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T OF ENERGY

DEPARTMEN

U

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NITED STAT S OFA ERICA

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May 2005 • NREL/TP-550-37731

The Hathaway “Solar Patriot” House: A Case 

Study in Efficiency and Renewable Energy

Paul Norton 

National Renewable Energy Laboratory

Ed Hancock and Greg Barker

Mountain Energy Partnership

Paul Reeves

Partnership for Resource Conservation
National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337

Technical Report 

NREL/TP-550-37731 

May 2005 

The Hathaway “Solar Patriot” 

House: A Case Study in 

Efficiency and Renewable 

Energy 

Paul Norton 

National Renewable Energy Laboratory

Ed Hancock and Greg Barker

Mountain Energy Partnership 

Paul Reeves

Partnership for Resource Conservation 

Prepared under Task No. BET5.8004 
NOTICE

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iii 

Table of Contents 

1 Abstract...................................................................................................................... 1 

2 Introduction ................................................................................................................ 1 

 2.1 The Context ......................................................................................................... 1 

 2.2 The “Solar Patriot” House .................................................................................... 2 

2.3 The Hathaway House Specifications ................................................................... 3 

 2.4 Study Objectives and Overview ........................................................................... 7 

3 Short-Term Testing................................................................................................... 8 

3.1 Air Exchange Characteristics.......................................................................... 9 

3.2 Heat Pump Coefficient of Performance (COP) ............................................ 15 

3.2.1 Heating Mode COP .............................................................................. 15 

3.2.2 Cooling Mode COP .............................................................................. 16 

3.2.3 Water-Heating COP ............................................................................. 17 

3.3 Other Short-Term Measurements and Observations.................................... 21 

4 Long-Term Monitoring.............................................................................................. 21 

4.1 Monitoring System and Data being Collected.............................................. 21 

4.2 Results from the Entire Monitoring Period ................................................... 24 

4.3 Results from the Calendar Year 2002 and 2003.......................................... 29 

4.4 Average Hourly Profiles .............................................................................. 30 

5. Simulated Energy Performance ............................................................................... 37 

5.1 Improving Accuracy: Modeling Specific Systems............................................... 38 

5.2 Building Confidence: Comparing Monitored Data and the Simulation Results .. 38 

5.3 Putting Performance in Context: Simulation Results.......................................... 41 

5.4 What if . . . Investigating Possible Improvements............................................... 43 

6. Discussion................................................................................................................. 45 

6.1 On the Path to Zero Net Energy........................................................................ 45 

6.2 Systems Operations and Interactions ............................................................... 46 

6.3 Modeling and Monitoring Approach .................................................................. 46 

7. References................................................................................................................ 48
iv 

List of Figures 

Figure 1. The “Solar Patriot” home on the National Mall in Washington, D.C................ 2 

Figure 2. The Solar Patriot house at its permanent location in Purcellville, Virginia 

(photo by Ed Hancock) ................................................................................... 3 

Figure 3. Floor plans for the first and second floors of the Hathway House .................. 5 

Figure 4. Duck supply trunk entering attic and continuing to trunk above the framing for 

the finished space........................................................................................... 6 

Figure 5. Supply register for attic space (top photo) and supply duct descending to 

ceiling register for second floor (bottom photo) .............................................. 6 

Figure 6. Alden Hathaway and blown-in cellulose insulation in attic ............................. 7 

Figure 7. Blower door installed in the Hathaway House................................................ 8 

Figure 8. Comparison of tracer gas ACH measurements to ACH modeled using blower 

door ELA measurement................................................................................ 14 

Figure 9. Space-heating COP determined from monitored data (fan power not 

included)...................................................................................................... 16 

Figure 10. Space-cooling COP determined from monitored data (fan power not 

included)...................................................................................................... 17 

Figure 11. Coefficient of performance (COP) of heat pump for water heating.............. 18 

Figure 12. Domestic hot-water (DHW) COP determined from monitored data............. 19 

Figure 13. Total energy consumption by end use for the entire monitoring period - 

September 2001 to February 2004.............................................................. 24 

Figure 14. Monthly energy consumption by end use.................................................... 25 

Figure 15. Monthly electrical energy (AC) produced by the PV system........................ 26 

Figure 16. Monthly net electrical energy bought........................................................... 27 

Figure 17. Monthly electrical energy (DC) produced by each of the three PV 

arrays and average outdoor temperature (west and east are arrays 

on the roof of the home)............................................................................... 28 

Figure 18. Energy produced by the PV system and net energy bought from the grid for 

2002 and 2003............................................................................................. 29 

Figure 19. End-use percentages for 2002 and 2003 .................................................... 30 

Figure 20. Outdoor temperature average hourly profile................................................ 31 

Figure 21. First-floor indoor temperature average hourly profile .................................. 31 

Figure 22. Irradiance in the plane of the collector average hourly profile ..................... 32 

Figure 23. DC power from the PV system average hourly profile................................. 32 

Figure 24. Total power use average hourly profile ....................................................... 33 
v 

Figure 25. Net power bought from the grid average hourly profile ............................... 33 

Figure 26. Power used for space conditioning average hourly profile .......................... 34 

Figure 27. Heat-pump power used for water heating average hourly profile................ 34 

Figure 28. Hot water draw average hourly profile......................................................... 35 

Figure 29. Clothes washer power use average hourly profile....................................... 35 

Figure 30. Clothes dryer power use average hourly profile.......................................... 36 

Figure 31. “Other” power use average hourly profile........................................................ 

Figure 32. DOE2 model of the Hathaway House, the dark area simulates 

the shading of the home by the garage and breezeway roof ...................... 37 

Figure 33. Monitored data vs. simulation results for total energy using 

Building America assumptions and TMY2 data........................................... 39 

Figure 34. Monitored data vs. the adjust model results, whole-building energy use .... 40 

Figure 35. Energy use by end-use category, beyond the Hathaway prototype ............ 44 

List of Tables 

Table 1. Summary of Specifications for the Hathaway House...................................... 4 

Table 2. Results of Blower Door and Duct Testing..................................................... 10 

Table 3. Register and Exhaust Fan Flow Results ...................................................... 12 

Table 4 Flow-Plate Measurements of the Total Air-Flow Rate at the Air-Handler ..... 13 

Table 5. Energy Balance for Domestic Hot Water (DHW) System ............................ 20 

Table 6. The COP in Each Mode ............................................................................... 20 

Table 7. Data Monitored at the Hathaway House ...................................................... 23 

Table 8. Summary of End-Use Site Energy................................................................ 42 

Table 9. Summary of End-Use Source Energy and Savings...................................... 42 

Table 10. Energy Savings of Energy Efficiency Measures.......................................... 42 

Table 11. Energy Reduction by End-Use Compared to the Building America 

Benchmark................................................................................................... 43 
vi 

List of Terms 

AC alternating current 

ACH air changes per hour 

ASES America Solar Energy Society 

BEopt Building Energy Optimization simulation model 

BESTEST simulation software 

cfm cubic feet per minute 

COP coefficient of performance 

DC direct current 

DHW domestic hot water 

DOE U.S. Department of Energy 

DOE2 simulation software 

EF Energy Factor 

ELA equivalent leakage area 

EPA ENERGY STAR® Environmental Protection Agency Energy STAR program 

HVAC heating, ventilation, and air conditioning 

HP heat pump 

HSPF Heating Seasonal Performance Factor 

IR infrared 

LBNL Lawrence Berkeley National Laboratory 

MEC 1995 Model Energy Code 

NREL National Renewable Energy Laboratory 

Pa Pascal 

PV photovoltaic 

SEER Seasonal Energy Efficiency Rating 

SF6 sulfur hexafluoride 

TMY typical meteorological year 

TRNSYS TRaNsient SYstem Simulation program 

UA overall heat-transfer coefficient (U) times area (A) 

ZEH zero energy home 
vii 

Acknowledgments

We express our gratitude to George James and Ed Pollock, U.S. Department of Energy, for the 

leadership and resources needed to complete this work. Many thanks to Alden Hathaway and the 

whole Hathaway family for inviting us into their home for testing and monitoring. Many thanks 

also to Tim Merrigan and Craig Christensen of the NREL Zero Energy Homes team for their 

comments and insights. We are indebted to Ren Anderson, Mark Eastment, and Ron Judkoff of 

NREL for their thoughtful reviews and comments and Nancy Wells of NREL for her technical 

editing and document preparation work support. 

We would like to acknowledge the help of two James Madison University students – Ashley 

Abbott and Kenneth Armstrong for their valuable assistance in instrumenting the Hathaway 

house. 
viii 
1 

1. Abstract 

This report details the monitored and modeled performance of a solar home outside of 

Washington, D.C. We modeled the home energy performance using DOE2.2, performed 

numerous short-terms tests on the home, and monitored its occupied performance for 29 months. 

The home used modular construction, solar water heating, a ground-coupled heat pump, efficient 

appliances, and compact fluorescent lighting to reduce its energy consumption by 35% compared 

to the Building America research benchmark home. The addition of 6kW of photovoltaics (PV) 

increased the savings to 67% compared to the Building America research benchmark. A more 

efficient shell to reduce space conditioning loads would have brought the home closer to its zero 

energy goals. However, even with efficient lighting and appliances, the lights, appliance and 

plug loads were a significant energy consumer. About 4 kW of PV was required to meet the 

needs of these loads alone. To achieve the zero energy goal with no further efficiency increases, 

the Hathaway house would need about 2.6 kW of PV in addition to the 6.0 kW it now has. 

Applying advanced efficiency measures available or being developed, such as heat-recovery 

ventilation, superinsulation, and electrochromic windows, could reduce the heating, cooling, and 

domestic hot water (DHW) energy use to less than 1700 kWh per year, an 88% reduction in 

these loads from the Building America Benchmark. At this efficiency level, the appliance and 

plug loads come to dominate energy consumption and account for nearly 70% of the total energy 

use. This analysis points out that even with highly effective energy-savings technologies (pushed 

beyond levels currently practical), whole-house energy use reduction by efficiency measures is 

only about 60% without also reducing the energy use of appliances and plug loads largely 

considered outside the designer’s jurisdiction. 

2. Introduction 

This report details the monitored and modeled performance of a solar home outside of 

Washington, D.C. The home uses modular construction, solar water heating, a ground-coupled 

heat pump, efficient appliances, and compact fluorescent lighting to reduce its energy 

consumption by 35% compared to the Building America research benchmark home1

. The 

addition of 6kWp of photovoltaics (PV) increased the savings to 67% compared to the Building 

America research benchmark. 

