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Project Economics

The objective of the project was to explore how the maximum feasible energy savings in a new Florida residence would perform when combined with PV electric power. As such, the project was research oriented and was not intended to be economic. Nevertheless, in cooperation with the builder we did track the cost of the various measures installed so that relative assessment of economic performance could be performed.

Table 11
Incremental Cost of Efficiency Measures in PVRES Home
Cost Differences vs. Control

Component $ Incremental Cost
Advanced Windows (Materials) $ 4,026*
Advanced Windows (Added Labor) $ 240
White Tile Roof (Labor) $ 5,301
White Tile Roof (Materials) $ 5,528*
Wider Overhang $ 1,882
High Performance AC $ 1,263
Interior Duct System $ 950
Exterior Wall Insulation $11,500
Propane lines & gas appliances $ 479
Solar Water heater w/propane backup $ 2,989
High Efficiency Lighting $ 525*
Programmable Thermostat $ 225
Refrigerator $ 298
Total $35,206
Total less donations* $25,127

Also, since measures were combined in a single package, it was necessary to use the DOE-2.1E building energy simulation (Energy Gauge USA) to estimate the relative contribution of the various measures. This was done by tuning to the model to reflect the actual conditions encountered (air handler leakage from the attic, unshaded windows etc.) and then using the model to estimate savings for the various measures.

The results of the parametric analysis used for this estimation is shown in Table 12. Starting with the base building, we analyzed how each measure influenced measured heating and cooling energy use. We then used each measure's results to predict the portion of the cooling savings coming from that particular measure. The simulation worked reasonably well at predicting the relative performance of the two buildings. Using the July TMY values for the extreme weather conditions seen in June of 1998, Energy Gauge USA predicted the Control home to use 60.5 kWh per day for cooling against the 61 kWh/day which was measured. When blinds were assumed in the PVRES home (as operated), the model predicted cooling energy consumption of 19.4 kWh/day against the 15.6 kWh which was measured in June. Overall, the model predicted the PVRES house would use about 68% less for cooling in June against the 74% savings actually measured.

Table 12
Parametric Analysis of Heating and Cooling Energy Use
in the PVRES Home DOE-2.1E Simulation

Case Fan Heat Cool Total Heat Total Cool Total % Cool Reduction
Base 1338 1,068 8,915 1,211 10,093 11,321 0.0
Hi Perf Windows 1,005 619 7,072 700 7,986 8,696 20.9
White Roof 1,115 1,119 7,376 1,266 8,328 9,610 17.5
R-10 Walls 1,297 945 8,539 1,074 9,691 10,781 4.0
3 Ft Overhangs 1,255 1,043 8,271 1,184 9,369 10,569 7.2
House Tightness 1,317 988 8,626 1,123 9,791 10,931 3.0
Duct Tightness 1,207 993 8,101 1,125 9,161 10,301 9.2
Hi-Effic. AC 1,367 319 5,709 391 6,988 7,395 30.8
Interior ducts 1,202 928 7,508 1,060 8,561 9,638 15.2
PVRES (All) 655 347 2,868 418 3,440 3,870 65.9
PVRES w/blinds 606 376 2,673 451 3,192 3,655 68.4

We then used each measure's results to predict the portion of the cooling savings from each particular measure as illustrated in Figure 57. This method has some important caveats, however. Many of the individual measures strongly interact with each other. For instance, duct tightness and interior duct location are strongly linked; also white roofs save considerably more when the ducts are located in the attic space. Wide overhangs save more when unimproved single glazed windows are assumed (and vice versa). All measures save less, once the high performance air conditioning system is assumed (the largest single savings measure). Regardless, the results do indicate the relative influence of the various improvements. We used results directly from the simulation for the lighting and water heating measures which did not involve heating and cooling.

figure 57

Figure 57. Estimated percentage of cooling energy savings (83%) attributed to each measure.

Table 13 shows the results on combining the cost and performance data from the above analysis for the various measures to estimate relative economics.

