A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

The NREL/Habitat for Humanity 

Zero Energy Home: 

A Cold Climate Case Study for 

Affordable Zero Energy Homes 

P. Norton and C. Christensen 

National Renewable Energy Laboratory 

E. Hancock and G. Barker 

Mountain Energy Partnership 

P. Reeves 

Partnership for Resource Conservation 

Technical Report 

NREL/TP-550-43188 

June 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
The NREL/Habitat for Humanity 

Zero Energy Home: 

A Cold Climate Case Study for 

Affordable Zero Energy Homes 

P. Norton and C. Christensen 

National Renewable Energy Laboratory 

E. Hancock and G. Barker 

Mountain Energy Partnership 

P. Reeves 

Partnership for Resource Conservation 

Prepared under Task No. BET88001 

Technical Report 

NREL/TP-550-43188 

June 2008 

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
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Acknowledgments 

The authors would like to thank the many individuals and organizations that supported 

this project: homeowner Amy Whalen; The U.S. Department of Energy Building 

Technologies Program; Midwest Research Institute; Battelle; Energy Outreach Colorado; 

Xcel Energy; The Governors Office of Energy Management and Conservation (now the 

Colorado Governor’s Energy Office); Secretary of Energy Samuel Bodman; Colorado 

Congressman Bob Beauprez; Lori Vaclavic, Bruce Carpenter, Andy Blackmun, Brady 

Nelson, Pandora Reagan, and Lynne Brown of Habitat for Humanity of Metro Denver; 

Paul Kreischer of Lightly Treading; Pete Beverly; Ren Anderson, Admiral Truly, Dan 

Arvizu, Jessie Harris, Byron Stafford, Kerry Masson, Lee Boughey, Sarah Barba, Jay 

Burch, and Scott Horowitz of NREL; Betsy Petit of Building Science Corporation; 

Barbara Miller of the National Affordable Housing Network; and Rob Dumont of the 

Saskatchewan Research Council. 

i
Executive Summary 

The design of this 1,280-square-foot, three-bedroom Habitat for Humanity of Metro 

Denver zero energy home carefully combines envelope efficiency, efficient equipment, 

appliances and lighting, and passive and active solar features to reach the zero energy 

goal. The home was designed with an early version (July 22, 2004) of the BEOpt 

building optimization software; DOE2 and TRNSYS were used to perform additional 

analysis. This engineering approach was tempered by regular discussions with Habitat 

construction staff and volunteers. These discussions weighed the applicability of the 

optimized solutions to the special needs and economics of a Habitat house—moving the 

design toward simple, easily maintained mechanical systems and volunteer-friendly 

construction techniques. A data acquisition system was installed in the completed home 

to monitor its performance. 

This report details the design of the home, presents detailed performance data from the 

first year of operation, and includes summary performance data from the second year of 

operation. 

The home appeared on Fox National News, was visited by Secretary of Energy Samuel 

Bodman and Congressman Bob Beauprez, and has been written up in Home Energy 

Magazine, Energy Design Update, countless Web pages, and several local newspaper 

stories. The home was also on the National Solar Tour in 2005. Habitat Metro Denver 

and NREL continue to receive queries about the home. 

Some overall conclusions from the project are listed below: 

• The NREL/Habitat ZEH exceeded its goal of zero net source energy and was a net 

energy producer for the first two years. The home produced 24% more energy 

than it consumed on a source energy basis in the first year of monitoring and 12% 

more energy than it consumed in the second year. 

• PV system sizing for ZEHs is challenging. 

o Total home energy use for a specific house becomes highly uncertain 

because of occupant choices and behaviors. 

o Meeting the ZEH design goal depends on occupant behaviors. 

o The economics of excess annual PV production depends on net metering 

agreements. 

• Zero energy does not necessarily mean a zero utility bill. 

o There are fixed monthly costs for NG and electricity service. 

o NG costs may not be displaced by net electricity production. 

• Efficient, affordable ZEHs can be built with standard construction techniques and 

off-the-shelf equipment. Meeting the BA goal of cost neutral ZEH in all housing 

sectors will require additional research on cost-effective efficiency options. 

ii
Acronyms 

BA Building America (Program) 

CFL compact fluorescent lamp 

DOE U.S. Department of Energy 

ERV energy recovery ventilator 

GCHP ground-coupled heat pump 

MEL miscellaneous electricity load 

NG natural gas 

NREL National Renewable Energy Laboratory 

PV photovoltaic 

SHGC solar heat gain coefficient 

TMY typical meteorological year 

ZEH zero energy home 

iii
Contents 

Acknowledgments................................................................................................................ i 

Executive Summary ............................................................................................................ ii 

Acronyms ........................................................................................................................... iii 

Contents ............................................................................................................................. iv 

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

Design Criteria and Process ................................................................................................ 2 

Project Design Criteria .................................................................................................... 2 

Using Computer Simulation in the Design ..................................................................... 3 

Final Home Design ............................................................................................................. 5 

Envelope Design ............................................................................................................. 5

Ventilation System .......................................................................................................... 8 

Space Heating System..................................................................................................... 9 

Water Heating System .................................................................................................. 11 

Photovoltaic System Sizing .......................................................................................... 11 

Construction Costs ............................................................................................................ 14 

Data Acquisition System Design ...................................................................................... 18 

Home Energy Rating......................................................................................................... 20 

First Year Home Performance .......................................................................................... 20 

Base Other Electricity Loads ........................................................................................ 23 

Photovoltaic Electricity Production .............................................................................. 24 

Peak Electricity Demand............................................................................................... 26 

Solar Water Heating ...................................................................................................... 27 

Utility Bills.................................................................................................................... 29 

Space Heating and Comfort .......................................................................................... 31 

Modeled versus Actual Performance ............................................................................ 32 

Second Year Performance Summary ................................................................................ 35 

Discussion and Conclusions ............................................................................................. 37 

References ......................................................................................................................... 40 

iv
v

Figures 

Figure 1. Comparison of annual source energy from the design-phase DOE2 model ....... 3 

Figure 2. Completed NREL/Habitat Zero Energy Home ................................................... 5 

Figure 3. Wall construction of NREL/Habitat ZEH ........................................................... 6 

Figure 4. Photograph showing stud wall and raised heel truss construction ...................... 7 

Figure 5. Window distribution for sun tempering in the ZEH ............................................ 8 

Figure 6. Graphs comparing energy use in typical Habitat house and in NREL/Habitat 

ZEH .................................................................................................................... 12 

Figure 7. Construction costs for the NREL/Habitat ZEH and standard practice home .... 17 

Figure 8. Monthly site electricity consumption by end use .............................................. 21 

Figure 9. Monthly site NG consumption by end use ........................................................ 21 

Figure 10. Monthly source energy consumption by end use ............................................ 22 

Figure 11. Annual average hourly profile of the other electric loads ............................... 24 

Figure 12. Daily and cumulative net site electricity use ................................................... 25 

Figure 13. Peak demand on June 15, 2006 ....................................................................... 26 

Figure 14. Peak demand on July 25, 2006 ........................................................................ 27 

Figure 15. Hourly average indoor and outdoor temperatures ........................................... 31 

Figure 16. Simulated and measured monthly electricity consumption ............................. 33 

Figure 17. Simulated and measured monthly NG consumption ....................................... 33 

Figure 18. Simulated and measured monthly NG consumption ....................................... 36 

Tables 

Table 1. Summary of NREL/Habitat ZEH Attributes ...................................................... 13 

Table 2. Construction Cost Data for the ZEH and Reference House ............................... 15 

Table 3. Construction Differences between the ZEH and the Reference House .............. 16 

Table 4. Measurements and Components of the Data Acquisition System ...................... 19 

Table 5. 12-Month Performance Summary of NREL/Habitat ZEH ................................. 20 

Table 6. Annual Source Energy by End Use .................................................................... 23 

Table 8. Predicted and Measured Performance of the Solar Water Heating System ....... 28 

Table 9. Comparison of Thermal and PV Solar Water Heating Systems Based on 

Measured Data from April 2006 through March 2007 ...................................... 29 

Table 10. Total Annual and Average Monthly Utility Bills for the Monitored Period .... 30 

Table 11. 24-Month Performance Summary of NREL/Habitat ZEH ............................... 35 

Table 12. Cost of Energy at the Oak Ridge, Tennessee Rate Structure ............................ 38 
Introduction 

In October 2005, Amy Whalen and her two sons moved into their new Habitat for 

Humanity home near Denver, Colorado. In doing so they became partners with the U.S. 

Department of Energy (DOE) Building America (BA) program in a case study that aimed 

to understand how to create affordable zero energy homes (ZEHs) in cold climates. The 

home was a result of collaboration between the National Renewable Energy Laboratory 

(NREL) and Habitat for Humanity of Metro Denver. This report will detail the design, 

construction, and performance of the home. 

The home appeared on Fox National News, was visited by Secretary of Energy Samuel 

Bodman and Congressman Bob Beauprez, and has been written up in Home Energy 

Magazine, Energy Design Update, countless Web pages, and several local newspaper 

stories. The home was also on the National Solar Tour in 2005. Habitat Metro Denver 

and NREL continue to receive queries about the home. This attention provides better 

visibility for project sponsors and equipment donors and may equate to more potential 

sponsors for Habitat. For the affordable home builders, this visibility is a benefit that 

should be considered in decisions to pursue super-efficient home projects. 

A ZEH is designed to produce as much energy as it consumes over the course of a full 

year. The home uses the utility power grid for storage—delivering energy to the grid 

when the photovoltaic (PV) system produces more energy than the home uses and draws 

from the grid when the PV system produces less energy than the house needs. This 

approach eliminates the need for battery storage and reduces the cost, complexity, and 

maintenance of the solar electric system. 