2.1 The Context 

How clean is clean enough? How efficient is efficient enough? These will be among the defining 

questions of the 21st century. As the human population pushes past 6 billion on the way to 9 

billion by 20502

 we are faced with increasing environmental consequences . . . mass species 

extinction; toxic air, water, and land pollution; and global warming to name a few. Many of these 

consequences are related to our energy use and choices. It is clear that we will need to reduce our 

per capita environmental impact, at least in relation to our population growth if we wish to 

stabilize or reduce environmental degradation. 

About a fifth of all energy consumed in the United States is consumed in the home.3

 This 

represents a huge opportunity to reduce our energy consumption and make cleaner choices for 

the energy we do consume. The U.S. Department of Energy’s Building America Program is 
2 

working to increase the energy efficiency of new and existing homes while increasing comfort, 

durability, and resource efficiency. As part of this program, we pursue opportunities to research 

highly efficient homes with the goal of understanding what works, what doesn’t work, and what 

are the most economic ways to reach very high efficiency targets. 

2.2 The “Solar Patriot” Home 

Imagine a home that produces as much energy as it consumes each year. That was the design 

goal of the “Solar Patriot” home constructed on the National Mall in Washington, D.C., as part 

of the American Solar Energy Society (ASES) annual conference in the spring of 2001 (Figure 

1).4,5,6,7 The home was designed by owner Alden Hathaway of the Environmental Resources 

Trust and builder Don Bradley of Solar Strategies Development Corporation. Construction of 

the home on the National Mall was accomplished in part with support from the Department of 

Energy (DOE)8

 through the National Renewable Energy Laboratory’s (NREL)9 Zero Energy 

Homes (ZEH) Program.10 After exposing more than 30,000 people to the potential of marketable, 

highly efficient, solar homes while it was on display on the National Mall, the home was moved 

to its permanent location outside of Washington, D.C., in Purcellville, Virginia. The Hathaway 

family moved into the home in August 2001. The following month staff from NREL visited the 

home and, with support from the DOE’s Building America Program,11 installed a data-collection 

system to measure the performance of the home. In addition to the measured performance, a 

detailed computer simulation of the house was created using DOE2.1 home energy simulation 

software. This simulation was also used in conjunction with a newly developed NREL research 

tool called Building Energy Optimization (BEopt) that uses DOE2.1 and the TRaNsient SYstem 

Simulation (TRNSYS)12 Program to explore optimum combinations of energy efficiency 

measures to reach energy savings goals. This report combines the information from all of these 

efforts to investigate the performance of the Hathaway house and use it as a case study for 

exploring highly efficient and zero energy homes. 

Figure 1. The “Solar Patriot” home on the National Mall 

in Washington, D.C.
3 

2.3 The Hathaway House Specifications 

The home is modular construction consisting of four modules. After its debut on the National 

Mall, it was moved to its permanent location in a rural area of Purcellville, Virginia (Figure 2.) 

The home was placed on an insulated full basement constructed from Superior WallTM precast 

concrete panels. 

A building-integrated PV system was added to a standing-seam metal roof on the home and a 

detached garage was built to the east of the home. Collectors for an active solar water-heating 

system were placed on the roof of the garage along with a second PV array. The house was 

equipped with a ground-coupled heat pump system that supplies heating and cooling as well as 

backup water heating. A summary of the home specifications is given in Table 1. Floorplans are 

given in Figure 3. 

Figure 2. The Solar Patriot house at its permanent location in Purcellville, 

Virginia (Photo by Ed Hancock) 
4 

Table 1. Summary of Specifications for the Hathaway House. 

Location Purcellville, Virginia 22132 

Builder Don Bradley, Solar Strategies Development Corporation 

Conditioned floor 

area 

2880 ft2 on two floors. Note that the basement and the third 

floor are both insulated and equipped with supply registers, 

but they were not being used as conditioned space during 

the time of this study. 

Orientation Front of home faces 10 degrees west of magnetic south 

(approximately true south) 

Wall construction 2x6 framing, 24-in., on center (OC), R-19 fiberglass batt 

insulation with R-5 insulating sheathing 

Basement wall 

construction 

Superior WallTM precast concrete panels, 8-ft ceiling 

1-1/2-in. of concrete, 1 in. of rigid polystyrene foam (R- 5), 

R-19 fiberglass batts between 2.25-in. x 7-in. concrete ribs 

Window type Seven-D Industries, Inc., Series 9000, double-glazed, 

U = 0.35, SHGC = 0.37 (south-facing windows have 

approximately 24-in. overhangs) 

Space-conditioning 

system 

Direct Axxess® ground-coupled heat pump with horizontal 

ground heat-transfer piping buried 10 ft under the front 

(south) yard of the home. Home is also equipped with a 

wood-burning fireplace. 

Water-heating 

system 

Two Duke Solar CPC-2000 compound parabolic 

concentrator collectors (4.5-m2

 total collector area) 

connected to a single 80-gallon storage tank. Backup water 

heating is accomplished with the ground-coupled heat 

pump. 

Appliances Refrigerator, clothes washer, and dishwasher are ENERGY 

STAR®-rated. Clothes dryer and oven are electric. Stove is 

propane. 

Lighting Compact florescent lighting is used throughout the home 

PV Systems Grid connected with battery back-up (no maximum power 

point tracker) 

Home array: Nominal 4 kW Unisolar amorphous 

Garage array: Nominal 2 kW BP Solar monocrystalline 

Inverters: Two Xantrex Model SW Series II inverters 
5 

Figure 3. Floor plans for the first and second floors of the Hathaway House 
6 

A utility room in the basement of the home contains the air handler, the heat pump, the water 

tank, breaker boxes, and PV inverters, charge controllers, and batteries. The ducts for the 

basement and first floor run in the floor between the basement and the first floor. A central 

supply trunk runs vertically through the home in the chase beside the stairs (see Figure 3 for the 

location of the chase). This supply trunk runs all the way to above the space in the attic that will 

eventually be finished and then runs the length of the house in this space (Figures 4 and 5) to the 

peak where supply ducts descend back along the roof sheathing to supply air to the second floor 

and attic. As shown in Figures 4 and 5, the floor of the attic is insulated with fiberglass batts. In 

the winter of 2003 the walls and ceiling of the framed-in attic space that will eventually be 

finished were insulated with sprayed-in cellulose (Figure 6). As of publication, the attic space 

was completed and was not being heated. Likewise the basement is insulated, but is not being 

actively conditioned. 

Figure 4. Duck supply trunk entering 

attic and continuing to trunk above the 

framing for the finished space 

Figure 5. Supply register for attic space 

(top photo) and supply duct descending 

to ceiling register for second floor 

(bottom photo)
7 

The 6-kW PV system is grid connected with a battery back-up that supplies power to the house 

during blackouts The controller does not include a maximum power-point tracker. 

The house has a unique single-tank water-heating system using the ground-coupled heat pump to 

back-up the solar water heater. During the day, the solar system exclusively warms the water in 

the 80-gallon storage tank. A timer controls the back-up heating from the heat pump. In the early 

morning hours, just before the Hathaways awake, the heat pump is used to raise the temperature 

of the water, preheated by the previous day’s solar gain, to the setpoint temperature. The heatpump water heating is then shut off. The Hathaway’s morning water use leaves the water in the 

tank at a temperature cool enough to take advantage of the solar heat gain for the remainder of 

the day. 

2.4 Study Objectives and Overview 

This study was funded by the Department of Energy’s Building America program 

(www.BuildingAmerica.gov). The Building America program is engaged in research with 

builders throughout the country to reduce energy consumption in new homes by an average of 

30% to 90% while increasing comfort and durability and decreasing material waste. In the past 8 

years, 26,000 energy-savings homes have been built as part of the program. The program is now 

focusing its substantial technical expertise and experience on cost effectively achieving higher 

energy-savings ratios. The Hathaway Solar Patriot house is an example of a home that was 

designed to achieve a high level of energy savings through energy-efficient features and to meet 

the remaining electrical energy needs through the use of solar energy (PV). The home provides 

an excellent case study for the Building America program. Building America’s long-term 

Figure 6. Alden Hathaway and blown-in cellulose 

insulation in attic
8 

research goal is to develop cost-effective, production-ready homes that achieve zero net energy 

by the year 2020. A net zero energy home produces as much energy as it consumes on an annual 

basis. 

The objective of the research outlined in this report is to answer the following questions: 

1. Does the home meet its design objective of reaching zero energy? Is so, how? If not, why 

not? 

2. How do the features of the home contribute to its overall energy efficiency? 

3. How much energy does the home consume compared to the Building America 

benchmark? 

4. What would be the most cost effective ways to improve the energy efficiency of the 

design? 

We used a combination of short-term tests, long-term monitoring, and computer simulation to 

answer these questions. 

3. Short-Term Testing 

Throughout the study, short-term tests were used to measure characteristics of the home that are 

not easy to determine from plans or specifications. Some of the results of these tests were later 

used to improve the accuracy of the computer simulations of the home and the confidence in the 

results from those simulations. Short-term tests also provide a quality check on some aspects of 

the home (such as air tightness and duct leakage) and assure that energy features (the PV system 

for example) are operating properly. The short-term tests performed on the Hathaway house are 

listed below then discussed in detail. 

Air-exchange Characteristics 

 Shell leakage measurement (blower door and tracer gas) 

 Duct leakage measurement (duct blaster and tracer gas) 

 Register flow measurement (flow hood) 

 Air handler flow measurement (flow plate) 

Heat-Pump Coefficient of Performance (COP) 

Other Short-term Measurements and Observations 

 IR photography 

 Fireplace operation 

Figure 7. Blower door 

installed in the 

Hathaway House 
9 

3.1 Air Exchange Characteristics 

On September 18, 2001, Glenn Chinery of the EPA’s ENERGY STAR® Homes Program and 

Don Mauritz of Lawrence Berkeley National Laboratory (LBNL) joined NREL staff at the 

Hathaway house. Glenn and Don brought a blower door to test the air tightness of the home and 

ducts. While they were there, we also used the blower door in conjunction with a duct blaster to 

measure the duct leakage. The blower door was placed in the kitchen door as shown in Figure 7. 

