RESIDENTIAL SOLAR WATER HEATING: AN ANALYSIS OF ENERGY SAVINGS

Richard S$ Barnes and Leslie GuliasiPacific Gas and Electric Company

ABSTRACT

Thi s paper presents the resul ts of a study conducted to estimate theenergy savings attributed to the use of solar water heating equipment inresidential householdso The study was undertaken to improve estimates of theimpact of solar water heating on residential energy demand as part of PacificGas and Electric Company's long-term forecasting process$ The results of thisstudy are based on an analysis of survey and energy consumption data collectedfrom a stratified random sample of approximately 3,500 single family dwellinghouseholds that use solar hot water heating equipmente

Using an econometric technique known as conditional demand analysis, acomparison was made of the average energy consumption of households with solarand conventional water heating equipment, while controlling for important

fferences between these households@ Conditional demand is a multipleregression technique which allows one to control for the effects of factorsassociated with energy consumption (e~g~, appliance holdings, householddemographic and dwelling characteristics, weather conditions, the price ofenergy) and thus isolate the savings attributed to the presence of a solarwater heating system0

The of this analysis show a reduction in water heating energyconsumption in households that repla a conventional water heater with a

ar water heating system$ The average energy savings in households thatreplaced a conventional el c water h er with a solar water heatingsystem 1,433 kWh per year, a reduction of 33 percent~ The average energy

ngs in hou 1 that replace a conventional natural gas water heaterwi a ar system was therms year, a reduction of 46 percent@

BARNES AND GULIASI

RESIDENTIAL SOLAR 'WATER HEATING: AN ANALYSIS OF ENERGY SAVINGS

Richard S0 Barnes and Leslie Guliasi*Pacific Gas and Electric Company

Thi s paper presents the resul ts of a study conducted to estimate theaverage net energy savings associated with residential solar water heatingequipment@ The study was undertaken to improve estima'tes of the impact ofsolar water heating on residential energy demand as part of Pacific Gas andElectric CompanyBs long-term forecasting process~ The results of this studyare based on an analysis of survey and energy consumption data collected froma s atifi random sample of 5,210 single fam; dwelling househol withsolar water heating equipment in Pacific Gas and ectric Companyts serviceterritorYe Each household in the st was mailed a six-page, 47-;questionnaire designed to collect information on household demographiccharacteristics, appliance holdings, and energy conservation measures~ Atotal of 3,437 households responded the , a response66 11"'"OJl'::l>llr'lr".lC.'IlRrll

HEATING EQUI

ar heating is increasea1 househo1ds wi th n9 equ i pmentnumber of domestic solar water ng systems in

in 1979 was estimated to be 5,337$ By 1981,increased sevenfold to 38,681, or slightly more

residenti households~ This increase occurred 'inmul ly 1ings, as well as in househol

ura1 to the; r wa heaters Pac;bwVSIVl1I cs cs Department, Res

) ~

Public 1; es Commission directed ofutilities to implement the Demonstration

service ter tory0 Thus, andE established aoffer rebates

version a report ginally published by thelysis Section of Paci c Gas and ectric CompanySs

cs Department~ The authors gratefully acknowledged Bonney, Rachel1e Canter, and Bill Levins ir

1 comments on ea ier drafts of this work~ This paper is the productstaff members Paci c Gas and ectric Company and does not

the views the Company or its management~

E....6

BARNES AND GULIASI

who installed a qualifying solar domestic water heating system. By the endof the programBs first year of operation a total of 16,526 systems had beeninstalled, accounting for approximately 43 percent of the total populationof solar water heating systems in the PGandE service territory& The vastmajority of these systems were installed in single family dwellings Thelargest number of rebates, 9,739 (59 percent), were given to the singlefamily gas sectorG Single family dwellings with an electric backup waterheater received 4,715 (29 percent) authorized rebatesf) In addition, 2,072(12 percent) multifamily dwellings participated in the Demonstration Program,each receiving a credit for a natural gas backup system.

HOUSEHOLD DEMOGRAPHIC AND APPLIANCE-HOLDING CHARACTERISTICS

Table 1 compares the demographic and appliance-holding characteristics ofsingle family households with solar and conventional water heating equipmenteAs the table illustrates, households with solar water heating equipment occupylarger homes~ have a higher median income range, and have more residents perhousehold than the average PGandE single family household0