Homes account for 37% of all U.S. electricity consumption and 22% of all U.S. primary 

energy consumption (DOE 2007). This makes home energy reduction an important part 

of any plan to reduce U.S. contribution to global climate change. The goal of the DOE 

BA Program is to create commercially viable ZEHs by 2020. This project is a case study 

in reaching that goal within the affordable housing sector in cold climates. Zero energy is 

especially important in this sector, where increasing energy costs can take a high toll on 

homeowners with limited economic resources. A ZEH guarantees long-term energy cost 

stability for these homeowners. 

1
Design Criteria and Process 

From its inception, the NREL/Habitat ZEH project focused on finding the balance 

between engineering ideals and real-world practicality. The team that designed the home 

included two NREL building energy researchers, two Habitat staff members (the 

construction manager and the real estate development manager), and two Habitat energy 

subcommittee volunteers. The NREL engineers made suggestions based on modeling 

results and analysis and presented them to the design team, who then grounded the 

discussion with practical concerns and insights. This mix of perspectives led to a design 

that balanced energy performance, ease of construction, and low cost, and maintained the 

zero energy goal. 

Project Design Criteria 

A Habitat for Humanity house is an unusual opportunity: thanks to volunteers, much of 

the labor comes at no cost and some of the equipment is donated or purchased at reduced 

cost. We established the following criteria for the home design: 

1. Its goal should be zero net energy. Zero net energy can be defined in terms of 

site energy (used at the building site) or source energy (sometimes called primary 

energy). For electricity purchased from a utility, the source energy used to 

produce and distribute the electricity is typically about three times as much as the 

delivered electricity. From a societal point of view, source energy better reflects 

the overall consequences of energy use. The home was designed to meet the 

definition of zero energy of the DOE’s BA residential energy efficiency research 

program (see the Building America Web site at www.buildingamerica.gov). It 

must have predicted zero net source energy consumption over the course of a year 

using typical meteorological year (TMY2) weather data (Marion and Urban 2005) 

and BA Benchmark (Hendron et al. 2007) assumptions about occupant behaviors 

based on average U.S. behavior in terms of temperature setpoints, miscellaneous 

electricity loads (MELs), and hot water use. 

2. It should be replicable by Habitat for Humanity. Construction techniques and 

energy efficiency technologies were vetted for their repeatability in future homes. 

3. It should take advantage of Habitat volunteer labor. When considering 

construction alternatives, we took into account that Habitat’s approach to building 

with volunteer labor presents a unique opportunity to reduce building costs. 

Construction techniques that were “volunteer friendly” and tended toward low 

material costs were favored. 

4. Tradeoffs for zero energy were done at full material cost. Although some of 

the equipment in the house was donated or bought with grants at no cost to 

Habitat, we considered the full value of these items to find the balance between 

efficiency and PV production. 

5. The home should require no special operation. This house was sold to a 

Habitat family. The design team wanted the home’s unique attributes to be as 

transparent to the family as possible. From the family’s perspective, it should be a 

normal home with no extra owner operating needs. 

2
6. No prototypes are used. We designed the home with off-the-shelf proven 

technologies. Although optimal research systems were discussed as part of the 

design process, the final design aimed to use commercially available products to 

come as close as possible to the ideal. Because the home is expected to outlive all 

its mechanical systems, we wanted these systems to be easily replaceable by 

technicians who could be found in the local yellow pages. 

7. Keep it simple. Many ZEHs being designed today involve complicated 

interconnected mechanical systems that are designed to maximize renewable 

energy use and distribution. We were often tempted in this direction; however, we 

tried to keep it simple. We believe a simpler system will have fewer problems and 

a greater chance at longevity. 

Using Computer Simulation in the Design 

The designers used a combination of computer simulations and heuristic judgment. Three 

simulation tools were used sequentially and iteratively during the design process: 

• TRNSYS Transient System Simulation software. TRNSYS is a highly flexible 

simulation program (Klein et al. 1996). It was used in this project to investigate 

solar water heating options and as part of the BEOpt program. 

• DOE2 Building Energy Model. DOE2 can be found on the Lawrence Berkeley 

National Laboratory Web site at www.doe2.com. It takes input on the 

construction of a buildings and typical weather data and runs hour-by-hour 

simulations of the energy performance over an entire year. Annual source energy 

use results from the design-phase DOE2 model are shown in Figure 1. 

Figure 1. Comparison of annual source energy from the design-phase DOE2 model 

3
• BEOpt Building Energy Optimization Program (Christensen, Barker, and 

Horowitz 2004; Christensen, Barker, and Tupper 2004). BEOpt is an advanced 

front-end program with a database of construction techniques, efficiency 

technologies, and costs that runs DOE2 and TRNSYS iteratively to find the least 

cost approach to reach zero energy performance. BEOpt is under development at 

NREL at this time. An early version (July 22, 2004) of the program was used in 

the NREL/Habitat ZEH project as a design tool and as a real-world test 

application of BEOpt. 

We ran the BEOpt program early in the design process. Among other things, BEOpt 

indicated that for this climate and current energy and PV costs, a superinsulated envelope 

and a large solar water heating system were economic choices. As the design advanced, 

BEOpt became unavailable as it was upgraded to a new version. We turned to DOE2 for 

some parametric studies of window size, window type. overhang size, and double dry 

wall for thermal mass. We used TRNSYS simulations in parallel to study the collector 

size and tilt and the storage size for the solar water heating system. 

4
Final Home Design 

The completed NREL/Habitat ZEH is shown in Figure 2. 

Figure 2. Completed NREL/Habitat Zero Energy Home 

(Credit: Pete Beverly) 

Envelope Design 

The home design began with a site evaluation. Habitat Metro Denver had acquired, or 

was in the process of acquiring, several plots at the project’s inception. We chose a site in 

Wheat Ridge that had fairly good solar access. A few large, well-established trees lined 

the south edge of the two-site plot, so we located the ZEH on the north side of the plot. A 

shading analysis at the site indicated that about 15% of the total solar energy available 

annually would be blocked by the trees. 

We began to design the envelope by looking at Habitat Metro Denver’s standard home 

plans. We sorted these plans for their applicability to the site and adaptability for a 

passive solar design. A standard three-bedroom, 26-ft × 46-ft design with a crawlspace 

was chosen. The floor plan was mirrored from its original design to accommodate the site 

Motivated by BEOpt’s recommendation for a superinsulated envelope, the design team 

considered a wide variety of approaches. Structural insulated panels and insulated 

concrete forms were eliminated because they tend to have high material costs and low 

labor costs—the opposite of what is needed to take advantage of Habitat volunteer labor. 

Straw bale construction was carefully considered because it has low material costs and 

high labor costs. However, after reviewing the literature about straw bale construction 

and speaking with other Habitat affiliates who have built with straw, we eliminated this 

option because standard techniques and details are lacking and because Habitat Metro 

Denver will be probably be unable to replicate it. We chose a double stud wall with 

fiberglass batt construction (see Figure 3) because of its low material costs, familiar 

5
volunteer-friendly construction techniques, and proven construction techniques and 

details, which are available from the National Affordable Housing Network. This type of 

Figure 3. Wall construction of NREL/Habitat ZEH 

6
construction has a long history in Montana and Canada where the cold climate justifies 

investing in highly insulated walls. When pursuing the zero energy goal, we compared 

efficiency option costs to the cost of energy generated by PV. Because PV energy is 

currently more expensive than energy from the utility grid, higher investments in 

efficiency are justified and double stud wall construction is justified in a much warmer 

climate than it has previously been widely used. 

The walls consist of an outer 2 × 4 structural stud wall on 16-in. centers with R13 

fiberglass batts in the cavities. Spaced 31⁄2 in. inside this wall, a second 2 × 4 stud wall on 

24-in. centers was built. Additional R13 fiberglass batts were placed horizontally in the 

space between the stud walls and vertically in the interior wall cavities. An outer vapor 

permeable house wrap and fiber cement siding and an inner poly vapor barrier and 

drywall complete the nominal R40 assembly. The clear wall R-value of this wall is much 

closer to its nominal value than a single stud wall because the thermal shorting of the 

studs is broken by the insulation in the space between the double stud walls. Figure 4 

shows the stud wall construction. 

Figure 4. Photograph showing stud wall and raised heel truss construction 

Raised heel trusses were designed to accommodate 2 ft of blown-in fiberglass in the attic, 

which gives the top of the thermal envelope an R60 rating. Figure 4 shows the raised 

heels of the roof trusses. Because these levels of insulation are difficult to reach in 

crawlspace walls and floors, and because we intended to have all mechanical equipment 

and ducts inside the home, we decided to insulate the floor above the crawlspace. We 

chose fiberglass batts the thickness of the TJI floor joists, which provided a nominal R30 

in the floors. The wastewater drainpipe slopes through the uninsulated, vented 

crawlspace. During normal operation this presents no problems even during a cold 

winter. However, if a slow drip occurs in the house during the winter, the drainpipe could 

freeze closed and cause a backup inside the house. The homeowners should be informed 

of this potential problem and encouraged to have faucet leaks fixed immediately. 