The conditions for the tests were as follows: 

 Attic door was closed and sealed (because the attic was unfinished at that time) 

 Attic registers were taped closed 

 Basement door was open and included in the measurement 

 Fireplace flue was closed 

 Unused PVC conduits in the basement were taped closed 

 Outside air was relatively still with occasional mild breezes. 

TECTITE blower-door software from The Energy Conservatory was used to determine the 

equivalent leakage area (ELA) of the home. The results of the blower door and duct blaster tests 

are given in the Table 2. 
10 

Table 2. Results of Blower Door and Duct Testing 

With duct 

registers 

taped 

CLOSED 

With duct 

registers 

OPEN Difference 

Blower Door Results 

CFM at 50 Pa 2241 2326 85 

ACH* at 50 Pa 3.73 3.88 0.15 

ELA (in.2 at 4 Pa) 132.5 146.8 14.3 

Correlation Coefficient (r2) 0.99935 0.99639 

Flow Coefficient (C) 197.7 227.0 

Exponent (n) 0.621 0.595 

Average Annual Infiltration** 

Average annual ACH 0.17 to 0.19 0.18 to 0.20 

Climate Factor (C) 20 to 23 20 to 23 

Height Factor (H) 0.8 0.8 

Wind Shielding Correction Factor (S) 1.2 1.2 

Leakiness Correction Factor (L) 1.0 1.0 

Duct Blaster Results @ 25 Pa 

Total (cfm) 555 

Total to outside*** (cfm) 219 

* air changes per hour 

** Estimated using the LBNL infiltration model and the assumed factors shown 

*** Measured with house pressurized to 25 Pa with the blower door 

 
11 

In the past, some modular homes have not achieved good air tightness because of leakage where 

the modules came together (the “marriage walls”). With less than four air exchanges per hour 

(ACH) at 50 Pascal depressurization, the Hathaway house had good air tightness. The projected 

average annual ACH is about 0.2. The addition of mechanical ventilation to the home should be 

considered. The ASHRAE standard 62.2, Ventilation and Acceptable Indoor Air Quality in LowRise Residential Buildings,

13 would recommend mechanical ventilation for this home. 

We measured the air flow rates at the registers throughout the house using an Alnor Low-Flow 

Balometer with a range of 10 to 500 cfm and a rated accuracy of + 3% of reading + 2 cfm. The 

flow rate of the supply registers on the first floor was measured with no filter, a dirty filter, and a 

clean filter in the air handler. The results of these measurements are given in Table 3. 
12 

Table 3. Register and Exhaust Fan Flow Results 

 Measured Air Flow Rates (cfm) 

Room (Register) Without filter With dirty filter With clean filter 

Supply Registers 

Dining room (1) 94 76 86 

Dining room (2) 87 67 88 

Entry 46 37 46 

Living room (1) 89 66 83 

Living room (2) 92 76 90 

Family room (1) 80 60 70 

Family room (1) 28 21 22 

Breakfast room (“Nook”) 58 48 54 

Kitchen (1) 30 20 25 

Kitchen (2) 70 51 63 

First floor total 674 522 627 

Master Bedroom (1) 71 

Master Bedroom (2) 79 

Master closet 38 

Master bathroom 48 

Hall bathroom 46 

NE bedroom 76 

SE bedroom (1) 73 

SE bedroom (2) 73 

Office (“Sitting room”) 71 

Second floor total 575 

Basement (1) 80 

Basement (2) 80 

Attic (1) - closed 0 

Attic (2) - closed 0 

Total for all supply registers 1409 

Return Registers 

1st floor entry > 500 

2nd floor entry > 500 

Office (“Sitting room”) 238 

Master Bedroom 65 

Exhaust Fans 

1st floor bathroom 13 

2nd floor hall bathroom 46 

Master bathroom 33 
13 

Table 4. Flow-Plate Measurements of the Total Air Flow Rate at the Air Handler 

Filter Condition 

Return Reference 

Pressure (Pa) 

Air-flow Rate 

(cfm) 

Dirty filter 34 1434 

No filter 53 1795 

Best filter 41 1580 

New filter 47 1672 

We measured the total air-flow rate at the air handler fan using a Energy Conservatory True 

Flow flow plate in the air filter slot. We measured the flow rates for different air filters and with 

different levels of filter dirt accumulation (Table 4). 

The "dirty" filter condition was for the filter condition as found at the start of the testing. It was 

an inexpensive ($1.50) glass-fiber filter that had not been changed for an extended time. The 

"best" filter was an expensive ($15) pleated-fabric filter that was intended to have significantly 

improved filtration, but also had a higher pressure drop. The "new" filter was a direct 

replacement for the inexpensive original filter. The measurements indicated that the dirty filter 

reduced the system flow rate by about 240 CFM or about 15% compared to the new filter. We 

recommended that Mr. Hathaway replace this filter about once every 3 weeks of normal 

operation. 

The Alnor flow-hood measurements indicated a similar reduction in air flow rate as the filters 

were changed. The total of the supply flows measured at each register with the flow hood was 

about 20% less the total flow measured with the flow plate. 

The total duct leakage for the home was quite high at 555 cubic feet per minute at 25 Pa 

pressurization. This was approximately 35% of the total air-handler flow. A tight duct system 

will leak at a rate of less than 10% of the total air-handler flow rate. Separate supply and return 

leakages were not measured. We measured the duct leakage to the outside by pressurizing the 

home to the same pressure as the ducts (25 Pa) and measuring the leakage from the ducts. The 

leakage to the outside was 219 cfm, approximately 14% of the total air-handler flow rate. 

Because the basement was included as indoor space in the blower door and duct blaster tests, the 

leakage to the outdoor was likely occurring in the attic. The effect of duct leakage will be 

discussed further in the Section 5: Simulated Energy Performance. 

A Bruel and Kjaer multi-point tracer gas-monitoring system was installed in the Hathaway home 

to measure air exchange rates from March 2-4, 2002. Sulfur hexafluoride (SF6) was periodically 

injected and mixed to a nearly uniform concentration. The decay in concentration of SF6 

indicated the rate of air exchange with the outside expressed as ACH. Two sampling points were 

located on the first floor, two on the second floor, and one in the basement. The average of the 
14 

ACH in the four zones from the first and second floor is plotted in the time-series graph in Figure 

8. Included in this graph is the calculated ACH using the LBNL model as described in the 

ASHRAE Handbook of Fundamentals 1997. The inputs to this model include the ELA from the 

blower door test, measured wind speed, outside temperature, and inside temperature during the 

tracer gas test. The LBNL model generally predicts the lower range of the measured air 

exchange rates. The measured higher air exchange rates on Saturday midday, Saturday evening, 

and Sunday midday were correlated with use of the fireplace and with family activity. This may 

imply that fireplace use causes increased air infiltration, or that increased infiltration may be 

caused by doors opened during family activity, or that the LBNL model is not accurate at this 

level of detail. 

Figure 8. Comparison of tracer gas ACH measurements to ACH modeled 

using blower door ELA measurement 

ACH Comparison of Measured and Calculated

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Sat 16 20 0 4 8 Sun 16 20 0 4

Hour

ACH

Measured

Model
15 

3.2 Heat-Pump Coefficient of Performance (COP) 

A ground-coupled heat pump was chosen to efficiently meet the home’s space heating, space 

cooling, and back-up water-heating needs. We were interested in performing a check of this 

systems coefficient of performance (COP). To estimate the COP during space-heating operation, 

we measured air flow and temperatures before and after the coil in the air handler. For selected 

hours of operation we used the flows and the changes in temperature to estimate delivered 

energy and compared this to the energy consumption of the heat pump. Because we measured 

only the sensible load, we estimated the latent load resulting from the moisture removed from the 

air during cooling to derive the COP in the cooling mode. The COP for water heating was 

calculated from tank temperature increase, measured tank losses, and heat-pump energy 

consumption. 

3.2.1 Heating Mode COP 

To measure the air-supply temperatures, we installed thermocouples in the supply air ducts in the 

basement mechanical room. The supply ducts branch into three parts immediately downstream 

from the air handler. The two branches supplying the first floor and basement had a location 

available to install the thermocouples at least 10 feet downstream from the air handler to allow 

for adequate mixing of the air. In these two branches, one thermocouple was installed in each 

branch. We used a hot wire anemometer to check for uniform air-temperature distribution at 

these locations before installing the thermocouples. In the branch that supplies the second floor 

and attic, the only easily accessible location for thermocouple installation was about 1.5 feet 

downstream from the air handler. Three thermocouples were installed across the section of the 

duct to account for any non-uniformity of air temperature. These three thermocouples were 

measured in parallel at the data logger to indicate an average temperature. Readings from these 

thermocouples while the heat pump was in heating mode indicated that all of the supply 

temperatures were within about 1°F of each other, implying a relatively uniform temperature 

leaving the heating coil. The return air temperature was assumed to be equal to the average of 

the indoor temperatures measured on the first and second floors. We used the results of the airhandler flow rate test described in section above for the airflow in the COP estimates. Mr. 

Hathaway changed his air filter frequently, so the clean filter airflow rate was used for the COP 

estimates. Monitored data for hours when the heating system was on nearly continuously and 

when the system did not deliver heat to the hot-water system were used to calculate the COP of 

the heat pump in heating mode. The results for the 65 hours during the monitored period that met 

the criteria stated above are shown in Figure 9. The average heating COP is 3.6, not including the 

supply fan power. If the supply fan power is included in the COP definition, the average heating 

COP is 2.9. 

We expected heat pump COP to be a function of ground temperature in the geothermal field and 

air temperature inside the house. The presence of the heat-extraction tubes may also affect the 

local ground temperature. As shown in Figure 9, the average calculated heating COP values in 

November and December were somewhat higher than the values determined for March. 

