Measured Performance of 

Occupied, Side-by-Side, 

South Texas Homes 

Dave Chasar and Valerie vonSchramm 

Building America Partnership for Improved 

Residential Construction

September 2012
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iii

Measured Performance of Occupied, 

Side-by-Side, South Texas Homes

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

Dave Chasar and Valerie vonSchramm 

Building America Partnership for Improved Residential Construction

Florida Solar Energy Center

1679 Clearlake Rd 

Cocoa, Florida, 32922 

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40339-00 

September 2012
iv

Acknowledgments

The authors wish to acknowledge the assistance of Lake Flato Architects, Inc., Woodside Homes 

of South Texas, and Builder’s Energy Rater. Staff at Lake Flato Architects performed the 

preconstruction modeling and developed the residential construction specifications to reduce the 

home’s energy consumption by 50%. Staff at Woodside Homes worked diligently to ensure the 

proper adoption of these modified building techniques. Thank you also to the staff at Builder’s 

Energy Rater for the extra assistance in photo documentation of the homes and assisting with the 

adoption of new construction practices.
v

Contents

Acknowledgments ..................................................................................................................................... iv

Contents....................................................................................................................................................... v

List of Figures ............................................................................................................................................ vi

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

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

2 Home Comparison................................................................................................................................ 2

2.1 Envelope Features................................................................................................................3

2.2 Envelope Air Sealing ...........................................................................................................3

2.3 Duct Tightness and Location ...............................................................................................4

3 Building America Benchmark Energy Analysis ................................................................................ 5

4 Electricity Demand ............................................................................................................................... 7

5 Cooling Energy ................................................................................................................................... 10

6 Photovoltaic Performance................................................................................................................. 15

7 Results and Conclusions................................................................................................................... 17

References................................................................................................................................................. 18
vi

List of Figures

Figure 1. Front view of control, high performance, and PV homes....................................................... 2

Figure 2. Infrared and visible images of high performance home (above) and PV home (below) ..... 4

Figure 3. Hub and branch ducts (left) and tapered ducts (right)............................................................ 4

Figure 4. BEopt source energy by end use.............................................................................................. 6

Figure 5. Measured electric load on the hottest summer day (July 8, 2009) ........................................ 8

Figure 6. Measured control home electric loads ..................................................................................... 9

Figure 7. Cooling energy versus outdoor-indoor temperature difference.......................................... 11

Figure 8. Average daily ambient and indoor temperatures (°F)........................................................... 13

Figure 9. Monthly electricity use in the PV home from July 2009 through March 2011 .................... 15

Figure 10. Comparison of measured and estimated monthly PV production .................................... 16

Unless otherwise noted, all figures were created by the Building America Partnership for 

Improved Residential Construction.

List of Tables

Table 1. Home Features Comparison........................................................................................................ 2

Table 2. BEopt Analysis Results ............................................................................................................... 5

Table 3. Projected “Non-PV” Site Energy Savings Versus BA 2010 Benchmark................................. 6

Table 4. Sensors and Datalogger ............................................................................................................ 10

Unless otherwise noted, all tables were created by the Building America Partnership for 

Improved Residential Construction.
vii

Definitions

A/C Air conditioning

ACH50 Air changes per hour at 50 Pascals pressure differential

AFUE Annual fuel utilization efficiency 

BA Building America

BEopt Building Energy Optimization

CFM25 Cubic feet per minute at 25 Pascals pressure differential

HVAC Heating, ventilation, and air conditioning

PV Photovoltaic

Qn,out Normalized duct leakage to the outside calculated by dividing the 

measured duct leakage to the outside at a test pressure of 25 pascals 

(CFM25,out) by the conditioned area of the home. For example, Qn,out 

of 0.03 represents a leakage of 3 CFM for every 100 ft2 of conditioned 

space.

