................ 2

Impact of Seasonal Control ............................................................................................. 4

Response to Controls ...................................................................................................... 5

Results ................................................................................................................................. 6

Savings in a 2006 IECC-Compliant Home ..................................................................... 6

Savings in a 50% Building America Home .................................................................... 8

Cost-Competitive EC Windows .................................................................................... 10

Conclusion ........................................................................................................................ 11

Future Work ...................................................................................................................... 11

References ......................................................................................................................... 12
v 

Figures

Figure 1. EC window tinting response to total solar incident on window ....................... 3

Figure 2. Heating and cooling source energy use in Atlanta reference home .................. 4

Figure 3. EC window tinting for each hour of the year—west-facing window ............... 5

Figure 4. EC window tinting for each hour of the day ..................................................... 6

Figure 5. Fully tinted equivalent hours for each hour, by orientation .............................. 6

Figure 6. BEopt output for source energy savings for EC windows relative to low-e, 

low-SHGC windows ......................................................................................... 7

Figure 7. Electricity demand for cooling with EC windows and code-compliant 

reference windows in 2006 IECC home on July 2 and 3 .................................. 8

Figure 8. BEopt output for source energy savings from EC windows in 50% Building 

America home ................................................................................................... 9

Figure 9. Electricity demand for cooling (compressor and fan) with EC windows and 

reference windows in 2006 IECC home on July 2 and 3 ................................ 10

Figure 10. BEopt output. EC glazing is competitive at approximately $20/ft2

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

Tables

Table 1. General Characteristics of the Prototype Energy Models ................................. 1

Table 2. Prototype Model Performance Levels .............................................................. 2

Table 3. Characteristics of SageGlass EC Glazing and DOE-2.2 Window Library 

Entry .................................................................................................................. 3

Table 4. Source Energy Impacts of Disabling EC Control During Heating Season 

(MMBtu/yr)....................................................................................................... 4

Table 5. Energy Cost Impacts of Disabling EC Control During Heating Season ($/yr)........ 5

Table 6. Source Energy Savings From EC Windows in 2006 IECC home (MMBtu/yr) ... 7

Table 7. Energy Cost Savings From EC Windows in 2006 IECC Home ($/yr) ............. 7

Table 8. July 3 Peak Electricity Demand Savings From EC Windows in 2006 IECCCompliant Home (kW) ..................................................................................... 8

Table 9. Source Energy Savings From EC Windows in 50% Building America Home 

(MMBtu/yr)....................................................................................................... 9

Table 10. Energy Cost Savings From EC Windows in 50% Building America Home 

($/yr) ................................................................................................................. 9

Table 11. July 3 Peak Electricity Demand Savings From EC Windows in 50% Building 

America Home (kW) ....................................................................................... 10
1 

Introduction

Electrochromic (EC) windows provide variable tinting that can help control glare and 

solar heat gain. The windows darken in response to changes in applied electrical voltage1

This report summarizes the results of a simple energy modeling exercise conducted to 

assess the impact EC windows might have in residential applications. 

and can be controlled manually or automatically. EC windows are available 

commercially, though currently their use is generally limited to commercial building 

applications and residential skylights. A study by Lee et al. (2006) that focused on 

commercial building applications reported significant energy and demand savings, based 

on empirical testing and energy modeling. 

Approach

Prototype Model Definitions 

We used energy modeling to assess the potential impact of installing EC windows in a 

single-family home and used the BEopt2

Table 1

software to develop and analyze prototype 

single-family home models. General characteristics of the prototype models are shown in 

. 

Table 1. General Characteristics of the Prototype Energy Models

Conditioned floor area 2,500 ft2

Conditioned volume 22,500 ft3

Number of stories 2 

Number of bedrooms 3 

Gross above-grade wall area 2,574 ft

2

Window area 450 ft2 (18% of conditioned floor area) 

Window orientation 50% west, 25% east, 12.5% north, 12.5% south

Window interior shading factors 0.85 winter, 0.7 summer* 

Door area 40 ft2

Set point temperatures 71 heating, 76 cooling

Mechanical ventilation Exhaust only, ASHRAE 62.2 levels

Space/water heating fuel Natural gas

Internal gains Building America Benchmark (Hendron 2008)

Lighting/appliance/plug schedules Building America Benchmark

* This is the standard assumption in Building America, HERS, and IECC analysis.