Whether this is a seasonal trend or a longer-term trend influenced by the introduction of the 

ground-source heat pump has not been determined 
16 

Hathaway - HP Space Heating Performance

based on full-load heating hours

2.5

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

61 93 125 157 189 221 253 285 317 349

Julian Day

COP

Figure 9. Space-heating COP determined from monitored data (fan power not included) 

3.2.2 Cooling Mode COP 

A direct calculation of the cooling COP of the heat pump system could not be made because the 

latent load of the cooling coil was not monitored. The COP of the sensible load only can be 

determined in a manner similar to the heating performance analysis above. The result of this 

analysis is shown in Figure 10. In this case, the average sensible cooling COP was 2.6 without 

and 2.1 with the fan power included. Assuming a sensible heat ratio of 0.8, the total cooling 

COP without and with fan power included is 3.2 and 2.6, respectively. The average outdoor 

temperature coincident with the cooling performance calculations was 85°F. To model the 

system performance in cooling mode, an estimate of the latent load resulting from moisture 

removal was added to the sensible heat COP calculation. 
17 

Hathaway - HP Space Sensible Cooling Performance

based on full-load cooling hours

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

161 168 175 182 189 196 203 210 217

Julian Day

COP

Figure 10. Space-cooling COP determined from monitored data (fan power not 

included) 

3.2.3 Water-Heating COP 

The ground-source heat pump, along with an active solar hot-water system, provided heat to a 

80-gallon domestic hot-water tank. Extensive monitoring of the system allowed for the 

determination of the tank loss, hot-water load, the HP performance and, ultimately, the overall 

HP/Solar system performance. 

We measured the water-storage tank temperature using a thermocouple attached to tank inside its 

insulation. We used the rate of change in hot-water storage tank temperature with respect to time 

(C/min), the nominal volume of water in the tank (80 gallons [303 kg]), and the heat capacity of 

water (4.18 kJ/kg) to calculate the heat flow in watts. The COP was the ratio of the thermal 

output of the heat pump to the electric input. The COP was expected to change as a function of 

water temperature and ground temperature. The ground temperature changed seasonally and 

may appear relatively constant over the period of a month, but the water temperature changed 

(from cold to warm) during every cycle of the heat pump. The COP for water heating was 

initially calculated from 1-minute data to observe the change in COP as a function of water 

temperature. Figure 11 shows that the measured COP in water heating mode is about 2.0 at 20o

C 

water and about 1.4 at 45o

C water temperature. 
18 

(total of 660 1 minute points)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

20 25 30 35 40 45 50

Tank Temperature (C)

CO

P

Jan 22 - Feb 1

Mar 6 - Mar 19

The tank UA (overall heat transfer coefficient [U] times area [A]) was determined by examining 

the hourly tank temperature and the basement temperature. During periods with no hot water 

usage and with no solar energy added to the tank, the tank UA could be estimated as the tank 

heat loss (as determined by the temperature drop of the tank) divided by the temperature 

difference between the tank and the basement. A plot of the calculated tank UA for every hour 

clearly showed a common value of 3.0 BTU/hr-F. This tank UA lead to an average tank loss of 

132 BTU/hr during the monitored period. 

The monitored data was examined to find periods when the heat pump charged the 80-gallon 

tank, without any significant hot water draw or solar heating contribution. The load of the hot 

water tank during these periods was defined only by the temperature rise of the water in the tank 

and the heat loss from the tank. The power use of the heat pump during these periods was used 

to determine the HP COP when charging the hot-water tank. 

Figure 11. Coefficient of performance (COP) of heat pump for water heating 
19 

Hathaway - HP DHW Heating Performance

based on time periods with tank charging only

0.0

0.5

1.0

1.5

2.0

2.5

62 94 126 158 190 222 254 286 318 350

Julian Day

COP

Figure 12. Domestic hot-water COP determined from monitored data 

The average COP for the thirteen periods identified in Figure 12 was 1.55. This performance was 

consistent with the 1-minute measurements discussed above and was comparable to dedicated 

air-to-water heat-pump water heaters. 

The actual hot-water energy demand was met by a combination of the solar collectors and the 

heat pump. As shown in Table 5, the total system COP of 3.5 included the heat-pump energy and 

solar-pumping energy used to deliver heat to the storage tank. This overall COP clearly varied 

as a function of weather conditions and use patterns. 
20 

Table 5. Energy Balance for Domestic Hot Water system 

Component Value Source/Description 

Hot Water Load 

 load of hot water used 8797329 BTU based on monitored flow and deltaT data 

load resulting from heat loss from 

tank 

1153305 BTU based on monitored temperature data 

 total heating load 9950634 BTU sum of above components 

HP energy used to heat water 576 kWh based on monitored power data 

HP energy delivered to water 3051232 BTU based on an average HPdhw COP of 1.55 

Solar energy delivered to water 6899402 BTU total load minus HP energy delivered 

Solar Energy Fraction 69% fraction of DHW heating load delivered by solar 

Solar pump energy used 256 kWh from monitored data 

 

 COP of delivered solar energy 7.9 (solar energy delivered)/(pump energy used) 

 COP of total system 3.5 (DHW energy delivered)/(energy used) 

The COP of the heat pump and solar water heating systems in each operating mode is shown in 

Table 6: 

Table 6. The COP in Each Mode 

Operating Mode Measured Average COP 

Space Heating 3.6 

Space Cooling 3.2 

Water Heating at 20o

C 2.0 

Water Heating at 45o

C 1.4 

Water Heating (average) 1.55 

Solar water heating 7.9 

Overall water heating 3.5 
21 

3.3 Other Short-Term Measurements and Observations 

Infrared (IR) Imaging. We used an infrared (IR) camera to examine the insulated surfaces of 

the house to determine whether any insulation might be missing or impaired. Several small 

(about 1-2 ft2

) areas in the ceiling were identified as locations where insulation batts in the attic 

had been moved during construction. Most of these areas were easily found in the attic and were 

repaired. One larger area (about 20 ft2

) was identified in the ceiling above the southeastern 

bedroom. It was located below the attic flooring and was not immediately repaired. The total 

amount of missing insulation was not a large percentage of the total area of the house. Repairing 

these areas was not expected to have a significant influence on energy use for heating and 

cooling, but it will be useful to correct these minor problems. 

Fireplace Operation. Mr. Hathaway reported that he used his wood-burning fireplace during 

heating months to reduce electricity used for space heating. The fireplace was probably used 

nearly every evening during cold weather and more frequently on weekends. The fireplace uses 

outside air for combustion and has a fan to circulate room air around the firebox. 

Propane Stove. The Hathaway’s oven is electric and is included in this analysis. However, 

propane is used for their kitchen stove. We did not collect information on the propane 

consumption. The kitchen stove energy consumption is not included in this analysis. 

4. Long-Term Monitoring 

The short-term tests at the Hathaway house revealed much about the shell of the home and the 

operation of some of its subsystems (which will be discussed in section 6). Measuring the longterm performance of the whole house and its subsystems required the installation of a system that 

continuously monitors the changing conditions of the home such as energy use, PV energy 

production, temperatures, hot water use, etc. In September 2001, we installed sensors inside and 

outside the Hathaway house to monitor its performance. This monitoring continued through 

February 2004. 

The energy consumed in a home is greatly affected by climate, thermostat settings, number of 

occupants, energy-use pattern of the occupants, the home design, and the equipment and 

appliance selection. The precise energy use of a home for a specific year and specific occupants 

cannot be predicted. Instead, predictions are based on typical weather data and typical occupant 

behavior. Monitored energy-use data reflect the influence of all of the factors listed above. 

Depending on the level of detail of the monitoring, some of these influences can be 

disaggregated. 

This section of the report summarizes data collected from the home from September 2001 to 

June 2003. 

4.1 Monitoring System and Data Being Collected 

Data from the installed sensors are collected using a Campbell Scientific CR10X data logger. 

The data logger is equipped with a Campbell SDM-SW8A pulse multiplexer and a Campbell 

ATM25T thermocouple multiplexer. Further information on the hardware used is contained in 
22 

Appendix 7.1. The system is connected by modem to a phone line in the home and data is 

downloaded daily. 

In designing the data collection system, we focused on understanding the performance of the PV 

system, the solar water-heating system, the heat pump, and the main home appliances. The 

home was equipped with sensors for the 39 measurements listed in Table 7. 

We had planned to directly measure the power production from the PV system on the AC side of 

the inverters. This measurement was not possible because of the wiring set-up. AC power from 

the PV system was not isolated in any of the wiring. We were measuring DC power from the PV 

system and applying the efficiency vs. power curve provided by Trace to estimate the AC power 

supplied by the PV system. 
23 

Table 7. Data Monitored at the Hathaway House 

Solar radiation (W/m2

) 

- In the plane of the collectors at the array on the house roof 

- Vertical south-facing 

Energy (kWh) 

- Energy entering the home from the grid 

- Energy exiting the home to the grid 

- Heat-pump energy consumption 

- Clothes-dryer energy consumption 

- Clothes-washer energy consumption 

- Air-handler fan energy consumption 

- Kitchen-oven energy consumption (stove is propane) 

Temperatures (o

C) 

- Outside air temperature (from T and RH sensor) 

- Outside air temperature (thermocouple) 

- House PV array temperature 

- Garage PV array temperature 

- Attic air temperature 

- Second-floor air temperature 

- First-floor air temperature 

- Basement air temperature 

- Supply-air temperature 

- Cold-water temperature 

- Hot-water temperature 

- Temperature after the mixing valve 

- Temperature of glycol fluid to the solar water collector 

- Temperature of glycol fluid from the solar water collector 

- Temperature of water from tank to heat pump 

- Temperature of water from heat pump to tank 

DC electrical measurements 

- Voltage of west house PV array (V) 

- DC current from west house PV array (A) 

- DC power from west house PV array (W) 

- DC current from east house PV array (A) 

- DC power from east house PV array (W) 

- DC current from garage PV array (A) 

- DC power from garage PV array (W) 

- Battery voltage (V) 

- Current to or from battery bank (A) 

- Current to well pump (A) 

Other measurements 

- First-floor relative humidity 

- Outside relative humidity 

- Hot-water consumption (liters) 

- Solar water heater pump run time (minutes each hour) 
24 

4.2 Results from the Entire Monitoring Period 

The energy consumption by end use for the entire 30-month monitoring period is shown in 

Figure 13. Space conditioning (HP,space; HP,standby; and AHfan) accounted for about half of 

the total energy consumption. Water heating (HP,DHW and SolPumps) accounted for less than 

10% of the total energy consumption. The clothes washer and refrigerator in the home were 

ENERGY STAR-rated, highly efficient appliances. Even with efficient appliances, the energy 

consumption from all the major appliances (Fridge, Washer, Dryer, and Oven) accounted for 

17% of the total energy consumption. The stove was propane and was not included in this 

accounting. 

Perhaps the most sobering result of this end-use accounting is the size of the “other” category. 