RH Relative humidity

SEER Seasonal energy efficiency ratio (for the efficiency of air conditioners 

and heat pump cooling)

SHGC Solar heat gain coefficient
viii

Executive Summary

The performance of three occupied homes built in 2009 in San Antonio, Texas with identical 

floor plans and orientation were evaluated through a partnership between the Florida Solar 

Energy Center, CPS Energy, and Woodside Homes of South Texas. Measurements included 

whole-house gas and electricity use as well as heating, cooling, hot water, major appliances, and 

indoor and outdoor conditions. One home built to the builder’s standard practice served as the 

control; the other homes demonstrated high performance features.

The goals of this research were to: (1) learn how energy systems affect peak load profiles during 

the hottest weather conditions; (2) inform the development of builder and homeowner incentive 

programs that manage demand and energy consumption; and (3) measure cooling energy use to 

determine savings from envelope and equipment improvements.

Data collection began in July 2009, which was at about the time of first occupancy, and 

continued through April 2011. One home (CP3) was unoccupied for the first two months of data

collection. Energy ratings for the homes yielded E-Scales (aka Home Energy Rating System

Indices) of 86 for the control home (CP1), 54 for one improved home (CP2) and 37 for the other 

improved home (CP3), which has a 2.4-kW photovoltaic array. Envelope improvements 

included:

• Sealed attic with R-28 open cell spray polyurethane foam at the roof deck

• Frame walls insulated to R-15 and R-3 rigid insulating sheathing

• ENERGY STAR® windows with a U-value of 0.34 and a solar heat gain coefficient 

(SHGC) of 0.33

• Enhanced air sealing

Equipment improvements include right-sized (per Air Conditioning Contractors of America 

Manual J) tankless gas water heaters (versus gas tank), two-stage air conditioning with a 

Seasonal Energy Efficiency Ratio (SEER) of 18 (versus right-sized SEER 14), ENERGY STAR

appliances (versus standard appliances), and 100% fluorescent lighting (versus 5% fluorescent).

The improved homes overall saved 55% –77% in cooling energy and, on the hottest day, a utility 

peak demand reduction of 6–8 kW (62%–83%). A 2.4-kW grid-tied photovoltaic array 

successfully offset total monthly heating, ventilation, and air conditioning electric energy and 

80% of the time on a daily basis during the summer months. Because the homes were occupied,

the impact of occupant behavior on the results is unclear.
1

1 Introduction

The Florida Solar Energy Center supports many Building America (BA) projects with long-term 

monitoring of building energy use and environmental conditions. Homes are typically monitored 

using 15–50 channels of data to measure indoor and outdoor environmental conditions and 

energy use of heating, cooling, water heating, whole-house, and other points (e.g., solar 

photovoltaic [PV] or solar domestic hot water) as needed. CPS Energy is the nation’s largest 

municipally owned energy company providing both natural gas and electric service. Acquired by 

the City of San Antonio in 1942, the company serves approximately 700,000 electricity

customers and more than 320,000 natural gas customers in and around America’s seventh largest 

city. Thirty-year weather averages for this location include 1,573 heating degree days and 3,038 

cooling degree days.

Research on these homes focused primarily on comparisons of peak electric load profiles and 

cooling energy performance. Several electric demand reduction strategies were used to limit 

demand, especially during utility peak periods. These included high efficiency electric 

equipment and replacement of electric appliances with gas and PV panels.
2 

2 Home Comparison

Construction of the three homes began in late 2008 and was completed in early 2009 (see Figure 

1). Each residence was built on the same street running north-northwest to south-southeast within 

300 ft of each other. All homes have identical 1,979 ft2

 floor plans and orientation. There are

differences in attic construction and wall insulation (as seen in Table 1), but otherwise the homes 

are similar. Gas appliances were used in the improved homes with the exception of a highefficiency, electric heat pump in the high performance home (CP2). The control home (CP1) had 

mainly electric appliances except for a gas water heater and furnace, all of standard efficiency. 

Standard appliances and lighting were used in the control home to represent higher internal 

cooling loads than found in the improved homes. (See Table 1 for details.) 