 

1 Nominally averages 0.5 W/m2 of glazing (Sbar 2009). 2 The BEopt software tool was developed at NREL to identify optimal building energy designs aimed at 

minimizing the total of the amortized cost of improvements and the cost of energy. It produces designs that 

minimize combined construction and energy costs by using the DOE-2.2 and TRNSYS energy simulation 

programs to automate a sequential search technique for locating least-cost solutions on a path toward net 

zero energy. The software and underlying methodology are described in detail by Christensen et al. (2005, 

2006) and Horowitz et al. (2008).
2 

Atlanta was selected as an appropriate climate for assessing the energy-savings potential 

of EC windows, as it produces fairly balanced and substantial heating and cooling loads 

in residential buildings. Solar gain is beneficial at times because it reduces heating loads 

in the winter and undesirable at times as it contributes to cooling loads in the summer. 

Two prototypical models were developed with the general characteristics outlined in 

Table 1, but differing performance characteristics. The first meets the requirements of the 

2006 International Energy Conservation Code (IECC) for Atlanta, climate zone 3. The 

second includes performance characteristics of an Atlanta home using 50% less source 

energy than the Building America Benchmark, as described by Anderson and Roberts 

(2008). The performance characteristics of both are shown in Table 2. 

Table 2. Prototype Model Performance Levels

Component 2006 IECC 50% Building America

Walls 2 × 4, R-13 cavity 2 × 6, R-21 cavity 

Ceiling R-30 R-40

Infiltration 0.00050 SLA 0.00015 SLA

Window U-value 0.65 0.30

Window SHGC 0.40 0.26

Lighting 14% fluorescent 90% fluorescent

Air conditioner SEER 13 SEER 15

Furnace 80 AFUE 92 AFUE

Ducts R-8, in crawl space In conditioned space

Water heater 0.59 EF 0.77 EF

Appliances Standard ENERGY STAR®

AFUE = Annual Fuel Utilization Efficiency

EF = Energy Factor

SEER = Seasonal Energy Efficiency Ratio

SHGC = Solar Heat Gain Coefficient

SLA = Specific Leakage Area

DOE-2 Modeling Capabilities

The BEopt software was modified to accommodate and model EC windows in the DOE2.2 simulation engine. DOE-2.2 provides a broad range of EC windows and controls. An 

EC window that best matches those currently available3

Table 3

was selected from the DOE-2.2 

window library (see ). 

 

3 Sage Electrochromics, Inc. is currently the only producer of commercially available EC glazing.
3 

Table 3. Characteristics of SageGlass EC Glazing and DOE-2.2 Window Library Entry

U-Value SHGC

Sage* DOE-2** Sage* DOE-2** 

Clear state 0.29 0.29 0.48 0.51

Tinted state 0.29 0.29 0.09 0.13

* SageGlass ClassicTM (source: Sage Electrochromics Web site: www.sage-ec.com/pages/technol.html) 

** DOE-2 2 library entries 2842/2843.

EC window controls available in DOE-2.2 include: 

• Direct or total solar incident on the window 

• Direct or total solar transmitted through the glazing 

• Total horizontal solar 

• Outside temperature 

• Space load

• Interior daylight level. 

These controls can also be combined with a seasonal/hourly time schedule.

We selected total solar incident on the window for control, as it allows the windows on 

each orientation to respond independently and is straightforward to implement in the 

field. 

DOE-2.2 provides for low and high limits for the control variable. We assumed that 

tinting begins when solar incident on the window exceeds 100 Btu/h·ft2; the window is

fully tinted at 150 Btu/h·ft2

. Figure 1 shows how the window tinting responds to incident 

solar in the DOE-2.2 model. 

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300

Total Incident Solar (Btuh/sf)

Fraction Switched 

(1 = Fully Tinted)

Figure 1. EC window tinting response to total solar incident on window
4 

Impact of Seasonal Control

As mentioned earlier, it is beneficial to maximize solar gain during the heating season. 