The superinsulated shell dramatically reduces heating energy needs; “sun tempering” 

reduces these needs further (see Figure 5). We increased the glazing area on the south 

7
side of the home and reduced it on other orientations, but added no additional thermal 

mass. We evaluated the use of double drywall on the ceilings, where it could easily be 

applied without changing trim details, but DOE2 modeling suggested it would have 

minimal benefit. We decided not to pursue other thermal mass options such as massive 

floors or double drywall because they complicated construction and reduced the chances 

of replicating the envelope design. As a policy, Habitat Metro Denver does not equip its 

homes with air-conditioning, so we were sensitive to overheating potential. We used 

DOE2 to evaluate different southern glazing areas and types and overhangs. We 

compared the heating energy and simulated cooling energy (as if there were an airconditioning system) in the ZEH and in the identical standard construction home for each 

window combination. We chose the design that maximized heating reduction without 

increasing cooling energy over the standard construction. We thus maximized heating 

energy displacement without increasing overheating potential. Double-glazed, lowemissivity (U-value = 0.30 Btu/h-F-ft2

) high solar heat gain coefficient (SHGC = 0.58) 

glass was chosen for the southern windows. Double-glazed clear windows would have 

provided more solar heating, but also would have increased the overheating potential. 

Double glazed low emissivity (U-value = 0.22 Btu/h-F-ft2) low SHGC (0.27) were used 

for the east, west, and north windows. The final design heating load for the home was 

15,000 Btu/h. 

Figure 5. Window distribution for sun tempering in the ZEH 

Ventilation System 

Because we intended to build the home with very low air leakage, a mechanical 

ventilation system was required. To provide fresh air to the home and minimize energy 

losses, we chose to use a balanced energy recovery ventilation (ERV) system. The ERV 

exhausts air from the kitchen and bathroom and supplies fresh air to the living room and 

bedrooms. The warmth of the exhaust air is used to heat the incoming fresh air. This 

8
significantly reduces the heat loss from ventilation. We chose an ERV with efficient 

electronically commutated motors. 

Space Heating System 

Having a very low design heating load is a blessing and a challenge. The blessing is 

obvious—very little energy is required to keep this home warm. The challenge is that 

most commonly available heating systems are too large for this home and the low heating 

energy needs cannot justify a complicated or expensive system. We considered a wide 

variety of heating systems for the home: 

• Active solar thermal with radiant floor, baseboard heaters, or fan coil in the ERV 

supply 

• Ground-coupled heat pump 

• Point-source natural gas (NG) furnace (no duct system) 

• Electric resistance baseboard heating 

The design team considered a solar “combisystem” that combines active solar thermal 

space heating and water heating. An active solar thermal system with a large collector 

array and a large, well-insulated water tank for thermal storage is an attractive idea for a 

ZEH. In fact, some ZEHs have employed this approach (Daub 2006). A properly sized 

system may completely eliminate the need for an electric or NG heating system. 

However, some challenges accompany this approach. The collectors, storage, pumps, and 

controls require a relatively large equipment investment. During the summer and at times 

during the fall, winter, and spring when the house is passively solar heated, there is no 

need for active heating, so the additional equipment investment for space heating delivers 

no energy benefit for most of the year. Also, a surplus of heat is collected in the summer 

when it is not needed. If the storage tank is indoors, the surplus summer heat can increase 

the cooling load or overheat the house. A distribution system is needed to move the heat 

into the home when it is needed. The pumps and fans that collect and distribute the heat 

can consume substantial electricity. Few integrated solar combisystems are commercially 

available in the United States, so most are custom designed and built and can be quite 

complicated. Combisystems in the BA Program homes have had operational challenges 

and have met only a small fraction of the heating load (Hendron et al. 2007). 

Combisystems are more widely used in Europe and have been studied extensively by the 

International Energy Agency (Weiss 2003). We decided that the high first cost, low use, 

complexity, and custom design of this approach were not consistent with our design 

criteria. 

We pursued options such as radiant floors, hydronic baseboard heaters, and a coil in the 

ERV supply air for distributing the heat from an active solar space heating system. 

Radiant floor heating is a good match for active solar heating because it requires a low 

water temperature. However, radiant floors were considered too expensive for affordable 

housing and are unlikely to be replicated. The supply air of the ERV can be used to 

distribute heat. Hot water from the solar tank is circulated through a coil downstream of 

the ERV supply fan. To supply enough heat the ERV would have to be operated in its 

9
high flow mode. Even at this rate, the amount of delivered heat would be barely enough 

for this small house. The ERV is a balanced system, so the air changes in the home would 

increase when the ERV is placed in high flow mode to deliver heat. This would 

overventilate the home and waste some of the delivered heat with the exhausted air. A 

solution might be to operate the ERV in defrost mode where dampers are positioned to 

pass inside air in place of outside air through the ERV. However, when the home was 

being designed, we were not able to locate any commercially available ERVs or heat 

recovery ventilators that implemented this strategy for space heating. Therefore, a custom 

control system—which violates the design criteria—would have had to be developed. If 

we had pursued active solar space heating, we would likely have used hydronic 

baseboard heaters to distribute the heat. This delivery system cost is quite low. The 

disadvantage is that high delivered water temperature is necessary, which means the solar 

heated water would often need backup heating to meet the required delivery temperature. 

In the end, we decided against an active solar water heating system because of its costs 

and complexities. 

For an all-electric ZEH, using a ground-coupled heat pump (GCHP) for heating has some 

attractive benefits. The GCHP can deliver three to four units of heat for each unit of 

electricity used. In contrast, electric resistant heat delivers one unit of heat for every unit 

of electricity consumed. The GCHP can also deliver cooling in the summer, but the heat 

pump and the ground loop are quite expensive and would require an air handler and duct 

system to deliver the heating. The compact size and superinsulated shell of the 

NREL/Habitat ZEH reduced heating needs to such a low level that the cost of the GCHP 

was not justified. 

The use of NG for heating, cooking, and clothes drying in a ZEH is somewhat 

controversial. Some believe that because a ZEH exports only electricity, it must consume 

only electricity. However, in most of the United States, the electricity consumed comes 

primarily from fossil fuels. So the home consumes fossil fuels when it uses electricity and 

offsets that consumption when it produces excess PV electricity. This is similar for a 

ZEH that consumes NG. The PV system is sized to produce and excess electricity to 

offset the NG used. The source energy use is net zero. 

The economics of these choices differs. An all-electric home that is too small and 

efficient to use a GCHP requires a larger PV system and is substantially more expensive. 

The all-electric approach has the advantage of eliminating the monthly fixed cost of 

having an NG hookup, which is about $9/month in the Denver, Colorado area. The 

NREL/Habitat ZEH design team decided to use NG to reduce the required PV array size 

and to take a hybrid approach to space heating. 

The space heating system combines a point-source direct vent NG furnace in the 

living/dining area of the home and small baseboard electric resistive heaters in the three 

bedrooms. This approach is relatively low cost, elegantly simple, and provides zone 

heating because each appliance has its own independent thermostat. 

10
Water Heating System 

Although we ruled out a solar combisystem, the results of the early BEOpt runs 

convinced us to incorporate a high solar saving fraction water heating system into the 

home design. We used TRNSYS to conduct parametric studies. We found that mounting 

the collectors flat at the roof pitch rather than raising them to their optimal angle incurred 

only a small energy penalty (Christensen and Barker 2001). TMY weather data and BA 

Benchmark hot water use indicated that a 96-ft2 collector area with 200 gal of water 

storage would result in an 88% annual solar saving fraction that includes pump energy 

losses. All summer hot water needs would be exceeded with this system. This means that 

the thermal storage will reach maximum storage temperature many times throughout the 

summer and the collectors will stagnate. If we used a glycol system, the frequent high 

stagnation temperatures may break down the glycol. The pressurized 200-gal storage tank 

would also be quite expensive, so we chose a drainback system. 

We specified an NG tankless water heater as a backup to the solar system. Unlike tank 

water heaters, the tankless system uses no heating energy when the solar water tank is at 

or above the 115°F hot water delivery temperature. The disadvantage of using the 

tankless system is the added cost compared to a tank system. We considered using the 

tankless water heater for space heating, but ultimately decided to use separate systems to 

avoid the complexity of the combined system. 

Photovoltaic System Sizing 

Once all possible energy loads in the house were significantly reduced, the PV system 

was sized to meet the remaining electricity needs and offset the expected NG use. In a 

similar home built to BA Benchmark standards, about one-fourth of the energy in the 

home is consumed by lighting, appliances, and MELs. We reduced the lighting load by 

using CFLs throughout the home. The appliance load was reduced with ENERGY 

STAR® appliances. This leaves the MELs, which include everything the occupants plug 

in—TV, hair dryer, toaster oven, computer, aquarium, etc. Because all other loads have 

been dramatically reduced, the MELs in the NREL/Habitat ZEH are expected to consume 

57% of all energy used annually (see Figure 6). Although the BA Program is researching 

ways to reduce these loads, they are currently out of the control of the home designer. 

Furthermore, these loads are highly unpredictable and vary substantially from household 

to household. Thus, the ZEH designer is faced with sizing a PV system for a home where 

the largest load is not known with any accuracy. 

The BA Benchmark includes assumptions that we used to estimate the MELs and size the 

4-kW PV system. These assumptions are based on the best available nationwide studies 

of energy use (Hendron 2005), so the home’s PV system is sized with the assumption that 

it will be occupied by a “typical” American household. If the household were typical, the 

home would achieve zero energy. If the household were atypical, the home may not 

achieve zero energy or may be a net producer. (See Table 1 for a summary of the 

NREL/Habitat ZEH attributes.) 