Figure 1. Front view of control, high performance, and PV homes

Table 1. Home Features Comparison

Control

CP1

Hi-Performance

CP2

PV

CP3

Foundation Uninsulated slab

on grade (1,979 ft2)

Uninsulated slab

on grade (1,979 ft2)

Uninsulated slab

on grade (1,979 ft2)

Roof Cladding Brown asphalt shingle Brown concrete tile Brown concrete tile

Attic Type Vented Sealed Sealed

Attic Insulation

R-30 blown fiberglass in 

ceiling plane,

roof deck radiant barrier

R-28 open cell spray 

foam under roof deck

R-28 open cell spray 

foam under roof deck

Wall Type 2 × 4 frame/brick veneer 2 × 4 frame/brick veneer 2 × 4 frame/brick veneer

Wall Insulation R-13 fiberglass batts R-15 blown-in fiberglass

+R-3 insulated sheathing

R-12 open cell spray 

foam 

+R-4 insulated sheathing

Windows SHGC: 0.37

U-value: 0.53

SHGC: 0.33,

U-value: 0.34

+1 ft roof line extension

SHGC: 0.33,

U-value: 0.34

+1 ft roof line extension

Heating 80% AFUE* gas furnace

9.5 HSPF** heat pump

+ 5kW b/u strip heat 94% AFUE gas furnace

Cooling SEER 14 SEER 17.8 SEER 17.7

Water Heating 40-gal Gas, EF = 0.59 Tankless gas, EF = 0.82 Tankless gas, EF = 0 .82

Ventilation None Passive runtime Passive runtime

Lighting Incandescent

+5% fluorescent

100% fluorescent,

timers and

occupancy sensors

100% fluorescent,

timers and

occupancy sensors

Cooktop Electric Natural gas Natural gas
3 

Control

CP1

Hi-Performance

CP2

PV

CP3

Refrigerator 775 kWh***/yr ENERGY STAR,

505 kWh/yr

ENERGY STAR,

505 kWh/yr

Washer Standard top-loader ENERGY STAR

Tier 3

ENERGY STAR

Tier 3

Dishwasher EF = 0.46 ENERGY STAR,

EF = 0.66

ENERGY STAR,

EF = 0.66

Dryer Electric Natural gas Natural gas

Thermostat Nonprogrammable Programmable Programmable

PV None None 2.4-kW roof tiles

Home Energy 

Rating System

Index

86 54 37

Envelope 

Leakage 5.84 ACH50  3.64 ACH50 1.95 ACH50

Duct Leakage 70 CFM** ***25,

Qn = 0.035

47 CFM25,

Qn = 0.024

65 CFM25,

Qn = 0.033

*Annual fuel utilization efficiency

**Heating seasonal performance factor

***Kilowatt hour

 Air changes per hour at 50 Pascals

** ***Cubic feet per minute

2.1 Envelope Features

All homes were built on uninsulated, slab-on-grade foundations with 2 × 4 frame walls and brick 

veneer. Wall insulation varied, with standard R-13 batts used in the control home, R-15 blown 

fiberglass plus R-3 foam sheathing in the high performance home, and R-12 spray foam as well 

as R-4 foam sheathing in the PV home. The window-to-wall ratio of 16% was identical in each 

home with double-pane low-emissivity used throughout, although those in the improved homes 

were of higher performance. An additional 12 in. of roof overhang is built into the improved 

homes over that of the control. The control home had a vented attic with R-30 blown fiberglass 

insulation on the ceiling and a radiant barrier roof deck. The improved homes had identical 

sealed attics with R-28 open cell foam sprayed on the roof deck and at the garage-home attic 

interface. Reflectance of the roof materials was similar, (medium to dark in color); the control 

home had asphalt shingles and the improved homes had concrete tile.