The energy consumption in the “other” category was calculated by subtracting every submetered 

category from the total house energy consumption. Despite submetering of the largest energy 

using appliances in the home, the “other” category was a whopping 23% of total energy 

consumption. This “other” category contained items such as lighting, computers, televisions and 

radios, toaster oven, microwave, hair dryers. Lighting was not submetered because it was 

distributed through many circuits in the home. However, this home already used compact 

fluorescent lights in every fixture, so the lighting energy should be quite low. The remainder of 

items in the “other” category was quite difficult for building scientists and researchers to reduce. 

As we continue to reduce space conditioning, water heating, and major appliance energy 

consumption, this “other” category will become an increasingly larger portion of the energy 

consumption pie.

Figure 13. Total energy consumption by end use for the entire 

monitoring period – September 2001 to February 2004 

Dryer

7%

Washer

0%

Oven

1%

SolPumps

2%

WellPumps

1%

H.P.,space

39%

H.P.,DHW

6%

H.P.,standby

2%

Fridge

7%

AHfan

11%

Other

23%

AHfan,standby

1%
25 

The monthly energy consumption by end use is shown in Figure 14. The substantial “other” 

category was relatively constant from month to month. Notice that the water-heating energy (HP, 

DHW) was several times larger in the first 3 months of data collecting than it was for the rest of 

the data-collection period. During those first 3 months, the water-heating mode of groundcoupled heat pump was switched on and off manually. It was switched on early in the morning 

and left on until all morning showering was completed. By the time it was switched back off, it 

had already heated the water in the single 80-gallon storage tank. Because the storage tank water 

was warm at the beginning of the day, the solar collectors were effectively defeated. This 

problem was alleviated by using a timer to control the heat-pump water-heating mode. By 

adjusting the run time of the heat pump in water-heating mode, the family was able to assure 

sufficient hot water supply for morning showering without leaving the storage tank full of heated 

water at the beginning of the day. The change reduced water-heating energy consumption 

dramatically. 

Figure 14. Monthly energy consumption by end use 

0

500

1000

1500

2000

2500

Sept 01

Oct 01

Nov 01

Dec 01

Jan 02

Feb 02

Mar 02

Apr 02

May 02

June 02

July 02

Aug 02

Sept 02

Oct 02

Nov 02

Dec 02

Jan 03

Feb 03

Mar 03

Apr 03

May 03

Jun 03

Jul 03

Aug 03

Sep 03

Oct 03

Nov 03

Dec 03

Jan 04

Feb 04

kWh/month

PkitchenGFI

PmasterBath

Solar Pumps

Well Pump

Dryer

Washer

Oven

HP, Space Conditioning

AH fan

HP, DHW

Other

Fridge

AH fan, standby

HP, Standby
26 

Figure 15 shows the AC electrical energy produced by the PV system. The PV system was 

oriented due solar south and tilted at about 49 degrees – near optimum for year-round energy 

production. The system was equipped with a battery back-up system that was kept charged to 

supply the home with power for essential needs during blackouts. The system was not equipped 

with a maximum power point tracker. With the exception of May 2002 (when there was a short 

interruption in service from one of the inverters) the PV system appeared to be operating as 

expected. Because there were fewer sunlight hours in the winter than the summer, the PV 

production was naturally lower in the winter. It is interesting to note how much the monthly PV 

power varied from year to year as a result of variations in the weather. For example, the PV 

system output in October 2002 was little more than half of the output in October 2001. In 

October 2002 much lower solar radiation levels were recorded in the plane of the collectors than 

in October 2001. (It was a cloudy October.) This should give pause to those who use TMY data 

to predict PV energy output. Without solar radiation measurements, low PV output like that seen 

in October 2002 could be misinterpreted as a problem with the system. 

Figure 15. Monthly electrical energy (AC) produced by the PV system 

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

Sept 01

Oct 01

Nov 01

Dec 01

Jan 02

Feb 02

Mar 02

Apr 02

May 02

June 02

July 02

Aug 02

Sept 02

Oct 02

Nov 02

Dec 02

Jan 03

Feb 03

Mar 03

Apr 03

May 03

Jun 03

Jul 03

Aug 03

Sep 03

Oct 03

Nov 03

Dec 03

Jan 04

Feb 04

kWh/month
27 

The total monthly energy consumption minus the monthly PV AC energy production yielded the 

net monthly electrical energy purchased, shown in Figure 16. During 8 months of the datacollection period the home produced more electricity than it consumed. These months occurred 

in the spring and fall “swing seasons” when the outdoor conditions are near the inside 

temperature setpoint and little space-conditioning energy was required to maintain comfort. In 

the summer and winter, the space-conditioning energy needs overwhelmed the PV production, 

and the home required more energy than it produced. The PV system appeared to be able to meet 

the needs of the auxiliary water heating, the major appliances, and the “other” category, but not 

the entire space-conditioning load. Because the space-conditioning equipment was already nearly 

the most efficient electrical equipment available, this indicated that it may be a good idea to look 

at more efficient shell designs, a better air-distribution system, or a larger PV array if we wish to 

reach zero net annual electrical energy consumption. We will revisit this topic in the section on 

building energy simulation. 

Figure 16. Monthly net electrical energy bought 

-500

0

500

1000

1500

2000

Sept 01

Oct 01

Nov 01

Dec 01

Jan 02

Feb 02

Mar 02

Apr 02

May 02

June 02

July 02

Aug 02

Sept 02

Oct 02

Nov 02

Dec 02

Jan 03

Feb 03

Mar 03

Apr 03

May 03

Jun 03

Jul 03

Aug 03

Sep 03

Oct 03

Nov 03

Dec 03

Jan 04

Feb 04

kWh/month
28 

The Hathaway house had three PV arrays employing two different technologies. The two arrays 

on the roof of the homes were both composed of building-integrated peel-and-stick amorphous 

silicon modules from Uni-Solar. The array on the garage roof was composed of rack-mounted 

monocrystalline panels from BP Solar. The maximum power point voltage behaved differently 

with temperature for these two different technologies, but the Hathaway array was operated at a 

constant voltage. In addition, the difference in physical characteristics of the arrays may have 

shed snow differently. Therefore, we would expect the outputs from the three arrays, each 

nominally rated at 2 kW, to differ. Figure 17 shows the monthly DC output from each array. 

Although they were nearly identical, the energy output from the east and west arrays on the 

home differed by an average of about 10% with the east array consistently outperforming the 

west array. We have no ready explanation for this difference other than variation in module-tomodule output manufacturing variability. The garage-array energy output was usually less, but 

sometimes more than the output from the home arrays. This variation was likely a result of the 

variation of maximum power-point voltage with temperature. In general, for the Hathaway house 

operating voltage of about 53 to 54 volts, the monocrystalline array performed better with 

respect to the amorphous arrays during months when the average outdoor temperature was low. 

 

Figure 17. Monthly electrical energy (DC) produced by each of the three PV 

arrays and average outdoor temperature (west and east are the arrays on the roof 

of the home) 

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

Sept

01

Oct

01

Nov

01

Dec

01

Jan

02

Feb

02

Mar

02

Apr

02

May

02

June

02

July

02

Aug

02

Sept

02

Oct

02

Nov

02

Dec

02

Jan

03

Feb

03

Mar

03

Apr

03

May

03

Jun

03

Jul 03 Aug

03

Sep

03

Oct

03

Nov

03

Dec

03

Jan

04

Feb

04

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

West

East

Garage

Average Outdoor

Temperature

o

C
29 

4.3 Results from the Calendar Year 2002 and 2003 

The two arrays summed to a nominal 6 kW of rated capacity. The average daily power 

production from the array in 2002 was about 20.3 kWh/day. This equates to an equivalent of 

about 3.4 equivalent full sun hours per day. 

The average daily power production from the array in 2003 was about 19.2 kWh/day. This 

equated to an equivalent of about 3.2 equivalent full sun hours per day. 

A home that is designed to have net zero net energy consumption for standard occupants under 

TMY weather data may not have zero net energy consumption with any specific occupants on 

any given year. Also, a home that achieves zero net energy consumption in one year, may not 

achieve it in other years even with the same occupants as a result of to year-to-year variations in 

available solar radiation. The Hathaway house design did not use rigorous energy simulation and 

optimization to attempt to achieve zero net energy. The approach employed by the builder and 

the homeowner was empirically based on past experience. The home did, however, achieve 

impressive efficiency and a very low net energy consumption. The total PV production and net 

purchased energy for calendar years 2002 and 2003 are shown in Figure 18. During 2002 the 

home produced an outstanding 70% of the total electrical energy used in the home. 

Figure 18. Energy produced by the PV system and net energy bought from the grid 

for 2002 and 2003 

2002 Calendar Year 2003 Calendar Year 

Net Energy Bought from the 

Grid

30%

Total PV AC Energy 

Produced

70%

Total PV AC 

Energy Produced 

= 63%

Net Energy Bought 

From the Grid = 

37%

Net energy bought 

from the grid = 30% 

Total PV AC 

energy produced = 

Net energy bought 

from the grid = 37%

Total PV AC 

energy produced = 
30 

 

Figure 19 shows the percentage of energy used for each monitored end use in the 2002 and 2003 

calendar years. Although there are some very minor shifts, the relative end use consumption was 

nearly identical for the two time periods. 

4.4 Average Hourly Profiles 

Hourly profiles chart the monthly average of a measurement for each hour of the day. They 

provide insight into the average variation of a quantity over the course of a day and the shift in 

daily profile from month to month. The hourly profiles for selected temperatures, energy uses, 

hot water use, and solar radiation are shown in Figures 20 to 31. 