11
Figure 6. Graphs comparing energy use in typical Habitat house and in NREL/Habitat ZEH 

Space Heating

35%

Water Heating

28%

Appliances and 

Plug Loads

28%

Lighting

9%

Annual energy use in typical Habitat house

Under the 

control of the 

occupant 

Space Heating

23%

Water Heating

12%

Appliances and 

Plug Loads

57%

Lighting

8%

Annual energy use in Habitat ZEH

Under the

control of the

occupant

12
Table 1. Summary of NREL/Habitat ZEH Attributes 

Square footage 1,280 ft2

Number of bedrooms 3 

Number of occupants 3 

Design heating load 15,000 Btu/h 

Walls 

Double stud wall 

Fiberglass batt insulation 

Nominal R-value = 40 h ft2

 F/Btu 

Ceiling 

2-ft raised heel trusses 

Blown-in fiberglass insulation 

Nominal R-value = 60 h ft2

 F/Btu 

Floor Fiberglass batt insulation 

Nominal R-value = 30 h ft2

 F/Btu 

South windows Low-e, high SHGC 

U = 0.30 Btu/h ft2

 F, SHGC = 0.58 

North, west, and east windows Low-e heat mirror 

U = 0.23 Btu/h ft2

 F, SHGC = 0.27 

Solar tempered 96 ft2 of south facing windows 

3-ft overhangs for summer shading 

Water heating 

Drainback solar system 

96-ft2 collectors with 200-gal storage tank 

NG tankless water heater for backup 

Ventilation ERV system with electronically computated motors 

Space heating Direct vent ductless NG heater in living room 

Electric baseboard heaters (750 W each) in bedrooms 

Lighting CFLs throughout the house 

Appliances ENERGY STAR clothes washer and refrigerator 

Solar electric Nominal 4-kWp DC PV system 

Other features 

All mechanical equipment is within conditioned space 

Light-colored roof shingles 

Increased attic ventilation 

13
Construction Costs 

Habitat Metro Denver tracks the construction costs of all the homes it builds. All site, 

material, and labor cost for trades such as plumbers and electricians, as well as the value 

of donated material, are recorded and categorized within the cost data. The value of the 

volunteer labor used to build the house is not included in the cost figures. Even if the 

volunteer hours put into the home were recorded, it would be difficult to value these 

hours because the experience of the volunteers varies considerably. The time Habitat staff 

work on the home is also not included in the cost data. 

In 2005, when Habitat Metro Denver built the ZEH, it also built a home that closely 

matches the ZEH in square footage and number of bedrooms. This home was built to 

Habitat Metro Denver standard building practices and is used as a reference for 

comparison to the ZEH. 

Table 2 shows the cost categories divided into two sets: those that may have been 

affected by the ZEH design and those that were not. Table 3 shows the differences 

between the ZEH and the standard practice home in the cost categories that may have 

been affected by the ZEH design. A bar chart of the costs affected by the ZEH design is 

shown in Figure 7. 

Overall, the ZEH cost 8% more per square foot than the standard practice home. 

However, this number is a bit misleading because the land, water, and sewer costs for the 

standard practice home were substantially higher than for the ZEH. The totals for the cost 

categories affected by the ZEH design were 42% higher for the ZEH than for the standard 

practice home. The home cost is affected nearly as much by the land and site-related 

costs as by its design to reach the zero energy goal. 

The main incremental costs and savings associated with the ZEH are highlighted in 

Figure 7. The actual system costs for the solar electric and water heating systems are 

shown along with incremental costs for framing, windows, and insulation. Incremental 

cost savings are seen in the mechanical category because of the simplified ductless 

heating system in the ZEH. Combining the costs and savings in only the six categories 

shown yields a total incremental cost of $28,054—about a 21% incremental cost over the 

total standard practice home cost. 

The cost shown for the solar electric system ($17,489) is the actual, deeply discounted 

price paid to Altair Energy. Before the Colorado Amendment 37 rebates went into effect, 

the installed price for a 4-kW solar electric system was $32,000 to $40,000. At the time 

of this writing, with the Amendment 37 rebates, a 4-kW system may cost somewhat less 

than the NREL/Habitat ZEH system. Because of the timing of the NREL/Habitat ZEH 

system installation, it was eligible for the rebates as well. The rebates entirely paid for the 

installed system cost. However, the benefit of the rebates was not included in this cost 

analysis. When developing plans for ZEH homes, it is important to check for local 

rebates. 

14
Table 2. Construction Cost Data for the ZEH and Reference House 

ZEH Standard 

Practice Home 

Incremental 

Costs 

Square feet 1,284 1,222 

Number of bedrooms 3 3 

May Be Affected by ZEH Design ($)

Excavation, foundation 10,630 10,543 87 

Joists, decking, framing 9,497 6,029 3,468 

Concrete flatwork 5,017 4,248 769 

Windows, exterior doors 3,099 1,561 1,538 

Trusses, roof, sheathing 3,496 6,137 –2,641 

Shingles, gutters 1,723 1,697 26 

Siding 4,423 4,696 –273 

Mechanical 2,600 5,805 –3,205 

Electrical 19,744 4,567 15,177 

Plumbing 14,002 6,340 7,662 

Insulation 2,893 1,197 1,696 

Drywall 3,174 2,473 701 

Painting, staining 1,498 1,548 –50 

Appliances 903 1,792 –889 

Subtotals 79,525 56,160 23,365

Not Affected by ZEH Design ($)

Vinyl floors 1,784 1,025 759 

Interior trim 537 2,177 –1,640 

Carpet 1,440 1,209 231 

Cabinets 1,384 1,384 0 

Land 18,797 39,729 –20,932 

Permits 4,529 1,565 2,964 

Temporary utilities 636 1,361 –725 

Landscaping 5,509 3,480 2,029 

Property taxes 33 313 –280 

Property development 6,101 4,978 1,123 

Soils, surveys 1,219 2,005 –786 

Water, sewer 24,603 13,013 11,590 

Punch list 80 355 –275 

Subtotals 69,826 75,067 –5,241

Grand totals 149,351 131,227 18,124

Cost per square foot 116 107 8%

15
Table 3. Construction Differences between the ZEH and the Reference House 

Categories Differences between the ZEH and Standard Practice Homes 

Excavation, foundation 

Joists, decking, framing 

Concrete flatwork 

ZEH foundation is larger to allow for the thicker walls and still 

create a floor plan identical to the standard practice house. The 

ZEH walls are double 2 x 4 stud walls. The standard practice 

home is a single 2 x 6 stud wall. 

Windows, exterior doors 

The windows in the ZEH are orientation specific with heat mirror 

windows on the north, east, and west and double-pane high 

SHGC low-e windows on the south. The southern window area is 

enlarged for solar gain. The standard practice home has double 

pane low SHGC, low-e windows throughout. 

Trusses, roof, sheathing 

Shingles, gutters 

The ZEH has a larger raised heel truss to allow for added 

insulation. The roof is also larger due to the thicker walls and to 

provide proper southern window overhangs. 

Siding 

Painting, staining 

The exterior of the home is somewhat larger than the standard 

practice home because of the thicker walls. 

Mechanical 

The ZEH uses inexpensive electric baseboard heaters and a 

point-source direct-vent natural gas heater. There is no air 

handler or heating duct system. The ZEH uses an energy 

recovery ventilation system. A small duct system for the 

ventilation air is contained in a drop ceiling in the hallway. The 

standard practice home includes a 90% efficient closed 

combustion furnace with sealed heating ducting in the crawlspace 

and continuous exhaust fan ventilation. 

Electrical The electrical for the ZEH includes the PV system. 

Plumbing The plumbing for the ZEH includes the solar water heating 

system. Both homes use tankless water heaters. 

Insulation 

The ZEH has fiberglass insulation in the ceiling, walls, and floor, 

(R-60,40, and 30 respectively). The standard practice home has 

R-38 fiberglass ceiling insulation, R-19 fiberglass insulation in the 

wall stud cavities with 1⁄2 inch of exterior foam sheathing, and R19 blanket insulation on crawlspace walls. 

Appliances 

Both homes use EnergyStar appliances and CFL lighting. The 

ZEH uses a stacked washer/dryer due to space limitations in the 

mechanical room. 

16
Figure 7. Construction costs for the NREL/Habitat ZEH and standard practice home 

17
Data Acquisition System Design 

A data acquisition system was installed to determine whether the home met its zero 

energy design goal. The system was designed to allow disaggregation of the PV energy 

production and some end uses. A summary of the data collected and the equipment used 

is given in Table 4. 

Data were collected at 1-min and 1-h intervals. The 1-h data were used to conduct most 

analyses of the home performance. The 1-min data were used for troubleshooting and to 

investigate transient behavior of the solar water heating system. We created an Excel 

spreadsheet with array formulas to aggregate daily and monthly averages and sums and to 

create graphics about the performance of the home. All electricity end use measurements 

were in place by February 2006. However, the water flow and NG end use monitoring 

was not completed until April 2006. Unless otherwise stated, all annual figures in this 

report include the period of April 2006 through March 2007. 