2.2 Envelope Air Sealing

All three homes have air-sealed envelopes, as reflected in the envelope leakage numbers, due to 

a concerted effort by the builder. Slab-to-wall connections were caulked in all homes, as were 

wall, window, and ceiling penetrations. Insulated sheathing in the improved homes was taped, 

and all three homes received a taped house wrap. Access to the vented attic in the control home 

was outside the conditioned space (garage). The control home envelope is reasonably airtight at 

5.84 air changes per hour at 50 Pascals pressure differential (ACH50), below the ENERGY 

STAR V3 requirement of 6 ACH50 for climate zones one and two. The improved homes are

considerably tighter but with noticeable variation. The PV home was fairly well sealed at 1.95 

ACH50, while the high performance home measured in at nearly twice that number. All the 

homes had a sealed roof-wall interface; however, infrared images of the improved homes on a 

cold December day indicated more leakage at this location in the high performance home than in 
4 

the PV home (similar wall locations in CP2 and CP3 are illustrated in Figure 2). The roof-wall 

interface of the high performance home was at approximately 45°F, while the roof-wall interface 

of the PV home was at approximately 51°F. This is thought to be the main contributor to higher 

envelope leakage in the high performance home. 

Figure 2. Infrared and visible images of high performance home (above) and PV home (below)

2.3 Duct Tightness and Location

Air distribution systems consisted of R-6 flex duct in the attic of each home. All ducts were 

sealed with mastic, which resulted in test numbers of no more than 0.035 CFM25/ft2

 of 

conditioned floor area. Measured attic conditions during the summer reflect one difference 

between the homes. Results show an average temperature of 95°F for the control home and 79°F 

for the two improved homes, which had similar attic conditions. This illustrates the effect of the 

sealed and insulated attics in the improved homes during the hottest months of the year—June 

through August—in both 2009 and 2010. Maximum attic temperatures reached 129°F and 85°F 

in the respective homes during these months. Another difference was that the improved homes 

had the ducts engineered for optimum distribution efficiency with tapered duct transition pieces; 

whereas the control home had a standard hub and branch design (Figure 3). 

Figure 3. Hub and branch ducts (left) and tapered ducts (right)
5

3 Building America Benchmark Energy Analysis

Analysis was performed using Building Energy Optimization (BEopt) version E+ 1.1 software 

(NREL 2011) and local typical meteorological year weather data following the Building America 

House Simulation Protocols (Hendron and Engebrecht 2010). Table 2 shows predicted source 

energy savings for each home over the reference benchmark. The analysis showed construction 

of the control home to be 14% better than the reference benchmark. Source energy savings,

without considering PV, totaled 45% for the high performance and PV homes. Inclusion of the 

2.4-kW array in the PV home increased its total source energy savings to 64%. The table also 

includes annual source energy use and savings based on measured data from September 2009 to 

August 2010, although these are not expected to correlate well with simulation results because of

differences in occupant behavior. CP2 housed only one occupant during this period; the other 

homes housed three. A home-based business accounted for larger than typical miscellaneous 

electric loads in CP3.

Table 2. BEopt Analysis Results

Annual Source Energy

(MBtu*)

Control

(CP1)

Hi-Performance

(CP2)

PV

(CP3)

PV 

(Building Only)**

(CP3)

Benchmark 205 198 202 202

As-Built (Simulated) 175 109 72 108

Savings (Simulated) 14% 45% 64% 46%

As-Built (Measured) 151 94 127 164

Savings (Measured) 26% 52% 37% 19%

*Million British thermal units

**PV excluded from analysis

Figure 4 shows the BEopt end-use energy breakdown for each mixed-fuel home. Miscellaneous 

energy use was a significant component in each home except in CP1, where the cooling energy 

of the control home, with a seasonal energy efficiency ratio (SEER) of 14, is roughly equivalent 

to annual miscellaneous energy use. The dark horizontal line in the CP3 bar represents the level 

of energy contribution from the PV system amounting to 36 MBtu/yr.
6

Figure 4. BEopt source energy by end use

Estimated costs for energy measures were derived from a mix of construction costs reported by 

the builder at the trade-level (plumbing, insulation, framing, etc) and BA database estimates. 