Figure 2 shows the monthly source energy use for space heating and cooling for the 

Atlanta prototype model with standard glazing. Ideally, the EC windows would not 

reduce solar gain during periods with large heating loads and small cooling loads. We 

“locked out” the EC window control during the heating season in the BEopt/DOE-2 

model and examined the impact. Based on the heating and cooling loads in Figure 2, the 

EC control was fixed in the “Clear” mode from October 15 through April 15, and then 

switched into “Auto” mode during the balance of the year. In “Auto” mode the glazing 

responds to total solar incident on the window.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0

5,000

10,000

15,000

20,000

25,000

30,000

Cooling (E) (Btu/hr)

Cooling (E)

Heating (G)

Figure 2. Heating and cooling source energy use in Atlanta reference home

The energy and cost impacts associated with locking out the EC windows during the 

heating season are shown in Table 4 and Table 5. It is clearly beneficial to lock out 

automated window tinting during the heating season. Based on these results, automated 

EC control is assumed to function from April 15 to October 15 only. 

Table 4. Source Energy Impacts of Disabling EC Control During Heating Season 

(MMBtu/yr)

Heating* Cooling* Total 

Automated year-round 71.2 29.3 100.5

Automated April 15 to October 15 67.1 29.9 97.0

Absolute savings 4.2 –0.6 3.6

Percent savings 5.9% –2.1% 3.5%

* Includes fan energy
5 

Table 5. Energy Cost Impacts of Disabling EC Control During Heating Season ($/yr)

Heating* Cooling* Total 

Automated year-round 1105 202 1307

Automated April 15 to October 15 1040 206 1246

Absolute savings 65 –4 60

Percent savings 5.9% –2.1% 4.6%

* Includes fan energy

Response to Controls

Figure 3 shows the level of window tinting on the west-facing windows during each hour 

of the year. The windows do not tint during the lock-out period between April 15 and 

October 15. 

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000 8000

Hour of the Year

Fraction Switched 

(1 = Fully Tinted)

Figure 3. EC window tinting for each hour of the year—west-facing window

Figure 4 shows window tinting for all orientations by hour of the day. This graph 

demonstrates how the windows tint in response to the position of the sun throughout the 

day. 
6 

0

0.2

0.4

0.6

0.8

1

1.2

1 6 11 16 21

Hour of the Day

Fraction Switched 

(1 = Fully Tinted)

Front/East

Left/South

Right/North

Back/West

Figure 4. EC window tinting for each hour of the day

Figure 5 quantifies the level of tinting that occurs on each orientation using “fully tinted 

hours”—the sum of all tinting that occurs throughout the year. As one might expect, with 

winter hours locked out, most tinting occurs in east- and west-facing windows. The 

north-facing windows never tint. 

0

20

40

60

80

100

120

140

160

1 6 11 16 21

Hour of Day

Fully Tinted Equivalent Hours

Front/East

Left/South

Right/North

Back/West

Figure 5. Fully tinted equivalent hours for each hour, by orientation

Results

Savings in a 2006 IECC-Compliant Home

Results of BEopt runs compare the source energy of the 2006 IECC prototype home with 

EC windows to the home with code-level windows (see Figure 6). The EC windows 

result in 9.1% savings in whole-house source energy. Table 6 shows 18.2% source 

energy savings for the heating and cooling end uses; Table 7 shows analogous cost 

savings of 19.5%.
 7 

Misc. (G)

Hot Water (G)

Heating (G)

Cooling (E)

Cool Fan (E)

Heat Fan (E)

Lights (E)

Lg. Appl. (E)

Misc. (E)

Source Energy Use (MBtu/yr)

36.3

28.3

32.2

28.4

77.9

19.9

36.3

28.3

32.2

25.7

61.5

19.9

0

50

100

150

200

250

Ref 1 - Design

236.6

215.0

Figure 6. BEopt output for source energy savings for EC windows relative to low-e, lowSHGC windows 

Table 6. Source Energy Savings From EC Windows in 2006 IECC home (MMBtu/yr) 

Heating* Cooling* Total 

2006 IECC-compliant home 85.2 33.3 118.5 

2006 IECC with EC windows 67.1 29.9 97.0 

Absolute savings 18.1 3.4 21.5 

Percent savings 21.3% 10.2% 18.2% 

* Includes fan energy

Table 7. Energy Cost Savings From EC Windows in 2006 IECC Home ($/yr) 

Heating* Cooling* Total 

2006 IECC-compliant home 1319 230 1549 

2006 IECC with EC windows 1040 206 1246 

Absolute savings 279 23 302 

Percent savings 21.1% 10.2% 19.5% 

* Includes fan energy

The BEopt/DOE-2 model predicts a peak-day electricity demand reduction of 0.7 kW, a 