2002 Calendar Year 2003 Calendar Year 

Figure 19. End use percentages for 2002 and 2003 

Dryer

8%

Washer

1%

Oven

2%

SolPumps

2%

WellPumps

1%

H.P.,space

37%

H.P.,DHW

5%

H.P.,standby

2%

Fridge

7%

AHfan

10%

Other

24%

AHfan,standby

1%

Dryer

7%

Washer

0%

Oven

1%

SolPumps

2%

WellPumps

1%

H.P.,space

39%

H.P.,DHW

6%

H.P.,standby

2%

Fridge

7%

AHfan

11%

Other

23%

AHfan,standby

1%
31 

Figure 20. Outdoor temperature average hourly profile 

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

Time

deg C

Sep_01

Oct_01

Nov_01

Dec_01

Jan_02

Feb_02

Mar_02

Apr_02

May_02

Jun_02

Jul_02

Aug_02

Sep_02

Oct_02

Nov_02

Dec_02

Jan_03

Feb_03

Mar_03

Apr_03

May_03

Jun_03

Jul_03

Aug_03

Sep_03

Oct_03

Nov_03

Dec_03

Jan_04

Feb_03

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

26.0

28.0

30.0

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

Time

deg C

Sep_01

Oct_01

Nov_01

Dec_01

Jan_02

Feb_02

Mar_02

Apr_02

May_02

Jun_02

Jul_02

Aug_02

Sep_02

Oct_02

Nov_02

Dec_02

Jan_03

Feb_03

Mar_03

Apr_03

May_03

Jun_03

Jul_03

Aug_03

Sep_03

Oct_03

Nov_03

Dec_03

Jan_04

Feb_03

Figure 21. First-floor indoor temperature average hourly profile 
32 

Figure 23. DC power from the PV system average hourly profile 

Figure 22. Irradiance in the plane of the collector average hourly profile 

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

Time

W/m2

Sep_01

Oct_01

Nov_01

Dec_01

Jan_02

Feb_02

Mar_02

Apr_02

May_02

Jun_02

Jul_02

Aug_02

Sep_02

Oct_02

Nov_02

Dec_02

Jan_03

Feb_03

Mar_03

Apr_03

May_03

Jun_03

Jul_03

Aug_03

Sep_03

Oct_03

Nov_03

Dec_03

Jan_04

Feb_03

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

Time

kW

Sep_01

Oct_01

Nov_01

Dec_01

Jan_02

Feb_02

Mar_02

Apr_02

May_02

Jun_02

Jul_02

Aug_02

Sep_02

Oct_02

Nov_02

Dec_02

Jan_03

Feb_03

Mar_03

Apr_03

May_03

Jun_03

Jul_03

Aug_03

Sep_03

Oct_03

Nov_03

Dec_03

Jan_04

Feb_03
33 

Figure 24. Total power use average hourly profile 

Figure 25. Net power bought from the grid average hourly profile 

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

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Figure 27. Heat-pump power used for water-heating average hourly profile 

Figure 26. Power used for space conditioning average hourly profile 

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35 

Figure 29. Clothes washer power use average hourly profile 

Figure 28. Hot-water draw average hourly profile 

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36 

Figure 30. Clothes dryer power use average hourly profile 

Figure 31. “Other” power use average hourly profile 

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37 

Some interesting observations from the hourly profile data are listed below: 

 The net power bought from the grid (Figure 25) showed a marked dip in the middle of the 

day. This was a result of the contribution from the PV system, which peaked midday 

(Figure 23). Many homes of this type could reduce the overall peak requirements on the 

energy grid, which would be a real benefit for the electrical utility. 

 The power use in the “other” category (Figure 31) almost never went to zero. At 4:00 am, 

the home was still consuming and average of between 100 and 300 watts. 

 The average indoor temperature (Figure 21) varied seasonally from a high of nearly 80°F 

in July and August to a low of around 62°F in January and February. These January and 

February temperatures were well below those typically found in homes. 

 Most hot water (Figure 28) was used in the early morning hours. 

5. Simulated Energy Performance 

We modeled the Hathaway house energy performance using DOE2 simulation software.14 DOE2 

is a widely used and accepted building energy-analysis program that was developed by the U.S. 

Department of Energy’s Lawrence Berkeley National Laboratory. DOE2 has been subjected to 

the BESTEST software validation procedure.15 16 The program uses a virtual model of the home 

based on the actual materials, size, shape, orientation, shading, mechanical systems, etc., of the 

test home. It subjects this virtual model to hourly weather data for the location of the home to 

calculate the hourly energy consumption of the home. This hourly data is summed to give the 

annual energy consumption of the home. The energy consumption is then compared to a other 

virtual models, called the “base cases,” that closely resembles the test home, but do not have all 

the energy-saving features of the test home. The different base cases used for comparison to the 

Hathaway house are discussed in section 6.3. Details on the modeling approach used by NREL 

in the Building America program can be found in reference.1

 Images of the DOE2 model are 

shown in Figure 32. 

Figure 32. DOE2 model of the Hathaway House, the dark area 

simulates the shading of the home by the garage and breezeway roof 
38 

5.1 Improving Accuracy: Modeling Specific Systems 

The residential system models available in DOE2 do not include a direct-expansion, groundcoupled heat pump, solar water heating system, or PV system as is found in the Hathaway house. 

The long-term monitored data, therefore, was essential in characterizing the performance of these 

systems. 

The measured COPs of the heat pump in heating and cooling modes discussed in section 3.2 

were used to characterize the performance of the heat pump in the DOE2 model. While this 

performance level of the heat pump is a function of the weather experienced during the 

monitored period, the overall average measured system performance was used in place of a 

component-based simulation within the model. Although this approach was a simplification, it 

allowed us to use the measured COP in the simulation. 

For water heating, a simplified approach based on the overall average COP of the solar/heat 

pump system (calculated in section 3.2) was implemented in the DOE2 model. The actual 

measured energy contribution of the PV system for calendar year 2002 was incorporated into the 

whole-house energy analysis independently of the DOE2 analysis. Note that the DOE2 analysis 

was driven using TMY2 weather data, but the PV performance is based on the actual 2002 

weather. 

5.2 Building Confidence: Comparing Monitored Data and Simulation Results 

The Building America prototype simulation model is based on the description of the house “asbuilt” along with standardized behavior and controls as defined in the Building America 

Performance Analysis Procedures.17 The model of the physical building attempts to be as true 

as possible to what was actually built and installed in the prototype house. The use of the 

building, however, and the energy consumption associated with it, must rely on established 

assumptions regarding average occupant behavior and typical hourly weather data. 

Monitored Data and the Building America Prototype. There is no reason to expect that the 

occupants of any particular house will behave according to the Building America benchmark 

operating conditions. While the mean behavior of the occupants (in the better sense of the 

adjective) is expected to be close to the behavior assumed for Building America, behavior may 

vary to a large degree. Comparing the monitored data (largely influenced by actual behavior) 

and simulation results (largely driven by assumed behavior) was, therefore, mainly a comparison 

of the actual occupants to the “assumed” occupants. 

Figure 33 below shows a monthly comparison of total electricity used at the Hathaway house in 

the year 2002 (not accounting for PV generation) and the total electric use of the Hathaway 

prototype simulation model based on TMY2 weather and standard occupant behavior. The 

model predicts that the house would use 62% more electricity than it actually used for the annual 

period. 
39 

The model predicts that the HVAC system would use approximately 23% more energy than was 

realized for the year and predicts that non-HVAC electricity use (lights, appliances, plug loads) 

would be nearly 65% more than was observed. Overall, the simulation predicted 44% higher 

whole-house annual energy consumption than was measured in the home in 2002. 

Some investigation was warranted to discover if differences in the standard assumptions and the 

actual operating conditions could explain this large discrepancy. 

Monitored data and the “Adjusted” Building America Prototype. Weather data used for the 

Hathaway site (TMY2 weather data for Sterling, Virginia) was significantly different than actual 

weather during February and October of the monitored period. 

Based on the monitored data, the “behavior” of the occupants and the operation of the house 

were adjusted in simulation in the following ways: 

• Internal Gains: plug loads were reduced by about 40% to match the monitored data 

Figure 33. Monitored data vs. simulation results for total energy use using Building 

America assumptions and TMY2 data 

Hathaway "Solar Patriot"

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40 

• HVAC Control: thermostat schedules were adjusted monthly to match observed actual 

setpoints (which were significantly lower in the heating season than assumed). Also, the 

thermostat schedule accounts for two week-long vacation periods. 

• DHW Load: a slight adjustment was made so that the simulated hot-water volume used 

matched the observed volume. 

• Infiltration: The average infiltration was adjusted based on measured data. 

When the actual behavior of the Hathaway house occupants and the measured 2002 weather was 

used to drive the simulation model, the predicted and observed energy use was much closer. The 

annual total energy use was less than 5% different between the model results and the monitored 

data. 

The simulation continued to overpredict the energy consumption in some months – especially the 

winter months. Some of this difference in energy use was explained by the occupants’ use of 

wood to heat the house in the winter months. 

Overall, it appears the simulation of the Hathaway prototype building was quite accurate. 

Differences in the monitored data and the Building America prototype simulation results shown 

in Figure 33 were mostly a result of differing assumptions regarding occupant behavior and 

weather rather than inaccuracies in the model of the home. 

Hathaway "Solar Patriot"

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Figure 34. Monitored data vs. the adjusted model results, whole-building energy use 
41 

5.3 Putting Performance in Context: Simulation Results 

The modeled energy performance of the Hathaway house was compared to two theoretical 

houses with the same basic architectural features and floor plans. One base case for comparison, 

called the Regional Standard Practice home, is constructed as a standard new home in the same 

area as the Hathaway home according to the knowledge and judgment of the authors. The other 

base case was the Building America Benchmark version 3.1. This Benchmark was designed to 

represent 1990s typical construction and to be in compliance with the 1995 Model Energy Code 

(MEC). The basic characteristics of these base case homes are given below: 

Regional Standard Practice: 

(assumes natural gas is not available on site) 

 Building Shell: 2 x 4 frame walls - R11 fiberglass insulation and wood sheathing 

 R30 ceiling insulation 

 R7 basement wall insulation 

double-pane clear windows with aluminum frames 

 Standard air-infiltration (0.51 ACH average) 

 HVAC: 10 SEER 5-ton air-conditioner 

 6.8 HSPF heat pump with electric backup 

 DHW: Electric DHW, 40 gallons, EF = 0.88 

Building America Benchmark 3.1: 

(assumes natural gas is not available on site) 

 Building Shell: 2 x 4 frame walls – overall R factor = 12.2 

 R33.3 ceiling insulation 

 R17 basement wall insulation 

 double-pane Low E windows with U = 0.40 

 Standard air-infiltration (0.51 ACH average) 