Construction and equipment specifications for the PV home (not including PV) were used to 

arrive at an amortized annual package cost of $1,019 and energy savings of $614, leaving a $405 

annual shortfall. The very low utility rates in San Antonio compared to other regions contribute 

to the negative cash flow situation.

Table 3. Projected “Non-PV” Site Energy Savings Versus BA 2010 Benchmark

PV Home*

(CP3)

Insulation $4,537

Windows $1,399

HVAC $6,000

Appliances $2,832

Water Heating $800

Lighting $250

Total Package Cost $15,818

Amortized Annual Package Cost (30 yr, 5%) $1,019

As-Built Annual Site Energy Savings Over Benchmark $614**

Net Annual Cash Flow -$405

*Incremental cost relative to CP1, Does not include PV costs or production

**CPS 2010/2011 utility costs: $0.08/kWh and $0.80/Therm
7

4 Electricity Demand

One goal of the research partnership with CPS Energy was to learn how the energy systems in 

the three South Texas homes affect their peak load profiles during the hottest weather conditions. 

That information can be used to help design incentive programs for builders and homeowners to 

help manage demand and energy consumption, where demand is defined as the peak load (in 

kilowatts) on any given day. Demand is important to energy-efficient operation of generating 

resources in the same manner as peak heating/cooling load is to a heating, ventilation, and air 

conditioning (HVAC) system designer. Both result in oversized equipment running inefficiently 

when the peak load is considerably larger than the average load. Although this is beyond the 

scope of the present study, the results from this work and similar studies could be used to 

develop tools to estimate source energy savings from more efficient power plant deployment and 

operation.

In San Antonio, the highest system-wide loads are encountered in the summer months. CPS 

Energy’s current demand management program runs from May through September between the 

hours of 3:00 p.m. and 7:00 p.m. Central Daylight Time.

Many strategies can be used to reduce demand, but the strategies of most interest of study to CPS 

Energy were:

• High efficiency air conditioning (A/C) paired with envelope upgrades

• PV panels

• Electric versus gas cooking

• Electric versus gas clothes drying

Figure 5 is a side-by side comparison of the electric load profiles on the hottest summer day 

(July 8, 2009) for the control home, the home with the upgraded envelope and SEER 18 heat 

pump, and the solar home with the upgraded envelope, SEER 18 A/C unit, and 2.4-kW solar 

array. Total energy (in kilowatt-hours) provides a relative measure of energy consumption or 

production for the components of each graph. Measured indoor temperatures averaged 76.7°F, 

75.5°F, and 76.0°F during this day in CP1, CP2, and CP3, respectively. The graphs show that the 

envelope and HVAC equipment upgrades effectively reduced the peak A/C loads by 1.17 kW, or 

28%, during the utility peak hours in the heat pump home and 2.88 kW, or 68%, in the solar 

home. Because only the control home and the high performance home were occupied during the

entire time of these measurements, it is unclear what portion of differences results from occupant 

behavior and what portion results from the energy efficiency features of the homes. The graph 

for the solar home also shows that the southwest-facing panels effectively remove the entire 

household electric load off the grid during the utility peak hours. They even export excess power 

to the grid to help reduce grid loads from other homes. Although these results vary from day to 

day depending on solar insolation conditions, the greatest system-wide utility peaks occur on hot 

and sunny days.
8

Figure 5. Measured electric load on the hottest summer day (July 8, 2009)

A/C loads contribute greatly to the system utility peak in San Antonio, but other intermittent 

loads such as large and small appliances and miscellaneous end uses contribute as well. Data on 

these intermittent loads are limited for a variety of reasons, including lack of widespread inhome monitoring systems, variable household behavior patterns, and differences among 

household miscellaneous load selections. The electric utility community currently responds to 

these challenges with data gathering efforts to estimate the system-wide demand contributions 

from these miscellaneous loads and consumer willingness to time-shift use of these loads. 