Table 13
Preliminary Economics of Efficiency Measures

Component Description Cost ($) Savings
kWh ($)
Simple Payback
Advanced Windows $ 4,266 1,610 ($129) 33
White tile Roof $10,829 1,342 ($107) 101
R-10 Walls $11,500† 307 ($ 25) 460
Wider Overhang $ 1,882 537 ($43) 44
Interior Duct System $ 950 1,150 ($80) 12
High Efficiency AC $ 1,263 2,376 ($190) 7
Efficient Lighting $ 525 1,479 ($118) 4
High Effic. Refrigerator $ 298 388 ($31) 10
Solar Water Heater $ 2,989 2,097 ($123)* 24
Utility Integrated PV System $40,000 5,600 ($448) 89

† Cost of the wall system was very large due to cost increases associated with a first time installation of the system. A mature market would be able to achieve half this cost.
* Computed on the basis of 37.8 gallons per day raised from 75 to 130oF with an EF = 0.88 base tank.

Annual back-up propane consumption estimated at 32 gallons.

Since the project was a technical research demonstration project, a number of the items did not appear cost effective. However, several measures were economically attractive, including both an interior duct system and a high efficiency air conditioner, high efficiency lighting and refrigeration.

Also, it must be pointed out that there are side benefits for some components. For instance a tile roof will have greater longevity than a shingle roof which makes the energy related savings a cost-effective by-product. Also, advanced insulated window units, such as those utilized in the project, will produce a more quiet home interior with rooms that are less prone to uneven temperatures during very hot or cold periods.

Further, there are construction methods by which the cost of the various measures might be considerably reduced over those experienced within the project. A fundamental scheme is to use surround porches in an altered building plan to keep solar radiation off walls and windows to allow for less rigorous treatment of these building components. Other strategies are to use less expensive white metal roofing and an integrated storage water heater. Potential cost reductions are summarized in Table 14:

Table 14
Potential Cost Reductions for Selected Measures

Measure Cost Potential Cost Reduction or Improvement
Advanced Windows $4,266 Utilize building plan with surround porches with insulated tinted glass
Drops added window cost to ~ $1700
White tile roof $10,829 White metal roof
Drops added cost to ~$3,500
R-10 walls $11,500 R-5 interior wall insulation
Drops added cost to ~$400
Solar Water Heater $2,989 Use integrated storage solar water heater
Drops added cost to ~$1,600

Through such an altered floor plan, it would be possible to reduce the incremental cost of the various improvements by over $22,000 and considerably improve economics while preserving the identified level of performance. The surround porches have a cost, but they also result in very useful exterior living space. Covered areas are of considerable amenity in Florida's climate where direct sun or afternoon rains can otherwise abbreviate outdoor activities. This is essentially a modern embodiment of "Cracker style" scheme utilized by the pioneers in Florida at the turn of the Century prior to the advent of air conditioning [12].


Based on a side-by-side evaluation, energy efficient housing incorporating utility integrated PV power can reduce total electrical consumption by 70% or more over traditional housing. Results also demonstrate that PV can be a viable means to eliminate the peak load posed by the cooling system on the utility during its coincident peak demand period.

Lakeland Electric & Water experienced their annual summer peak power demand at 5 PM on June 18th, 1998. On this day, the occupied PVRES home showed dramatically lower cooling and total electricity requirements than the unoccupied control house. Over the 24 hour period, the PVRES home only used 28% of the air conditioning power that the Control required. During the utility coincident peak period the Control home air conditioner required 2,980 Watts as opposed to 833 W for the PV home - a 72% reduction. Moreover, when the PV electric generation is included during the peak period, the PVRES home net demand was only 199 W - a 93% reduction in electricity requirements over 3 kW required for the control home.

The project had successfully demonstrated its fundamental objective - the ability to greatly reduce space cooling loads and when matched with PV electric power production, to bring the house utility coincident peak demand close to zero. Under matched unoccupied conditions during several hot weeks in May 1998, space cooling energy use was shown to be reduced by 84%. Moreover, average PV power production to the grid averaged 17.1 kWh/day over a 26 day period from April 22nd to May 17th. With the control home this level of power production would have provided less than half the electricity used by the cooling system, while in the efficient PV home, three times as much electricity was produced as the cooling system used.