 HVAC: 10 SEER 5-ton air-conditioner 

 6.8 HSPF heat pump with electric backup 

 DHW: Electric DHW, 40 gallons, EF = 0.88 

The Building America program has created a standard set of three output tables that are used 

with any building modeling effort. The standard output tables for the Hathaway house modeling 

are given in Tables 8 through 10. 
42 

End-Use kWh therms kWh therms kWh therms

Space Heating 6276 0 7195 0 5030 0

Space Cooling 3540 0 2005 0 1092 0

DHW 3522 0 3525 0 987 0

Lighting 3110 3110 1204

Appliances + Plug 7227 0 7227 0 7017 0

OA Ventilation 000

Total Usage 23675 0 23062 0 15330 0

Site Generation 0 0 0 0 7402 0

Net Energy Use 23675 0 23062 0 7929 0

BA Base BA Prototype Region Standard

Annual Site Energy

Base Region Proto BA Reg BA Reg

End-Use MBTU/yr MBTU/yr MBTU/yr Base Base Base Base

Space Heating 64 74 52 20% 30% 5% 9%

Space Cooling 36 21 11 69% 46% 10% 4%

DHW 36 36 10 72% 72% 11% 11%

Lighting 32 32 12 61% 61% 8% 8%

Appliances + Plug 74 74 72 3% 3% 1% 1%

OA Ventilation 000 0% 0% 0% 0%

Total Usage 243 236 157 35% 34% 35% 34%

Site Generation 0 0 -76 31% 32%

Net Energy Use 243 236 81 67% 66% 67% 66%

Annual Source Energy Percent of End-Use Percent of Total

Source Energy Savings

Measure Package

Increment kWh therms MBTU Savings % $/yr Savings % $/yr Savings % value ($/yr) savings $/yr

Base 

(Bldg America) 23675 0 242.6 2,368 $ 2,332 $ 

Base 

(Regional Std Practice) 23062 0 236.3 3% 2,306 $ 3% 3,053 $ 

Base 

(Builder Std Practice) 27851 0 285.4 -18% 2,785 $ -18% 2,743 $ 

Base + 

 improved walls 25642 0 262.7 -8% 2,564 $ -8% 2,526 $ 8% 217.6 $ 218 $ 

Base ++

 Low-E Windows 23140 0 237.1 2% 2,314 $ 2% 2,279 $ 17% 246.4 $ 464 $ 

Base ++

 Smaller A/C (5 -> 4 tons) 22806 0 233.7 4% 2,281 $ 4% 2,246 $ 18% 32.9 $ 497 $ 

Base ++

 Inc. Bsmt Wall Insulation 22499 0 230.5 5% 2,250 $ 5% 2,216 $ 19% 30.2 $ 527 $ 

Base ++

Ground Source HP (+DHW) 18380 0 188.3 22% 1,838 $ 22% 1,810 $ 34% 405.7 $ 933 $ 

Base ++

 Solar DHW 17202 0 176.3 27% 1,720 $ 27% 1,694 $ 38% 116.0 $ 1,049 $ Base ++

 Lighting, Appl. & Plug 15330 0 157.1 35% $ 35% 1,510 1,533 $ 45% 184.4 $ 1,233 $ 

Site Generation

Base ++

 PV 7928 0 81.2 67% 793 $ 781 $ 72% 729.0 $ 1,962 $ 

National Average Regional Standard (Local Costs)

Site Energy Source Energy Energy Cost Energy Cost

Table 8. Summary of End-Use Site Energy 

 

Table 9. Summary of End-Use Source Energy and Savings 

Table 10. Energy Savings of Energy Efficiency Measures 
43 

5.4 What if . . . Investigating Possible Improvements 

The Hathaway house reduced total energy use by 35% as compared to the Building America 

Benchmark (not including the contribution of the PV system). The energy-use reduction by enduse is given in the second column of Table 11. While impressive in its application of individual 

technologies, the overall result leaves two-thirds of the energy use untouched. 

Lighting and DHW energy use was reduced by a large fraction and was close to the limits of 

practical energy-saving technologies. The space-conditioning energy use could be reduced 

further by eliminating some of the heating and cooling loads with more efficient shell 

technology, as well as by applying even more efficient space-conditioning equipment. The 

appliance and plug load category was nearly untouched by the energy-reduction measures. 

Table 11. Energy Reduction by End-Use Compared to the Building America Benchmark1

Category Hathaway 

Prototype 

“Super Building” 

Indoor Lighting 61% 61% 

Appliances and Plug 

Loads 

3% 3% 

Space-Heating 

Equipment 

20% 96% 

Space-Cooling 

Equipment 

69% 89% 

DHW Equipment 72% 72% 

Exterior Lighting 71% 71% 

Total 35% 58% 
44 

Figure 35 shows the change in energy use with each added technology. The column labeled 

“Ltg/Appliance” is the Hathaway prototype model. The last four columns apply additional 

technologies in an attempt to reduce the heating and cooling loads as much as practical (and 

beyond). In the end, the heating, cooling and DHW energy use was reduced to less than 1700 

kWh per year, an 88% reduction. 

The last column in Table 11 shows the energy savings for the “Super House” that includes all the 

technologies in Figure 35. The application of building shell and other measures currently not 

cost effective, including super-insulated walls, electrochromic glazing, and highly effective heat 

recovery ventilation, can eliminate much of the remaining space-conditioning load. However, the 

appliance and plug loads come to dominate energy consumption and account for nearly 70% of 

the total energy use. 

This analysis points out that even with highly effective energy-savings technologies (pushed 

beyond levels currently practical), whole-house energy-use reduction for efficiency measures is 

only about 60% without reducing the energy use of appliances and plug loads largely considered 

outside the designer’s jurisdiction. 

Hathaway "Solar Patriot"

Maximum Savings Estimate for Whole Building Energy Use

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Electro Windows

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Hathaway

Figure 35. Energy use by end-use category, beyond the Hathaway prototype 
45 

6. Discussion 

The 2020 research goal for the Building America program is to develop cost-effective 

production-ready homes that achieve zero net energy. The Hathaway house is an early example 

of a home on the path to zero net energy. A zero net energy home is one that produces on-site as 

much energy as it consumes on an annual basis. The goal of this home was to achieve zero 

energy. Through energy efficiency and solar energy the home achieved remarkable performance 

– reducing its whole house energy needs 35% and meeting nearly 70% of the remaining 

electricity needs with solar electricity. 

6.1 On the Path to Zero Net Energy 

The Hathaway case study provides many insights into the path to commercially viable zero 

energy homes. It also raises more questions than it answers. 

It is clear that modeling tools will be required to sift through the many options for approaching 

zero energy and choose the options that achieve the goal as cost effectively as possible for the 

climate zone and builder preferences. The target is most certainly not a singularity but a 

spectrum of options. For example, many different shell designs such as wood frame, structural 

insulated panels, insulated concrete forms, and others may be parallel paths to the zero energy 

goal. The most appropriate path may depend on costs, local codes, local material availability, 

builder preferences, and market preferences. Similar parallels exist in equipment strategies. 

The zero energy home design approach will likely employ typical occupant behavior 

assumptions and typical annual weather data. A home designed to consume zero net energy 

under these assumptions may be a net consumer or a net producer with any real occupants in any 

specific year. In fact it could easily be a net producer one year and a net consumer the next. The 

task of the designer is to evaluate the range of possible home construction details near the 

economically optimum design and choose one most appropriate for the circumstances. Hence the 

design will be a combination of science and art. 

A tool, call the Building Energy Optimization (BEopt) program is currently under development 

at NREL. This tool will allow researchers to choose possible shell and equipment options and 

automatically run hundreds or thousands of hourly building energy simulations to identify the 

lowest cost combinations to achieve a given energy savings goal for a specific home geometry in 

a specific location. The program is being designed to allow the user to investigate not only the 

optimal design, but also designs that are near the optimal. 

The Hathaway house did not have the advantage of energy modeling during the design phase and 

yet achieved impressive energy efficiency. Subsequent analyses indicate that a more efficient 

shell to reduce space-conditioning loads would have brought the home closer to its zero energy 

goals. However, even with efficient lighting and appliances, the lights and appliance and plug 

loads were a significant energy consumer. About 4 kW of PV were required to meet the needs of 

these loads alone. To achieve the zero energy goal with no further efficiency increases, the 

Hathaway house would need about 2.6 kW of PV in addition to the 6.0 kW it had during this 

study. 
46 

6.2 Systems Operations and Interactions 

The Hathaway house included several advanced systems including PVs, solar water heating, and 

a ground-coupled heat pump. The PV system worked well during the monitoring period 

consistently delivering the expected performance with no maintenance issues. In May 2002 the 

power output of one of the PV inverters was interrupted when the homeowner inadvertently 

threw a fuse during a tour. We noticed the drop in power in the data, informed the homeowner, 

and the power was quickly restored. This in an advantage of monitoring home performance, but 

it also raises the question of how long it would take an average homeowner to notice if their PV 

system was not operating properly. 

The ground-coupled heat pump also performed well throughout the monitoring period with an 

average heating COP of 3.6 and cooling COP of 3.2. System performance data from the 

manufacturer list a COP of about 4.1 for heating for a horizontal tube field and a ground 

temperature of 55°F. The average water-heating COP of the system was about 1.55, which is 

similar to dedicated air-source heat-pump water heaters. Early in the project, the water-heating 

energy for the household was much higher than expected. In talking with the Hathaways, we 

learned that they switched the heat pump to heat the water in the storage tank until well after 

their morning hot water draws. Because the tank was now hot at the beginning of the day, the 

solar water-heating system operated at a very low efficiency. The situation was corrected by 

adding a timer to the water heating mode of the heat pump. The time was set to top off the tank 

temperature before the morning hot water draws. The morning use of hot water then left the tank 

filled with cooler water and the solar water heater operated much more effectively. The overall 

energy use for water heating dropped dramatically when this strategy was implemented in 

November 2001 (Figure 14). 

Although the heat pump worked well, the overall energy performance of the space-conditioning 

system was reduced because of high air-handler fan energy consumption. The air handler 

consumed more than 25% as much energy as the heat pump. This was a result of two factors: the 

occupants sometimes ran the air-handler fan when the heat pump was not operating to mix the air 

in the home, and the air handler did not use an efficient ECM motor. 

6.3 Modeling and Monitoring Approach 

We use a combination of computer modeling, testing, and monitoring to understand the 

performance of prototype houses. As a Building America project, the performance of a home is 

required to be measured against the Building America Benchmark home. The benchmark home 

is based on construction practices of the mid-1990s. To make the comparison, the prototype 

home and the matching benchmark home must be modeled to compare their energy performance. 