Additional efforts include the development of a smart grid infrastructure that can either directly 

control the miscellaneous loads or send price signals to consumers to alter their behavior. 
9

The research team studied demand contributions from electric cooking and clothes drying in 

detail, because the associated appliances use large amounts of electricity when they are on, and

they have the potential to significantly increase the utility peak load. In addition, fuel switching 

(e.g., electricity to natural gas) could control demand, and fuel switching to gas cooking and 

clothes drying is much less common in San Antonio than gas water heating. 

Figure 6 shows the electric monitoring data from the control home on a day when cooking, 

baking, and laundry occurred during the utility peak hours on a hot day. Above the 4-kW A/C

load, cooking and baking added another 1 to 2.5 kW of load to the total. This day may not be 

typical, but it conveys the significance of the miscellaneous loads toward the utility peak. For 

example, an examination of the demand reductions on the hottest day (Figure 5) shows that peak 

electricity use was reduced by more than 6 kW for the high performance home and more than 8 

kW for the PV home during the utility peak period. Fortunately, the system-wide utility peak 

benefits from averaging the differing behaviors among many homes. Incorporating gas cooking 

and gas clothes drying in the high performance homes greatly reduced these spikes in electricity

grid use and had a relatively small impact on the natural gas infrastructure. This also accounts for 

most demand reduction.

Figure 6. Measured control home electric loads
10

5 Cooling Energy

Summer 2009 was one of the hottest on record in San Antonio, Texas. On-site instruments 

showed 34 days at 100°F or higher for July and August of that year, with an average daily 

temperature during those months of 86.3°F compared to 76.6°F for September. The second 

summer of data collection (2010) had a more typical weather profile. Data collection for all 

homes was established in late June 2009, so cooling season analysis began in July of that year.

Initial occupancy in the newly constructed homes was somewhat staggered, with the control 

home first occupied in early May, the high performance home first occupied July 1, and the PV 

home first occupied on September 1. All three homes were fully occupied during summer 2010.

Cooling equipment consisted of split systems with ducted central air handlers. Submetered 

energy consumption data from the condenser and air handler were stored at 15-min intervals and 

subsequently combined and totaled on a daily basis. Energy generated by the PV home is not 

factored into its cooling energy total; it consisted solely of equipment energy use. Daily cooling 

energy totals (Figure 7) were plotted against the average daily temperature difference between 

outdoors and indoors for the 24-h period starting at midnight. Weather measurements were 

collected at one of the homes and consisted of dry bulb temperature, relative humidity (RH), and 

solar radiation. Indoor temperatures were taken very close to the thermostat. The use of 

temperature difference is intended to account for indoor temperature variations caused by

occupant-determined thermostat settings. Table 4 shows the sensors and datalogger used.

Table 4. Sensors and Datalogger

Measurement Equipment Used

Indoor and attic T/RH Vaisala resistance temperature 

detector/thin-film probe

Outdoor T/RH Vaisala resistance temperature 

detector/thin-film probe

Whole-House and HVAC Energy Use WattNode electric energy meters

Data Collection and Storage Campbell scientific datalogger
11 

Figure 7. Cooling energy versus outdoor-indoor temperature difference

The cooling performance levels shown in Figure 7 were determined by comparing the areas 

under the least-squares line. This assumes that the areas are directly proportional to energy use 

and are affected by the length chosen to make up the bottom edge of the area along the x-axis (–5 

to 14 for this analysis). Figure 7 also shows the coefficient of determination (R2) for each 

regression line. This measure of “goodness of fit” of the line to its associated data points ranged 
12

from 0.62 to 0.92. For the 92-day period in 2009, a total of 4 days were removed from each 

home’s dataset, three of which resulted from a temporary cold front and the other because of 

datalogger collection errors. Two additional days in 2009 and 2010 were removed from only the 

high performance home data set due to collection errors. Five days of data were lost to 

datalogger downtime from August 27 to August 31, 2010 but all collected measurements were

otherwise included through the two summer periods. Also during 2009, only two of the homes 