Even during June's extreme heat wave, reduction in air conditioning use was over 70% even with the PVRES home occupied and the Control unoccupied. When solar electric power production was included, the PVRES home had a net electric demand on the grid was near zero. On the utility peak day of June 18th, the PVRES project conclusively demonstrated it is possible to build very efficient homes in Florida with PV which exert little net demand on the grid during utility coincident peak periods.

Project Update

Addition of a Swimming Pool

A very large change in the electrical loads on the PVRES building took place on August 11th. On this day, construction on the new 14 x 28 ft 14,000 gallon pool was complete and the circulation pump was energized. The pool includes a one horsepower pump which has a 1.6 kW electrical demand. A pump of this size was added in spite of the use of low friction 2" piping for the pool circulation installed. Currently, the pool pump's daily consumption comprises the largest electrical load at the site - with approximately 12.5 kWh/day used during its 7.5 hours of operation from 10 AM to 5 PM. Figure 58 shows the pool pump load profile over the month of September. Daily PVRES household electrical consumption increased from 30 kWh/day to over 42 kWh/day after the pool was installed. FSEC is exploring methods in which pool pump power might be reduced through the use of two speed pumps or other means to cut this load.

figure 58
Figure 58

Control Home

The control home remains unsold, but is shown to prospective customers on Sundays. On August 16th the temperature inside was altered so that a maximum temperature of 80oF is maintained. Project members intend to reset the Control home thermostat in the near future so that comparable temperatures are maintained to that in the PVRES home.

Follow-up Objectives

Our future objectives for the project include:

  • Determination of comparative performance after the Control is occupied
  • Long term assessment of utility interactive PV performance
  • Load reduction of pool pumping electrical load in PVRES hom

Improvements to Concept

Based on findings within the project, the following methods are suggested to enhance concept performance and improve cost effectiveness:

Improves Performance Impact
Greater fraction of tile flooring the floor plan

Greater use of ground as a heat sink

Use sealed recessed cans for ceiling fixtures

Reduces house air leakage

Use greater portion of PV array on west face

Improves peak period power (2 kW) production

Consider higher efficiency water to air

Improves cooling efficiency by heat pump 10%-20%

Reduces Costs
Use white metal rather than white tile roof

Reduces incremental cost of roofing and simplifies PV installation

Use R-5 insulation on interior of masonry walls

Greatly reduces wall insulation cost

Use large diameter flex duct for interior ducts

Simplifies sealing and reduces cost

Use integrated storage solar water heater

Single tank system reduces cost


This project represented a very large effort by many individuals, firms and institutions - a fact reflected in its accomplishments. Special recognition goes to the project sponsors: Sandia National Laboratory, the Florida Energy Office and Lakeland Electric and Water Company. The project would not have been possible without the cooperation of Rick Strawbridge of Strawbridge Construction and his able assistants, Mr. Gary Morrison and Ms. Maureen Warren. Many companies and their representatives also assisted with acquiring superior efficiency products for the project: Dick Edwards of the Celotex Corporation (exterior wall insulation system), Pat Kenny of Pittsburg Plate Glass (advanced windows), Rod Hirschberger of PGT/VinylTech (window fabrication), Keith Wesche of Monier Tile (reflective roofing tiles), Mark Adams and Keith Ledford of the Trane Company (high efficiency air conditioner), Tim Rice of Ward's Air Conditioning (cooling system installation and interior duct system), Wayne Wallace of Solar Source (solar water heating system), Don Lewinski of Panasonic Corporation (high efficiency lighting), and Maggie Baker with Sears contract sales (efficient side-by-side refrigerator). With Lakeland Electric and Water, thanks to Bob Reedy, Jeff Curry, Al Lukhaub and Mimi Fernandez. At FSEC, thanks to Mike Murden for assisting with the PV system wiring and Kevin Lynn and Brian Farhi for assisting with module assembly and testing. Dr. Jerry Ventre provided able overall direction for the project. Finally, our sincere appreciation to the new owners of the PVRES home, Harry and Nancy Adam, for their continued patience.


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