18
Table 4. Measurements and Components of the Data Acquisition System 

Measurements Component Make Model 

Electrical Energy Measurements 

PV energy production 

Baseboard electric heaters 

Hard-wired lights Pulse output Wattnode 

Kitchen range Watt-hour 

transducers 

Continental 

Controls WNA-1P-240-P 

Ventilation system 

Solar pump 

Space and water heating controls 

All other loads 

NG Measurements 

Space heater Diaphragm NG 

meters 

Backup water heater with pulse output American Meters AM250TC 

Indoor and Water Temperatures 

Living room 

North bedroom 

Southeast bedroom 

Cold water supply 

Solar tank Type T 

thermocouples Omega FF-T-20S-TWSH 

Solar—water to collectors 

Solar—water from collectors 

Solar—water to backup heater 

Hot water supply to house 

Water Flow 

Hot water use Water meter Omega 

Engineering FTB-6107-A-PS 

Weather-Related Measurements 

Outdoor temperature and relative 

humidity 

T&RH sensor 

with shield 

Campbell 

Scientific 

CS500-L 

and 4020 

Solar radiation—horizontal Pyranometer Li-Cor, Inc. LI-200SZ 

Solar radiation—plane of collectors Pyranometer Li-Cor, Inc. LI-200SZ 

Data Logging Equipment 

 Logger Campbell 

Scientific CR-10 

 Thermocouple 

multiplexer 

Campbell 

Scientific AM25T 

 Switch closure 

multiplexer 

Campbell 

Scientific SDM-SW8A 

Communications 

 Cell phone 

modem Redwing Airlink 100 

19
Home Energy Rating 

The home received a Colorado E-star rating of 95. Blower door results yielded a leakage 

rate of 460 cfm at 50 Pa (2.7 ACH 50). This corresponds to a natural ventilation rate of 

about 0.15 ACH, which indicates that the constuction crew did an excellent air sealing 

job. 

First Year Home Performance 

The home’s net source energy performance exceeded expectations. The PV system was 

sized to achieve net zero annual source energy with TMY2 weather data for Boulder, 

Colorado (Marion and Urban 1995) and BA Benchmark assumptions for occupant effects 

such as temperature setpoints and miscellaneous energy use (Hendron et al. 2005). The 

BA Benchmark represents U.S. average occupancy choices and behaviors. The occupants 

of the NREL/Habitat ZEH use less energy than the BA Benchmark occupants average 

energy users, so the home performed beyond zero and was a net source energy producer. 

A summary of the overall home performance is given in Table 5. 

Table 5. 12-Month Performance Summary of NREL/Habitat ZEH 

kWh (MBtu) 

Site Energy Summary 

Total site electricity consumption 3,585 (12) 

Total AC site PV electricity production 5,127 (17) 

Net site electricity production 1,543 (5.3) 

Total site NG consumption 1,665 (5.7) 

Source Energy Summary*

Total source energy consumption 13,025 (44) 

Total source energy offset 16,201 (55) 

Net source energy offset 3,176 (11) 

Percent of source energy consumption 

offset via on-site renewable production 124%

* The site-to-source energy conversions are U.S. national averages according to the BA Analysis 

Procedures (Hendron et al. 2004): site-to-source multiplier for electricity = 3.16; site-to-source multiplier 

for NG = 1.02. 

The monthly site electricity and NG consumption by end uses are shown in Figures 8 and 

9. The monthly source energy consumption by end use is shown in Figure 10. These 

figures are consumption only—they do not include the electricity generated by the PV 

system. Rather than being separately monitored for the entire year, the average daily 

refrigerator energy use over an 84-day period was measured and applied to each day of 

the year. 

20
0.00

100.00

200.00

300.00

400.00

500.00

600.00

Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07 Feb 07 Mar 07

Month

Monthly Site Electricity (kWh)

Electric Baseboard Heaters

Miscellaneous Electric Loads

Refrigerator

Lights

Ventilation

Cooking

Space and Water Controls

Solar Pump

Figure 8. Monthly site electricity consumption by end use 

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07 Feb 07 Mar 07

Month

Monthly Natural Gas Use (Therms)

NG Space Heater

NG Clothes Dryer

NG Backup Water Heater

Figure 9. Monthly site NG consumption by end use 

21
0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07 Feb 07 Mar 07

Month

Monthly Source Energy (MBtu)

NG Space Heating

Electric Baseboard Heaters

Miscellaneous Electric Load

Refrigerator

NG Clothes Dryer

Lights

NG Backup Water Heater

Ventilation

Cooking

Space and Water Controls

Solar Pump

Figure 10. Monthly source energy consumption by end use 

The ventilation energy use in the home was lower than expected. We found that the 

adjustment for the continuous ventilation rate installed in the mechanical room turned off 

the ventilation system when it was at the “low” setting. The ventilation system was often 

turned off during the monitoring year, so many of the monitoried ventilation data 

represent only the standby power draw. A stop on the adjustment that maintains the 

minimum ventilation rate at ASHRAE 62.2 recommendations would solve the problem. 

As expected, space heating is largest electricity, NG, and source energy consumer during 

the winter months. During the design phase, we assumed the NG heater in the living 

room would provide the bulk of the home heating. This assumption was based on 

conversations with builders who had built similarly sized double stud wall homes in 

colder climates and used point source heating with favorable results. The NG heater was 

sized to meet the entire design heating load. The baseboard heaters were seen as backups 

to the NG heater if the distribution of the heat to the bedrooms was inadequate. However, 

in reality the baseboard electric heaters accounted for 60% of the total space heating site 

energy and 82% of the total space heating source energy. This indicates the heat 

distribution to the bedrooms from the NG heater was not adequate. Additional NG 

heaters or a heat distribution system would be needed for the house to rely more on NG 

for heating. Meeting all of the heating load with natural gas would have lowered the 

source energy use in the first year by about 12%. 

Despite submetering of most large end uses, the other electricity loads was the largest 

single year-round end use category. The annual average power draw of the other 

electricity loads was about 164 W. 

22
The annual source energy by end use is given in Table 6. Generally, the end uses within 

the control of the building designer include the space conditioning, water heating, 

ventilation, and lighting. If we sum all other loads (often referred to as appliance and plug 

loads), they account for 58% of the total source energy consumption. These loads result 

primarily from occupant choices and behaviors. They vary substantially with homeowner 

and time, and presents a challenge for ZEH designers. The PV system output must be 

sized to match all energy consumption to reach the ZEH goal, but the energy 

consumption is dominated by loads that are out of the designer’s control, vary 

substantially with different homeowners, and are unknowable in advance for a specific 

home. 

Table 6. Annual Source Energy by End Use 

End Use 

Annual Source 

Energy 

MBtu (kWh) 

Percent of Total 

Other electricity loads 15.5 (4,550) 34% 

Electric baseboard heaters 9.2 (2,690) 21% 

Refrigerator 5.6 (1,630) 13% 

Lights 3.3 (970) 7% 

NG clothes dryer 2.8 (830) 6% 

NG space heating 2.0 (590) 5% 

Ventilation 1.6 (460) 4% 

Space and water controls 1.5 (420) 3% 

Cooking 1.3 (370) 3% 

Solar pump 1.0 (300) 2% 

NG backup water heating 0.7 (220) 2% 

Totals 44.5 (13,030) 100% 

The other electric loads, refrigerator, and lights add up to 7,150 kWh source energy or 

about 2,260 kWh site energy per year. This equates to an annual average power draw of 

about 258 W. This energy eventually appears as heat in the home. Based on a monthly 

energy analysis, the average total site space heating energy used during the months 

requiring heating is about 330 W. This indicates that the heat generated by the other 

electric loads, refrigerator, and lights is meeting over 40% of the average heating load. 

Base Other Electricity Loads 

The annual average hourly profile of the other electricity loads is shown in Figure 11. 

The line in this figure shows the other electricity load at a given hour of the day averaged 

over every day of the year. The annual average power draw of the other electric loads is 

164 W. About 40% of the this power draw (64 W) varies hour by hour; peaks occur in the 

morning before the occupants leave for school or work and in the evening when they 

return but before they retire for the day. The remaining 100 W is drawn continuously, day 

and night, whether or not the occupants are home. We used plug-in energy meters that 

can measure energy draws greater than 5 W to investigate these loads. The results are 

given in Table 7. The measured end uses account for 40 W of the baseline electricity 

loads. The remaining baseline other electric loads were not identified. Some hard-wired 

end uses that may contribute to the remaining load include ground fault interrupters, 

23
doorbell transformers, smoke alarms, and our data acquision system (estimated to be 7 to 

9 W). 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

100

200

300

400

500600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

19002000

2100

2200

2300

2400

Hour of the Day

kWh

Average Other Electric Loads = 164 W

Average Base Other Electric Loads = 100 W

Figure 11. Annual average hourly profile of the other electric loads 

Table 7. Measured Baseline Electricity Loads 

End Use 

Power Consumption 

(W) 

Entertainment center standby* 26 

Additional TV 6 

Computer, monitor, printer standby 5 

Digital clock (rated power draw) 3 

Microwave oven standby 0 (<5) 

Clothes washer standby 0 (<5) 

Clothes dryer standby 0 (<5) 

Totals 40

* Includes TV, stereo, cordless phone, DVD player, and digital clock. 

Photovoltaic Electricity Production 

A free PV performance calculator, called PVWatts, is available on NREL’s Renewable 

Resource Data Center Web site (http://rredc.nrel.gov). The PVWatts simulation of the 4-

kWp DC PV system using TMY2 weather data from Boulder, Colorado predicts the 

system will deliver 5,756 kWh (19.6 MBtu) of AC electricity per year with no shading. 

The PVWatts default DC-to-AC derate factor of 0.77 was used for this prediction. A 

24
Solar Pathfinder shading analysis indicated a 15% loss of solar radiation because of 

shading from mature trees on the site that reduce the expected annual PV production to 

4,892 kWh (16.7 MBtu) The actual energy delivered was 5,127 kWh (17 MBtu), which 

exceeds the prediction by 5%. The production exceeded expectations, even though the 

measured total horizontal radiation was about 4% lower than that in the TMY2 data and 

the PV system was covered in snow and produced no electricity for 35 days during 

December 2006 and January 2007. This indicates that the PVWatts default derate factor 

may be conservative or that the Solar Pathfinder shading analysis overestimated the 

impact of the shading. 