(control and high performance) were occupied during the entire three-month period. For 2009, 

the PV home was occupied only during September. All three homes were fully occupied during 

summer 2010, making it the preferred comparison over 2009. A cooling energy analysis similar 

to this was performed on eight high performance homes in a previous publication (Chasar 2006)

Cooling savings over the control home were considerable, with the high performance home 

saving 55%–56% and the PV home saving 71%–77% for the two summer periods. These savings 

numbers were derived solely from cooling equipment energy use with no impact from the PV 

system in the PV home. 

There was an unexpected difference in cooling energy savings between the two improved homes 

compared to the control, especially in summer 2009 (55% versus 77%). Each improved home 

had cooling systems with nearly identical SEER 18 ratings, although the PV home had a straightcool system with gas heat and the high performance home had a heat pump. Diagnostics 

performed in November 2009 showed the heat pump to be operating within specifications, which 

alleviated concerns that the heat pump system was underperforming. Some of the savings 

discrepancy in 2009 can be attributed to occupancy and occupant behavior, as the PV home was 

unoccupied during the hottest months (July and August). The cooling energy discrepancy 

between the improved homes was somewhat less during 2010 when all homes were fully 

occupied (56% versus 71%). The 2010 graph shows Regression line comparisons for 2009 

(dashed) and 2010 (solid).

Figure 8 shows the average daily indoor and outdoor temperatures for each home. The hot 

weather in June and July of 2009 changes to much cooler temperatures in September, where 

results show the difference between outdoors and indoors as negative for several days. The 

coolest weather near the end of the data period is removed from analysis, but all other September 

data in both years is used and contributed to the points making up the far left portion of the trend 

lines in Figure 6. Figure 6 most clearly shows the unusually hot weather pattern in 2009, with 

many more data points visible between 10°F and 14°F on the x-axis during 2009 versus 2010.
13

Figure 8. Average daily ambient and indoor temperatures (°F)

The thermostats in all three homes were kept at relatively stable set point temperatures 

throughout the summer. It appears none of the occupants used programmable functions but there 

are a few notable days where the control home set point was raised considerably, possibly 

because of a period of vacancy. These periods of high indoor temperature settings are consistent 

with reduced energy use as illustrated in Figure 7 by a spread of seven days in the control home 
14

data set (red triangles) with low energy use relative to the other days. The days on which these 

data points fell were sometimes followed by days with relatively high energy use. While these 

outliers, both above and below the trend line, caused a reduced coefficient of determination for 

the control home, they effectively offset one another in terms of their impact on final savings

calculations. Removing these outliers changed the improved home’s savings values by only one 

percentage point for the 2009 season.
15

6 Photovoltaic Performance

The 2.4-kW grid-tied PV array was activated in mid-June 2009, providing energy for the PV 

home and feeding unused energy back to the utility. Twenty-one months of data were analyzed 

from July 2009 through March 2011. The home was unoccupied during the first two months of 

this period, during which the air conditioner was set to maintain an interior temperature of 

approximately 77°F, similar to that of the occupied control home. The PV home was otherwise 

continuously occupied from September 2009 to March 2011.

Each stacked bar in Figure 9 represents the total monthly electric energy used by the PV home 

and the percentage offset by the grid-tied system. The components of each bar are composed of

the net-grid energy used by the home, the PV-generated energy used directly by the home, and 

the portion fed to the utility grid. 