13.5% reduction in whole-house demand. Figure 7 shows the electricity demand for 

cooling for the two days with the highest predicted demand. Table 8 shows the demand 

savings at 7:00 p.m. on July 3, the peak hour for the year. 
8 

0

0.5

1

1.5

2

2.5

3

3.5

4

7/2 0:00 7/2 12:00 7/3 0:00 7/3 12:00 7/4 0:00

EC

Ref

Figure 7. Electricity demand for cooling with EC windows and code-compliant reference 

windows in 2006 IECC home on July 2 and 3

Table 8. July 3 Peak Electricity Demand Savings From EC Windows in 2006 IECCCompliant Home (kW)

Cooling* Whole-House

2006 IECC-compliant home 3.7 5.5

2006 IECC with EC windows 3.0 4.7

Absolute savings 0.7 0.7

Percent savings 19.9% 13.5% 

* Includes fan energy

Savings in a 50% Building America Home

Results of BEopt runs comparing the source energy of the 50% Building America 

prototype home with EC windows, to the home with low-e, low-SHGC, argon-filled 

windows, are shown in Figure 8. The EC windows result in 3.2% savings in whole-house 

source energy. Table 9 shows 8.2% source energy savings for the heating and cooling end 

uses; Table 10 shows analogous cost savings of 9.4%.
 9 

Misc. (G)

Hot Water (G)

Heating (G)

Cooling (E)

Cool Fan (E)

Heat Fan (E)

Lights (E)

Lg. Appl. (E)

Misc. (E)

Source Energy Use (MBtu/yr)

36.3

24.9

13.4

18.5

31.1

12.1

36.3

24.9

13.4

17.7

27.7

12.1

0

30

60

90

120

150

Ref 1 - Design

142.9

138.3

Figure 8. BEopt output for source energy savings from EC windows in 50% Building 

America home 

Table 9. Source Energy Savings From EC Windows in 50% Building America Home 

(MMBtu/yr) 

Heating* Cooling* Total 

50% Building America home 33.6 21.1 54.7 

50% Building America home with EC windows 30.0 20.2 50.2 

Absolute savings 3.6 0.9 4.5 

Percent savings 10.7% 4.2% 8.2% 

* Includes fan energy

Table 10. Energy Cost Savings From EC Windows in 50% Building America Home ($/yr)

Heating* Cooling* Total 

50% Building America home 524 145 670 

50% Building America home with EC windows 468 139 607 

Absolute savings 57 6 63 

Percent savings 10.8% 4.2% 9.4% 

* Includes fan energy

The BEopt/DOE-2 model predicts a peak-day electricity demand reduction of 0.4 kW, a 

10.3% reduction in whole-house demand. Figure 9 shows the electricity demand for 

cooling for the two days with the highest predicted demand. Table 11 shows the demand 

savings at 7:00 p.m. on July 3, the peak hour for the year. 
10 

0

0.5

1

1.5

2

2.5

7/2 0:00 7/2 12:00 7/3 0:00 7/3 12:00 7/4 0:00

EC

Ref

Figure 9. Electricity demand for cooling (compressor and fan) with EC windows and 

reference windows in 2006 IECC home on July 2 and 3 

Table 11. July 3 Peak Electricity Demand Savings From EC Windows in 50% Building 

America Home (kW)

Cooling* Whole House

50% Building America home 2.2 3.4

50% Building America home with EC windows 1.8 3.1

Absolute savings 0.4 0.4

Percent savings 16.4% 10.3%

* Includes fan energy

Cost-Competitive Electrochromic Windows

We used the Atlanta 2006 IECC-compliant prototype model to run BEopt with a number 

of above-code features to identify the optimal non-EC windows for this home. BEopt 

identified low-e, low-SHGC, argon-filled windows as the most cost-effective upgrade at 

$16/ft2

. This window was selected as an appropriate “competing technology” and the 

basis of this analysis. These same windows are in 50% Building America home.