The daily and cumulative net electricity use is shown in Figure 12. The PV system 

produced more electricity than the home used nearly every day throughout the spring, 

summer, and fall. Despite the long period of net use with no production in January 2007, 

the home completed the 12-month period with a net electricity production of 1,543 kWh 

(5.3 MBtu). 

We calculated a simple monthly average PV system efficiency by dividing the monthly 

total AC electricity production by the monthly total solar radiation on the plane of the 

collectors times the area of the collectors. The monthly average efficiency varied from a 

low of 2.1% in January 2007, to a high of 13.1% in November 2006. The annual average 

efficiency was 10.2%. 

-40

-30

-20

-10

0

10

20

30

40

Feb 06 Mar 07 Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07 Feb 07 Mar 07

Month

Daily net kWh production

-4000

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

Cumulative net kWh consumption

Daily Net Electricity Production

Cumulative Net Electricity Production

Production

Consumption

Blizzards

Figure 12. Daily and cumulative net site electricity use 

25
Peak Electricity Demand 

Late summer afternoon air-conditioning loads in Colorado cause peak electricity demand. 

ZEHs often reduce peak demand compared to standard practice homes because their 

higher efficiency envelopes and equipment lead to lower energy demand and because the 

PV system may produce energy during the peak period. In the summer of 2005, a 

community of near ZEHs in California was compared to nearby homes of the same 

vintage and found to have about half the peak demand during the local utility’s peak 

demand period (Keesee and Hammon 2006). 

Because Habitat Metro Denver does not install air-conditioning systems, these homes 

inherently do not contribute to the utilities’ summer afternoon peak demand problem. 

We investigated the alignment of PV power production and peak demand during the two 

highest demand days in 2006: June 15 and July 25. Xcel Energy supplied data for its total 

Colorado demand on these days. The total Colorado demand is plotted with the ZEH 

demand and PV production in Figures 13 and 14. The peak periods are shown by the 

ligher shaded regions on the graphs. The peak demand period for Colorado occurred 

between about 2:00 p.m. and 6:00 p.m. The figures show that PV production at the 

NREL/Habitat ZEH led to net electricity production during the peak periods in 2006. 

However, at this time in the afternoon, PV production declines well beyond its own peak, 

which is around noon. This decline is exacerbated by frequent cloudy summer afternoons 

in the Denver area. July 25 was partially cloudy all day and June 15 had a cloudy 

afternoon. If peak reduction were to become a more central goal for ZEHs, the PV 

systems could be oriented toward the west rather than due south. This would reduce 

overall energy production, but would bring the peak PV production more in line with the 

peak system demand. 

Figure 13. Peak demand on June 15, 2006

26
Figure 14. Peak demand on July 25, 2006 

Solar Water Heating 

We used TRNSYS modeling software (Klein et al. 1996) to develop design expectations 

for the solar water heating system. We used the model to investigate tradeoffs with tilt 

angle, collector size, and storage tank size. The initial BEOpt results indicated that an 

investment in a high savings fraction system was justified. The final design incorporated 

a drainback system with 96 ft2

 collector lying directly on the roof (tilt angle = 27 

degrees), with 200 gal of water for thermal storage. 

We tracked the energy delivered to the backup water heater by the solar system by 

measuring the water temperature entering the solar tank heat exchanger, the water 

temperature entering the backup water heater from the solar tank heat exchanger, and the 

water flow rate. The flow*ΔT calculation is perfomed continuously by the data logger 

and stored on a 1-min basis. We also logged the electricity used by the solar pump and 

the NG used by the tankless backup water heater. We used this information to define 

three solar saving fractions: 

1. Thermal site solar saving fraction = Qs /( Qs + Qng) 

2. Total site solar saving fraction = (Qs - Ep)/Qs + Qng) 

3. Total source solar saving fraction = (Qs -(EpMe))/((Qs + QngMg)) 

Where: 

27
Qs = Thermal energy delivered by the solar system to the backup water heater 

Qng = Energy content of the NG consumed by the backup water heater 

Ep = Electrical energy used by the solar pump 

Me = 3.16 = site to source multiplier for electricity (Hendron et al. 2004) 

Mg = 1.02 = site to source multiplier for NG (Hendron et al. 2004) 

Table 8 lists the predicted and measured perfomance characteristics of the solar thermal 

system. The period of the analysis is only 10 months water flow and NG data for 

February and March 2006 are lacking. 

Table 8. Predicted and Measured Performance of the Solar Water Heating System 

Predicted Measured Percent 

Difference 

Average daily hot water use 63.4 gal 20.5 gal –68% 

Delivered energy 12.29 MBtu 

(3,602kWh) 

2.21 MBtu 

(647 kWh) –82% 

Pump energy 0.638 MBtu 

(187 kWh) 

0.321 MBtu 

(94 kWh) –50% 

Ratio of pump energy to delivered energy 0.052 0.145 179% 

Maximum monthly thermal site solar 

saving fraction 1.00 0.95 –5% 

Annual thermal site solar saving fraction 0.92 0.75 –18% 

Annual site solar saving fraction 0.88 0.64 –27% 

Annual source solar saving fraction 0.78 0.40 –49% 

The delivered energy of the solar water heater was a small fraction of the predicted value. 

The main reason for this appears to be that the occupants used less than one-third of the 

predicted average daily hot water. The prediction is based on the BA Benchmark, which 

represents national average hot water use. Although the thermal site solar saving fraction 

was nearly unity during the summer months, the delivered energy was low because of 

small hot water demand. Thus, the pump energy becomes more significant and the total 

site solar saving fraction was only 0.66 compared to the prediction of 0.88. On a source 

energy basis, the savings fraction drops to 0.39 because of the site-to-source multiplier 

for the electricity used by the pump. 

We calculated a simple overall system efficiency for the solar water heater by dividing 

the thermal energy delivered from the solar tank to the backup water heater by the total 

solar radiation on the plane of the collectors times the area of the collectors. The monthly 

average efficiency varied from 2.8% in August to 7.4% in December. The annual average 

efficiency was 4.8%. 

The low delivered energy of the solar thermal system begs the question of whether the 

investment is justified. The installed cost of the solar thermal system was $7,068. The 

tankless backup water heater cost $1,340 plus installation (the cost of installation is not 

available). We used measured PV and solar water heating data to pose the following 

question: “What would it cost to increase the size of the PV system and use an electric 

tank heater?” Conventional wisdom hold that the solar water heating system is a better 

28
investment. A comparison of the two systems is shown in Table 9. For both cases, all 

source energy use is displaced by the solar systems. 

Table 9. Comparison of Thermal and PV Solar Water Heating Systems Based on Measured 

Data from April 2006 through March 2007 

Thermal Solar Water Heating 

Sytem with Tankless Backup 

(EF = 0.84) and PV to 

Displace NG Use 

Incremental PV with an 

Electric Tank Water 

Backup Heater (EF=.95) 

Site energy from solar system 2.21 MBtu (647 kWh) 0 

Site pump energy 0.321 MBtu (94 kWh) 0 

Site energy to water heater 0.727 MBtu(213 kWh) 2.97 MBtu (870 kWh) 

PV energy needed1 0.549 MBtu (161 kWh) 2.97 MBtu (870 kWh) 

PV needed (Wp)

2 125 672 

Solar water heater installed cost $7,068 n/a 

Conventional water heater cost3 $1,340 $400 

Incremental PV installed cost4 $881 $4,702 

Total system cost estimate $9,289 $5,102 1

 For solar water heater case, this includes the pump energy plus the PV energy required to displace the 

source energy from the water heater (= 94 kWh + 213/3.16 kWh). 

2 Annual PV production was 1.295 kWh per rated peak Watt of the PV system. 

3 Installation costs for the conventional systems are not included. 

4

 In both cases we assume that the balance of system investments such as inverter, combiner box, and 

disconnects have already been made and the full retail installed incremental cost for PV of $7/Wp. 

This analysis depends on specific system costs and the 12 months of weather, hot water 

use, and NREL/Habitat ZEH solar water heater system performance on which these are 

based. If the low hot water use of the household had been known in advance, a smaller 

solar water heating system could have been installed at lower cost. PV costs would be 

higher if an additional inverter were needed. Cost rebates, which are not considered in 

this analysis, vary considerably around the country. Colorado currently has a rebate of up 

to $4.50/Wp for PV. The size of the PV in the incremental PV option could be reduced by 

using a heat pump water heater in place of the conventional electric tank heater. The only 

conclusion that can be drawn from this simple analysis is that the conventional wisdom 

may not be true in all cases and additional investigation into the comparison of solar 

water heating and PV investments may be warranted. 

Utility Bills 

Zero energy performance does not necessarily equate to zero utility bills. The 

NREL/Habitat ZEH was designed to use NG for space heating, backup water heating, 

and clothes drying. The Xcel Energy net metering arrangement calls for any excess 

energy accumulated by the end of the calendar year to be zeroed out and compensated for 

at the “average hourly incremental cost of electricity supply over the most recent calendar 

year” (Public Service Company of Colorado 2006). In a heating-dominated climate, a 

ZEH produces more energy than it consumes during the summer when daylight hours are 

long and consumes more energy than it produces during the winter when daylight hours 

are shorter and energy is consumed for space heating. Because the accumulated excess 

energy is zeroed out in the winter when PV production is low, the homeowner will likely 

have to pay for net electricity consumption during January and February. Because the 

29
cost of production is lower than the retail cost of the electricity, the compensation the 

homeowner receives for the excess energy accumulated by December 31 will be less than 

the cost of the net electricity used in Febuary and March. A more ideal time (for the 

homeowner) to zero out the accumulated net production would be near the spring 

equinox, when the accumulation would be closest to zero. In addition to charges for 

energy use, utility bills include a fixed monthly charges for electricity and NG. In the 

design phase of the project we used simulated energy performance to estimate a monthly 

average utility bill of $30 for the house under the current Xcel rate structure. 