0

200

400

600

800

1000

1200

1400

1600

Jul-10 Aug-10 Sep-10 Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11

Total Monthly Energy Use (KWh)

PV Home Monthy Energy Use

and Percentage Offset by PV 

Net Grid to House PV to House PV to Grid

July 2010 to March 2011

0

200

400

600

800

1000

1200

1400

1600

Jul-09 Aug-09 Sep-09 Oct-09 Nov-09 Dec-09 Jan-10 Feb-10 Mar-10 Apr-10 May-10 Jun-10

Total Monthly Energy Use (KWh)

PV Home Monthy Energy Use

July 2009 to June 2010

Figure 9. Monthly electricity use in the PV home from July 2009 through March 2011

Total electricity use in the PV home increased once occupancy began on September 1. The 

noticeable spike in December 2009 was attributed to extensive holiday lighting and the addition 

of a 1-kW kiln and electric resistance space heating for a garage-based glass-making operation.

The extreme electric energy use continued in January and February 2009, presumably due 

primarily to the glass-making business; however, subsequent months showed more moderate use.
16

On average, PV energy production made up 29% of total electric energy use; about two-thirds of 

the energy was used directly by the home and the remaining one-third fed back to the utility.

The 2.4-kW array, with its roughly southwest azimuth, was well suited to offsetting the total 

monthly HVAC electric load. During the 21-month data collection period, PV production 

completely offset monthly HVAC electric energy except for August 2010, when it made up 97% 

of the HVAC load. On a daily basis, PV production offset HVAC energy 80% of the time for the 

210 days recorded between June 1 and September 30 in 2009 and 2010.

Figure 10 provides a comparison of measured PV energy production to estimated performance as 

stated by the manufacturer on an equivalent 2.4 kW system with an azimuth of 230 degrees and a 

6:12 roof pitch in San Antonio, Texas. Overall, for the 21-month period of measured data, the 

system shows 4.6% less energy production than the manufacturer’s predictions. Generally, 

variations associated with weather data can cause measured and modeled PV performance to 

vary by as much as ±40% for individual months and ±20% for individual years (NREL 2006).

Figure 10. Comparison of measured and estimated monthly PV production
17

7 Results and Conclusions

Three occupied homes with identical floor plans and orientation in San Antonio, Texas 

demonstrated reduced energy use and electricity demand by comparing high performance 

construction with standard building practices. Because the homes were occupied, the impacts of 

occupant behavior on the results are unclear. Results show that cooling energy savings were

55%–77% in two improved homes over the control home. Total demand reductions between the 

control and improved homes were 6–8 kW (62%–83%) on the hottest day during the utility peak 

period. Peak A/C loads in the improved homes on the same day were reduced by 1.2–2.9 kW 

(28%–68%) over the control. A 2.4-kW grid-tied PV array on the PV home provided 29% of 

total electric energy needs on average, with about two-thirds of the energy used directly by the 

home and the remaining one-third fed back to the utility. The southwest-facing array was well

suited for offsetting total HVAC electric energy, which was regularly accomplished on a 

monthly basis and 80% of the time on a daily basis during summer months.

The results of this research provided new knowledge of peak load profiles during the hottest 

weather conditions in new homes with various energy measures, and in those built according to 

standard practice for the local area. The study also provides important feedback for builder and 

homeowner utility incentive programs that manage demand and energy consumption.
18

References

Chasar, D.; Chandra, S., Parker, D.; Sherwin, J.; Beal, D.; Hoak, D.; Moyer, N.; McIlvaine, J.

(2006). Cooling Performance Assessment of Building America Homes. Proceedings of the 

Fifteenth Symposium on Improving Building Systems in Hot Humid Climates. FSEC-CR-1673-

06. Cocoa, FL: Florida Solar Energy Center.

NREL. (2006). PVWATTS: Cautions for Interpreting the Results. Golden, CO: National 

Renewable Energy Laboratory.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/interp.html. Accessed January 2011.

Hendron, R.; Engebrecht, C. (2010). Building America House Simulation Protocols. Golden, 

CO: National Renewable Energy Laboratory. 

NREL. BEOpt Software. Golden, CO: National Renewable Energy Laboratory.

http://beopt.nrel.gov. Accessed July 2011.
DOE/GO-102012-3577 ▪ September 2012

Printed with a renewable-source ink on paper containing at 

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