To determine the cost at which EC windows will be competitive, we ran BEopt with EC 

windows at different price points: from $40/ft2 down to $10/ft2. Figure 10 shows the 

BEopt curves for these windows along with the “competing technology”: low-e, lowSHGC, argon-filled at $16/ft2

. The graph indicates that EC windows would have to reach 

a price point of approximately $20/ft2 before they would be competitive with this 

window. EC windows currently cost $50–$100/ft2

; residential applications are on the 

higher end (Sbar). 
 11 Mortgage + Utilities ($/yr)

 

Source Energy Savings (%)

0

600

1200

1800

2400

3000

3600

4200

4800

5400

6000

0 10 20 30 40 50 60 70 80 90 100

Low-e low SHGC arg

Electrochromic $40

Electrochromic $20

Electrochromic $10

Electrochromic $30

$20/ft2 ECbecomes

cost competitive

2006 IECC

Figure 10. BEopt output. EC glazing is competitive at approximately $20/ft2.

Conclusion 

A simple modeling study was undertaken to assess the potential energy savings of EC 

windows in residential applications. We used BEopt software to evaluate the performance 

of EC windows in prototypical energy models of a single-family home in Atlanta. The 

windows were assumed to operate automatically, tinting in response to incident solar 

during the cooling season. The model predicted the EC windows would produce wholehouse source energy savings of 9.1%, and whole-house electricity demand savings of 

13.5% in a 2006 IECC-compliant home and source energy savings of 3.2% and demand 

savings of 10.3% in a 50%-level Building America home. Comparing the cost and 

modeled performance of EC windows to “competing technology” indicates EC windows 

would be competitive at a price point of approximately $20/ft2

. 

Future Work 

Suggested future work to further assess the potential impact of EC windows in residential 

applications includes: 

 Additional modeling 

o Additional climates 

o Compare performance to orientation-tuned glazing 

o Examine other control strategies 

o Optimize controls 

 Monitoring installed performance 

o Test facility 

o Unoccupied lab homes 

o Occupied homes. 
12 

References

Anderson, R.; Roberts, D. (2008). Maximizing Residential Energy Savings: Net Zero 

Energy Home (ZEH) Technology Pathways. Golden, CO: National Renewable Energy 

Laboratory, NREL/TP-550-44547. 

Christensen, C.; Horowitz, S.; Givler, T.; Courtney, A.; Barker, G. (2005). BEopt: 

Software for Identifying Optimal Building Designs on the Path to Zero Net Energy.

Golden, CO: National Renewable Energy Laboratory, NREL/CP-550-37733. 

Christensen, C.; Anderson, R.; Horowitz, S.; Courtney, A.; Spencer, J. (2006). 

BEopt(TM) Software for Building Energy Optimization: Features and Capabilities.

Golden, CO: National Renewable Energy Laboratory, NREL/TP-550-39929. 

Horowitz, S.; Christensen, C.; Brandemuehl, M.; Krarti, M. (2008). Enhanced Sequential 

Search Methodology for Identifying Cost-Optimal Building Pathways. Golden, CO: 

National Renewable Energy Laboratory, NREL/CP-550-43238. 

International Code Council, Inc. (2006). 2006 International Energy Conservation Code. 

ISBN-13: 978-1-58001-270-6. 

Lee, E.; Selkowitz, S; Clear, R; DiBartolomeo, D; Klems, J; Fernandes, L; Ward, G; 

Inkarojrit, V.; Yazdanian, M. (2006). Advancement of Electrochromic Windows, 

Sacramento, CA: California Energy Commission, PIER, 500-01-023, LBNL-59821. 

Sbar, N. (2009). Vice President Technology, Sage Electrochromics, Inc. Personal 

communication, October 23. 
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Preliminary Assessment of the Energy-Saving Potential of 

Electrochromic Windows in Residential Buildings

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DE-AC36-08-GO28308

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6. AUTHOR(S)

D.R. Roberts

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NREL/TP-550-46916

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14. ABSTRACT (Maximum 200 Words)

Electrochromic (EC) windows provide variable tinting that can help control glare and solar heat gain. We used BEopt 

software to evaluate the performance of EC windows in prototypical energy models of a single-family home in 

Atlanta. The windows were assumed to operate automatically, tinting in response to incident solar during the cooling 

season. The models predict the EC windows will produce whole-house source energy savings of 18.2% and wholehouse electricity demand savings of 13.5% in a 2006 IECC-compliant home, and source energy savings of 8.2% and 

demand savings of 10.3% in a 50%-level Building America home.

15. SUBJECT TERMS

electrochromic; ec; windows; tinting; glare; solar heat gain

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