As energy use decreases, fixed charges become a larger percentage of the utility bill. For 

the NREL/Habitat ZEH, there was no use charge for electricity most months because it 

was a net producer, but the fixed charge for electricity still applied. This made it easy to 

determine the fixed charge for electricity. Disaggregating the NG fixed charge from the 

use charge on the utility bill was surprisingly difficult. Instead, we applied the fixed 

charge from the rate tariff and assumed the remainder was the use charge. 

Some billing problems occurred with the house, probably because it was one of the first 

net metered houses under Colorado’s renewable portfolio Amendment 37. The home 

began with an analog meter that ran backward as the PV produced more electricity than 

the house consumed. The first bill was not received until the home had been occupied for 

four months. When it arrived, the meter reading was interpreted as indicating a large 

positive number rather than a small negative number and the occupant received a $939.68 

electricity charge on her first bill. Billing continued to be erratic throughout the first year. 

An additional hitch came when the analog meter was replaced by a digital net meter. An 

incorrect final analog meter reading was later corrected. Rather than having the 

accumulated net positive electricity zeroed out at the end of December, it was zeroed out 

when the analog meter was replaced on November 8, 2006. At this time the home had 

generated 2,517 kWh more than it had consumed since the meter was installed in October 

2005. The homeower was reimbursed for this excess generation at a rate of $.04291/kWh. 

In January 2006 she received a check from Xcel Energy for $108. 

The total annual and average monthly electricity and NG costs are given in Table 10. The 

average total utility bill was about $17/month. 

Table 10. Total Annual and Average Monthly Utility Bills for the Monitored Period 

Fixed Charge Use Charge Total 

Total annual electricity $94.69 $69.58 $164.27 

Reimbursement for net production –$108.00 –$108.00 

Total annual NG $106.43 $43.03 $149.46 

Total annual bill $201.12 $4.61 $205.73 

Average monthly electricity $7.89 $5.80 $13.68 

Reimbursement for net production –$9.00 –$9.00 

Average monthly NG $8.86 $3.58 $12.46 

Average monthly total utility bill $16.75 $0.38 $17.14 

30
Space Heating and Comfort 

The hourly average temperatures in the living room, two of the three bedrooms, and 

outdoors are shown in Figure 15. The temperature distribution in the home appeared to be 

fairly uniform; the temperature difference between any two of the three rooms monitored 

was less than 3oF for 97% of the hours during the year. The indoor winter temperatures 

averaged about 68oF; however, the indoor temperature commonly oscillated by about 6o

to 8oF because of solar gain on sunny winter days. 

During the summer, the indoor temperatures tended to track outdoor temperatures and 

often exceeded standard comfort conditions. The indoor temperature remained lower than 

outdoor temperatures during hot sunny periods, which indicates the southern overhangs 

prevented overheating. On two occasions the indoor temperatures exceeded 90oF for 

several hours. The homeowner reported that indoor temperatures were similar to those in 

a Habitat Metro Denver standard practice house next door. She was coached to open the 

windows during the cooler evenings and close them during the hot days. However, 

security concerns prevented her from opening the windows at night. 

In recent years the Denver area has experienced outdoor temperatures of 90o

 to 100oF for 

several periods each summer. This trend may necessitate re-evaluation of airconditioning in affordable homes in Denver. 

0

Feb 06 Mar 07 Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07 Feb 07 Mar 07

-10.00

10.00

30.00

50.00

70.00

90.00

110.00

Month

Temperatures (Degrees F)

Outdoors

Living Room

NW Bedroom

SW Bedroom

000

Figure 15. Hourly average indoor and outdoor temperatures 

31
Several paths may provide efficient cooling for homes such as the NREL/Habitat ZEH. If 

the security concerns could be overcome (perhaps with security grills on specific 

windows), a whole house fan could take advantage of Denver’s diurnal temperature 

swings and reduce the cooling load. A small, high seasonal energy efficiency ratio 

minisplit AC system could then be used to provide any additional cooling needed. 

Modeled versus Actual Performance 

DOE2 software was used to conduct the final design energy simulation of the home. This 

simulation is driven by TMY2 weather data, and uses assumptions for setpoints, 

appliance and plug loads, lighting and plug load schedules, and hot water use based on 

the BA Performance Analysis Proceedures. After collecting a year of monitored data, we 

reran the simulation, leaving the building and equipment models unchanged but driving 

the simulation with measured weather and occupant effects. The changes made to “tune” 

the model to actual weather and occupants are listed here: 

• Hot water used was reduced to 20.4 gal/day (BA assumption = 65.6 gal/d). 

• Appliance and plug loads were reduced to 2,079 kWh/yr (BA assumption = 3053 

kWh/yr). 

• Dryer energy use was reduced to 28 therms/yr (BA assumption = of 76 

therms/yr). 

• Cooking was changed from NG (which was originally anticipated) to electric 

(which was installed). 

• Base lighting kilowatt-hours were adjusted down by 30% and the impact of CFLs 

was increased from a 60% reduction to a 75% reduction based on measured data. 

• The lighting schedule was adjusted based on monitored data. 

• The plug load and miscellaneous electricity use schedule was adjusted based on 

monitored data. 

• The Hot water use schedule was adjusted based on monitored data. 

• Thermostat settings were adjusted based on monitored data. 

• Monthly PV was adjusted to monitored values (from 5,274 kWh/yr to 5,127 

kWh/yr). 

• Ventilation energy was lowered from 298 kWh/yr to 144 kWh/yr. 

• Solar domestic hot water effectiveness was adjusted to 80% solar saving fraction 

annually. 

The monthly electricity and NG consumption predicted by the original and the tuned 

simulations are shown with the measured data in Figures 16 and 17. 

32
0

100

200

300

400

500

600

Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07

Month

Montly total electricity consumption (kWh)

Simulated - BA assumptions, TMY2

Simulated - Measured drivers, actual weather

Measured

Figure 16. Simulated and measured monthly electricity consumption 

0.0

5.0

10.0

15.0

20.0

25.0

Apr 06 May 06 Jun 06 Jul 06 Aug 06 Sep 06 Oct 06 Nov 06 Dec 06 Jan 07

Month

Monthly total natural gas consumption 

(therms)

Simulated - BA assumptions, TMY2

Simulated - Measured drivers, actual weather

Measured

Figure 17. Simulated and measured monthly NG consumption 

The simulation that used BA assumptions and TMY2 weather overestimated the annual 

electricity consumption by 19%. However, when the simulation used measured occupant 

33
and weather drivers, it agreed with the measured data on annual electricity consumption 

to within 3%. 

The simulation that used BA assumptions and TMY2 weather overestimated the annual 

NG consumption by more than 200%. The simulation overestimated all NG end uses: 

clothes drying, backup water heating, and space heating. In the tuned simulation, the 

clothes drying NG and the hot water uses were set to the measured value. The measured 

annual average solar saving fraction was used to simulate backup water heater NG 

consumption. Measured room temperatures were used to generate more representative 

thermostat settings. With these changes, the difference between simulated and measured 

NG consumption decreases to 17 therms. Because the NG consumption of the home is 

light, this still represents a 32% difference between tuned simulation and measurement. 

The simulation still overpredicts the space heating NG consumption during the coldest 

months. This difference is probably due to imperfect modeling of the NG heater and 

remaining differences between simulated and actual daily temperature setpoints. 

When NG and electricity are combined, the tuned simulation is within 8% of the 

measured annual energy consumption. 

34
Second Year Performance Summary 

This section contains a summary of the second year performance compared to the first 

year detailed in the previous section. 

The overall performance of the home in the second year followed similar patterns to the 

first year performance. The home exceeded the zero energy goal again in the second year, 

though by a narrower margin. The two-year performance summary is given in Table 11. 

The daily and cumulative net electricity performance pattern for the second year also 

closely followed the pattern established in year 1. The daily and cumulative net electricity 

performance for both years is shown in Figure 18. 

Table 11. 24-Month Performance Summary of NREL/Habitat ZEH 

April 2006 to 

February 2007 

April 2007 to 

February 2008 

Percent

Change 

kWh (MBtu) kWh (MBtu) 

Site Energy Summary 

Total site electricity consumption 3,585 (12) 4,224 (14) 18% 

Total AC site PV electricity production 5,127 (17) 5,388 (18) 5% 

Net site electricity production 1,543 (5.3) 1,164 (4.0) –25% 

Total site NG consumption 1,665 (5.7) 1,811 (6.2) 9% 

Source Energy Summary 

Total source energy consumption 13,025 (44) 15,195 (52) 17% 

Total source energy offset 16,201 (55) 17,025 (58) 5% 

Net source energy offset 3,176 (11) 1,830 (6) –42% 

Percent of source energy consumption 

offset via on-site renewable production 124% 112% 

* The site-to-source energy conversions are U.S. national averages according to the BA Analysis 

Procedures (Hendron et al. 2004): site-to-source multiplier for electricity = 3.16; site-to-source multiplier 

for NG = 1.02. 