Therefore, modeling is an essential tool to track the progress of homes toward the Building 

America program goals for energy savings. 

Computer models provide other useful information that cannot be gained by testing or 

monitoring. The model provides generalized energy performance of a home; performance that is 

based on average occupancy behavior and typical weather patterns. In contrast, monitoring 

provides information on the performance of the home with specific occupants and actual weather 

conditions. The model can also be used to generate “what-if” scenarios (as discussed in section 

5.3) to see how the home would perform if it incorporated different building techniques and 
47 

energy technologies. These attributes make the model a powerful tool for understanding the 

performance of the home. So why not just create a computer model and skip testing and 

monitoring? 

Testing and monitoring data increase confidence in the computer model. There are some areas, 

such as shell leakage or performance of newly developed equipment, that are difficult to know in 

advance and, therefore, difficult to accurately model. In these cases, the measurements become 

input to the model and improve its accuracy. Measurements also provide a reality check for the 

computer simulation. If the simulation can accurately predict the behavior of the home given the 

measured inputs, it again increases our confidence in the model. Finally, testing and modeling 

provide information on home performance that modeling cannot provide. Testing and monitoring 

provides quick feedback on the performance of new technologies. It provides insights into 

equipment installation issues, interaction between occupants and technology, and occupant 

behavior. It can also provides the means to interpret higher level results such as energy bill 

comparison. 

Modeling, testing, and monitoring nicely complement each other and provide different insights 

into the performance of the home. We have found that a combination of them tailored to the 

objectives of the project is the best approach to understanding advanced homes. 

 
48 

7. References 

 

1 Hendron, B., et. al., 2004, Building America Research Benchmark Definition, Version 3.1, 

NREL/TP-550-36429, Golden, CO: National Renewable Energy Laboratory

2

 World Population Prospects: The 2002 Revision, February 2003, ESA/P/WP.180, United 

Nations Population Division. 

3

 United States Energy Information Administration, see http://www.eia.gov/. 

4

 Rastelli, Linda, Jan/Feb 2002, “Energy Independence—With all the Comforts,” Solar Today, 

Boulder, CO: American Solar Energy Society, Vol. 16, No. 1. 

5

 Sklar, Scott, Sept/Oct 2001, “Solar House Goes to Washington,” Home Energy Magazine, 

Berkeley, CA. 

6

 Home Buyer Publications, Inc., Mar/Apr 2001, “Solar Patriot on Parade on the National Mall,” 

Building Systems Magazine, Lakewood, CO. 

7

 “Solar Home add to Earth Day landscape,” April 18, 2001, Environmental News Network, 

www.enn.com. 

8

 For more information on the Building Energy Program of the Department of Energy see 

www.eere.energy.gov/buildings

9

 For more information on the Center for Buildings and Thermal Systems at the National 

Renewable Energy Laboratory see www.nrel.gov/buildings_thermal

10 This program no longer exists as a separate entity. It is part of the Building America program. 

For more information on the current research pertaining to zero energy homes see 

www.buildingamerica.gov.

11 For more information on the Building America Program see www.buildingamerica.gov

12 Klein, S., et al., 1996, TRNSYS: A Transient System Simulation Program – Reference 

Manual, Solar Energy Laboratory, University of Wisconsin. 

13American Society of Heating, Refrigeration and Air Conditioning Engineers, 2004, Ventilation 

and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, Fourth Public Review, 

ASHRAE Standard 62.2, Atlanta, GA: American Society of Heating, Refrigeration and Air 

Conditioning Engineers. 

14 Lawrence Berkeley National Laboratory, DOE2.2 Building Simulation Software, Berkeley, 

CA: Lawrence Berkeley National Laboratory. available at http://www.osti.gov/estsc/ or 

http://www.doe2.com 

15 Judkoff, R., and Neymark, J., 1995, International Energy Agency Building Energy Simulation 

Test (BESTEST) and Diagnostic Method, NREL/TP-472-6231, Golden, CO: National 

Renewable Energy Laboratory. 

16 Judkoff, R., and Neymark, J. Home Energy Rating System Building Energy Simulation Test 

(HERS BESTEST) Volumes 1 and 2, Nov 1995, NREL/TP-472-7332. 
F1147-E(12/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

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May 2005 

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Technical Report 

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5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

The Hathaway "Solar Patriot" House: A Case Study in Efficiency 

and Renewable Energy 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/TP-550-37731 

5e. TASK NUMBER

BET5.8004 

6. AUTHOR(S)

Paul Norton, Edward Hancock, Greg Barker, Paul Reeves 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

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REPORT NUMBER

NREL/TP-550-37731 

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Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

This report details the monitored and modeled performance of a solar home outside of Washington, D.C. We 

modeled the home energy performance using DOE2.2, performed numerous short-terms tests on the home and 

monitored its occupied performance for 29 months. The home uses modular construction, solar water heating, a 

ground-coupled heat pump, efficient appliances and compact fluorescent lighting to reduce its energy consumption 

by 35% compared to the Building America research benchmark home. The addition of 6kW of photovoltaics 

increases the savings to 67% compared to the Building America research benchmark. A more efficient shell to 

reduce space conditioning loads would have brought the home closer to its zero energy goals. However, even with 

efficient lighting and appliances, the lights, appliance and plug loads are a significant energy consumer. About 4 kW 

of PV are required to meet the needs of these loads alone. To achieve the zero energy goal with no further efficiency 

increases, the Hathaway house would need about 2.6 kW of PV in addition to the 6.0 kW it now has. 

15. SUBJECT TERMS

Building America; DOE2.2; solar home; solar patriot; hathaway house; energy performance; case study; PV; 

photovoltaics 

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Standard Form 298 (Rev. 8/98) 

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Bringing you a prosperous future where energy is clean, abundant, reliable, and affordable

A Strong Energy Portfolio 

for a Strong America

Energy effi ciency and clean, renewable 

energy will mean a stronger economy, 

a cleaner environment, and greater 

energy independence for America. 

Working with a wide array of state, 

community, industry, and university 

partners, the U.S. Department of 

Energy’s Offi ce of Energy Effi ciency 

and Renewable Energy invests in a 

diverse portfolio of energy technologies.

Research and Development 

of Buildings

Our nation’s buildings consume more 

energy than any other sector of the 

U.S. economy, including transportation 

and industry. Fortunately, the opportunities to reduce building energy use—

and the associated environmental 

impacts—are signifi cant.

DOE’s Building Technologies Program 

works to improve the energy effi ciency 

of our nation’s buildings through innovative new technologies and better 

building practices. The program

focuses on two key areas:

• Emerging Technologies 

Research and development of the 

next generation of energy-effi cient 

components, materials, and 

equipment

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Integration of new technologies 

with innovative building methods 

to optimize building performance 

and savings

For more information contact:

EERE Information Center

1-877-EERE-INF (1-877-337-3463)

www.eere.energy.gov

An electronic copy of this publication is 

available on the Building America Web 

site at www.buildingamerica.gov

U.S. Department of Energy

Energy Effi ciency

and Renewable Energy

Building America Program

George S. James • New Construction • 202-586-9472 • fax: 202-586-8134 • e-mail: George.James@ee.doe.gov

Terry Logee • Existing Homes • 202-586-1689 • fax: 202-586-4617 • e-mail: terry.logee@ee.doe.gov

Lew Pratsch • Integrated Onsite Power • 202-586-1512 • fax: 202-586-8185 • e-mail: Lew.Pratsch@hq.doe.gov

Building America Program • Offi ce of Building Technologies, EE-2J • U.S. Department of Energy • 1000 Independence Avenue, S.W. • 

Washington, D.C. 20585-0121 • www.buildingamerica.gov

Building Industry Research Alliance (BIRA)

Robert Hammon • ConSol • 7407 Tam O’Shanter Drive #200 • Stockton, CA 95210-3370 • 209-473-5000 • fax: 209-474-0817 • 

e-mail: Rob@consol.ws • www.bira.ws

Building Science Consortium (BSC)

Betsy Pettit • Building Science Consortium (BSC) • 70 Main Street • Westford, MA 01886 • 978-589-5100 • fax: 978-589-5103 • 

e-mail: Betsy@buildingscience.com • www.buildingscience.com

Consortium for Advanced Residential Buildings (CARB)

Steven Winter • Steven Winter Associates, Inc. • 50 Washington Street • Norwalk, CT 06854 • 203-857-0200 • fax: 203-852-0741 • 

e-mail: swinter@swinter.com • www.carb-swa.com

Davis Energy Group

David Springer • Davis Energy Group • 123 C Street • Davis, CA 95616 • 530-753-1100 • fax: 530-753-4125 •

e-mail: springer@davisenergy.com • deg@davisenergy.com • www.davisenergy.com/index.html

IBACOS Consortium

Brad Oberg • IBACOS Consortium • 2214 Liberty Avenue • Pittsburgh, PA 15222 • 412-765-3664 • fax: 412-765-3738 •

e-mail: boberg@ibacos.com • www.ibacos.com

Industrialized Housing Partnership (IHP)

Subrato Chandra • Florida Solar Energy Center • 1679 Clearlake Road • Cocoa, FL 32922 • 321-638-1412 • fax: 321-638-1439 • 

e-mail: subrato@fsec.ucf.edu • www.baihp.org

National Association of Home Builders (NAHB) Research Center

Tom Kenney • National Association of Home Builders (NAHB) Research Center • 400 Prince George’s Boulevard • 

Upper Marlboro, MD 20774 • 301-430-6246 • fax: 301-430-6180 • toll-free: 800-638-8556 • www.nahbrc.org/ 

National Renewable Energy Laboratory

Ren Anderson • 1617 Cole Boulevard, MS-2722 • Golden, CO 80401 • 303-384-7433 • fax: 303-384-7540 • 

e-mail: ren_anderson@nrel.gov • www.nrel.gov

Tim Merrigan • 1617 Cole Boulevard, MS-2722 • Golden, CO 80401 • 303-384-7349 • fax: 303-384-7540 • 

e-mail: tim_merrigan@nrel.gov • www.nrel.gov

Oak Ridge National Laboratory

Pat M. Love • P.O. Box 2008 • One Bethel Valley Road • Oak Ridge, TN 37831 • 865-574-4346 • fax: 865-574-9331 • 

e-mail: lovepm@ornl.gov • www.ornl.gov

Produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory, a DOE national laboratory.

May 2005 • NREL/TP-550-37731

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