35
-40

-30

-20

-10

0

10

20

30

40

Feb 06

Mar 07

Apr 06

May 06

Jun 06

Jul 06

Aug 06

Sep 06

Oct 06

Nov 06

Dec 06

Jan 07

Feb 07

Mar 07

Apr 07

May 07

Jun 07

Jul 07

Aug 07

Sep 07

Oct 07

Nov 07

Dec 07

Jan 08

Feb 08

Mar 08

Month

Daily net kWh production

-4000

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

Cumulative net kWh consumption

Daily Net Electricity Production

Cumulative Net Electricity Production

Production

Consumption

Figure 18. Simulated and measured monthly NG consumption 

36
Discussion and Conclusions 

The NREL/Habitat ZEH is a success story on many levels. It has enabled a small, singleparent family to buy an affordable home that is efficient and comfortable. It has enabled 

NREL and Habitat Metro Denver to work together to explore ideas about how to make 

energy efficiency and renewable energy work in this context. And because of the 

overwhelming public response, it has proven to be a wonderful public relations and 

educational tool. 

Installation problems were encountered with the ERV system. In addition to the control 

problem described previously, some of the ducts were incorrectly connected during the 

installation. As ERV systems become more common, some ERV commissioning is 

recommended if the installation contractor is not familiar with these sytems. 

The built-in thermostats included with the baseboard heaters proved to be imprecise at 

best. In 2007 we installed wall-mounted line-voltage thermostats for each heater. 

The economics of a ZEH is a function of the specific net metering tariffs for its location. 

Some of these tariff structures are more favorable than others. For example, the Tennesee 

Valley Authority buys 100% of the PV-generated electricity from home PV systems at 

$0.15/kWh. The cost of electricity varies with the area. In Oak Ridge, Tennessee, the 

electricity use charge is $0.07543/kWh, so homeowners are paid nearly twice their 

electricity rate for their PV production. Table 12 shows what the energy costs for the 

house would be if the Oak Ridge electricity and NG rate structures were available in 

Denver. Rather than having to pay the utility bills, the homeowner would have received

an average of $24/month from the utility. In locations with incentive programs that are 

less favorable than Tennessee’s, a ZEH can be seen as a hedging strategy against 

uncertainty in energy prices. Owners of affordable homes are generally less able to 

absorb energy price shocks and would benefit from the low and stable home energy costs 

of ZEHs. 

The PV sizing for this project was based on BA Benchmark appliance and plug load use 

designed to represent mid-1990s national averages. With this strategy, one could expect 

the home’s chances of achieving zero energy performance to be 50/50—half of the 

occupants will be above average energy consumers and half will be below average. The 

NREL/Habitat ZEH appliance and plug load energy use was 32% less than the 

Benchmark level and still accounts for 58% of all energy used in the home. This is one of 

the main reasons the home exceeded the net zero energy goal and was a net energy 

producer. Yet the occupants’ lifestyle is not one of deprivation for the sake of energy 

savings. Another sizing strategy that could be adopted would be to size the PV system for 

a below average user and provide educational material to the occupant that outlines the 

energy budget to achieve zero energy. An inexpensive whole-house energy meter can be 

installed for feedback. 

37
Table 12. Cost of Energy at the Oak Ridge, Tennessee Rate Structure 

Value Units Oak Ridge 

Cost/Unit Total Cost 

PV reimbursement 5127 kWh –$0.15 –$769.05 

Electricity fixed charge 12 months $7.46 $89.52 

Electricity use charge 3595 kWh $0.07543 $270.42 

NG fixed charge 12 months $3.50 $42.00 

NG use charge 57 therms $1.4030 $79.97 

Total annual cost –$287.14 

Total average monthly cost –$23.93 

It was a design decision to use NG in the NREL/Habitat ZEH and displace the NG use 

with excess PV electricity generation to achieve net zero source energy. The PV system 

required for this approach is smaller than for an all-electric house with resistive water and 

space heating, and reduces overall home cost to achieve net zero source energy with the 

same societal benefits. However, because the occupants are below average energy users, 

the net site energy use was nearly zero. This means the occupants could use electric 

resistance heat to meet the loads currently served by NG and still come very close to the 

zero energy goal without additional PV panels. Eliminating the NG would further 

simplify the mechanical equipment and reduce the already small utility bill by 

eliminating the fixed charges for natural gas. Making the home all-electric and using the 

PV sizing strategy may be a reasonable approach for cold climate affordable ZEHs. 

The person-to-person variability of appliance and plug load energy use makes sizing the 

PV system for zero energy challenging. One advantage of net metered PV is a 100% 

utilization factor. If the occupant does not need the energy being provided by the PV, it is 

sent to the grid for others to use. (Economic compensation for this energy varies 

considerably.) In contrast, if the homeowner uses less hot water than expected, the solar 

thermal system stagnates at its maximum temperature and cannot take advantage of 

additional solar resource. In effect, the energy that could have been collected is lost. 

Because water use is highly variable, it presents a sizing challenge that is similar to the 

PV system. If the water use is lower than expected, the savings drop off substantially and 

the economic value of the system is reduced. For a ZEH that must supply all its energy 

from renewable resources, the economic value of solar thermal and PV needs to be 

carefully weighed, and the uncertainty of the occupant effects needs to be taken into 

account. This area warrants further investigation. 

Some final conclusions from the project are listed below. 

• The NREL/Habitat ZEH exceeded its goal of zero net source energy and was a net 

energy producer for the first two years. 

• PV system sizing for ZEHs is challenging. 

o Total home energy use for a specific house becomes highly uncertain 

because of occupant choices and behaviors. 

o Meeting the ZEH design goal depends on occupant behaviors. 

38
o The economics of excess annual PV production depends on net metering 

agreements. 

• Zero energy does not necessarily mean a zero utility bill. 

o There are fixed monthly costs for NG and electricity service. 

o NG costs may not be displaced by net electricity production. 

• Efficient, affordable ZEHs can be built with standard construction techniques and 

off-the-shelf equipment. Meeting the BA goal of cost neutral ZEH in all housing 

sectors will require additional research on cost-effective efficiency options. 

39
References 

Christensen, C., Barker, G., Horowitz, S. 2004. “A Sequential Search Technique for 

Identifying Optimal Building Designs on the Path to Zero Net Energy,” Proceedings of 

the Solar 2004 Conference Including Proceedings of 33rd ASES Annual Conference and 

Proceedings of 29th National Passive Solar Conference, American Solar Energy Society, 

NREL/CP-550-36099. 

Christensen, C., Barker, G., Tupper, K. 2004. “Optimal Building Designs on the Path to 

Zero Net Energy,” Proceedings of the Solar 2004 Conference Including Proceedings of 

33rd ASES Annual Conference and Proceedings of 29th National Passive Solar 

Conference, American Solar Energy Society, NREL/CP-550-36100. 

Christensen, C.; Barker, G. 2001. “Effects of Tilt and Azimuth on Annual Incident Solar 

Radiation for United States Locations,” Proceedings of Solar Forum 2001: Solar Energy: 

The Power to Choose.

Daub, E. 2006. “Solar Harvest: Better Than Net Zero Energy,” presented at Solar 2006 in 

Denver, Colorado, American Solar Energy Society. http://www.ecofuturesbuilding.com/

DOE. 2007. Building Energy Data Book, U.S. Department of Energy, 

http://buildingsdatabook.eere.energy.gov/

DOE2.2 Building Simulation Software, developed by Lawrence Berkeley National 

Laboratory, Berkeley, CA, available at http://www.doe2.com 

Hendron, R.., Eastment, M., Hancock, E., Barker, G., Reeves, P. 2007. Evaluation of a 

High-Performance Solar Home in Loveland, Colorado, Journal of Solar Energy 

Engineering. 129:2, pp. 226-234. 

Hendron, B., et al. 2005. Building America Benchmark Definition, Updated December 

29, 2004. 

Hendron, B., et al. 2004. Building America Performance Analysis Procedures, Revision 

1, National Renewable Energy Laboratory, NREL/TP-550-35567 

Keesee, M. and Hammon, R. 2006. “Impact of Distributed Solar on SMUD’s Peak Load,” 

presented at ASES Solar 2006 conference in Denver, CO. 

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

Manual, Solar Energy Laboratory, University of Wisconsin. 

Marion, W. and Urban, K. 1995. User’s Manual for TMY2s Typical Meteorological 

Years, Derived from the 1961-1990 National Solar Radiation Data Base, National 

Renewable Energy Laboratory. 

40
41

Public Service Company of Colorado. 2006. Electric Rates: Net Metering Service, 

Schedule NM, Advice letter number 1451, Decision number c06-0155, Effective date 

March 1, 2006. 

Weiss, W. ed. 2003. Solar Heating Systems for Houses: A Design Handbook for Solar 

Combisystems, International Energy Agency, Solar Heating & Cooling Programme, 

ISBN 1 902916 46 8. 
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The NREL/Habitat for Humanity Zero Energy Home: A Cold Climate 

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The design of this 1,280-square-foot, three-bedroom Habitat for Humanity of Metro Denver zero energy home 

carefully combines envelope efficiency, efficient equipment, appliances and lighting, and passive and active solar 

features to reach the zero energy goal. The home was designed with an early version (July 22, 2004) of the BEOpt 

building optimization software; DOE2 and TRNSYS were used to perform additional analysis. This engineering 

approach was tempered by regular discussions with Habitat construction staff and volunteers. These discussions 

weighed the applicability of the optimized solutions to the special needs and economics of a Habitat house—moving 

the design toward simple, easily maintained mechanical systems and volunteer-friendly construction techniques. A 

data acquisition system was installed in the completed home to monitor its performance. 

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zero energy home; habitat for humanity; building optimization; doe2; trnsys; beopt 

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