Compared to the general population of single family PGandE households,households with solar water heaters splay a higher saturation of centralspace heating and central ectric cooling systems~ Moreover, households withsolar heating equipment have a higher saturation of appliances that usehot water, pa cularly el c dishwashers and othes washers~

same is install on of energy conservation measures:househol s ar water heating equipment have a substantially higher

on t water heater blan ts, hot water pipe insulation, low-flowheads, ceiling and wall insulation, caulking and weatherstripping, and

duct i ation compared PGandE single family households in

heating systems differPGandE single family households

ngs (i nc1udi ng those app1i ancesrn~~eO,~\J:~T'on measures~ These differences, as

on, account for significant differencesr average levels of energy consumption~

BARNES AND GULIASI

TABLE 1

Selected Household Statistics

93@S%71.0%68@8%61~9%

59.3%12@717.21.8

41~3%

7$789.4%98.0%

0.0%Q.O

100eO

81.7%67*0%

~5%

1,948$25,000-$30,000

3~22

64~1%

Single Family ,Households with Solar

Water Heating Equipmentb

9809%

~8%

18@7%36@5%

66.9%43*4%55~O%

00%

82@6%

21~O%

1.5.7%

8ge4%

PGandESingle FamilyHouseholds

1,676$20,000-$25,000

3.0355.9%

74.8%4e87.50.2

Own HomeAverage Square Footage

of ResidenceMedian Income RangeAverage Number of ResidentsCentral Space Heating SystemMain Space Heating Fuel:

Natural GasElectricityWoodSolar

Space Cooling System:Central ectric

ectric Heat Pumpectric shwashervate Clothes Washer

Primary WaterNatura1 Gas

ectriar

rce: and Statistics Department~

Vol@ II: Catalo ue of

rce: c Company~ Economics and Statistics(April 1984)0-------------

BARNES AND GULIASI

MONTHLY ENERGY CONSUMPTION

This section of the paper examines the levels and patterns of monthlyenergy consumption of households with solar and conventional water heatingequipment. Table 2 presents information on the seasonal and annual averagemonthly electricity and natural gas consumption during 1982 for householdswith solar and conventional water heating equipment in both the electricityand natural gas market segments@

TABLE 2

Average Monthly Energy Consumption, 1982(Single Family Dwellings)

ElectricityMarket Se menta

onventl0na S--

Natural GasMarket setment

ectri ty (kWh)Annual Average 974 1,163 523 7Summer Season 837 998 517 716Wi Season 1~110 1,328 530 714

Natural ( )Annual Average 69 63Summer Season 38 24Winter Season 100 101

'Ie ty but no gas service from PGandEQ

BARNES AND GULIASI

Monthly Electricity Consumption

Figure 1 depicts the monthly electricity consumption during 1982 of fourhousehold groups: households in the electricity or natural gas marketsegments, and households with solar or conventional water heating equipment&

1~ Monthly on

two groups inci consumption of

consumption is similar for all four~

winter months when demand fordeclines during the spring and 11summer due to the demand space

rna -- with ar oripment -- di ay a higher saturation of

cooling systems as well as other electrici oners, than households in the gas market

r higher level monthly electricity

in pattern of monthly electricityin gure 1, is that households with solar water

a higher average level of consumption than comparable~~~~unnT' 1 water heating equipment in both the electricity

market segments~ When compared to households withanal water ng equipment, solar water heater households differally in many ways which affect electricity consumption. For example,

scussed above, househol with solar water heaters are more affluent thannono~2l popul ion of PGandE households. They tend to live in larger

homes, use ntensive central electric space heating and cooling systems

BARNES AND GULIASI

and electric heat pumps, and have more electrical appliances such as electricdishwashers, clothes washers, and clothes dryers than PGandE households withconventional water heating equipment$

Mo.nthly Natural Gas Consumption

Figure 2 similarly depicts the pattern of monthly natural gas consumptionduring 1982 for households with solar water heaters and gas backup systems andhouseholds with conventional gas wa..ter heating equipment.* Here the resultsdiffer from what we observed in the pattern of monthly electricityconsumption~ Single family households with conventional gas water heatingequipment consumed more natural gas than households with solar water heatersduring the warmer months of the year, but during the cooler months (Novemberthrough March) the leve'l of consumption was generally similar for the twogroups* This pattern suggests that while solar water heating systemscontribute the conservation of natural gas throughout the year, the savingsappear most dramatically in the summer when there is little or no demand forgas space heatinge Furthermore, as illustrated above, the 'dwellings ofhouseholds with solar water heating equipment are typically larger than thedwellings of other single family households@ And since, in general, largerdwellings require more energy for space heating than smaller dwellings, onewould expect households with solar water heating equipment to consume a higherlevel natural gas, on average, than households th conventional water

•

..

'48

~\ I

l

\ J~\ I,~ .. [/

1\ v~

~ COrMmticnal- Gaa Mark9i~ A'~ hi\\ /l

-- _. " ,

~"SoW - Oms MwicGt~ '\

~,.....,.,

~

-~~-I~l

gure 2@ Monthly Natural Gas Consumption

*Households th conventional ectric water heaters or solar water heatingsystems and an ectric backup water heater rarely receive gas service~

Therefore, no data are presented for these household groups~

[-11

BARNES AND GULIASI

The patterns of total monthly electricity and natural gas consumptiondiscussed here reflect the complex composition of a household 8 s applianceholdings as well as many other factors that influence energy consumption,such as household income, lifestyle, geographic location, and dwelling size~

It was shown that households with solar water heating systems use moreelectricity per month than households with conventional water heaters. Inaddition, it was shown that households with solar water hea,ting equipmentand natural gas backup water heaters use about the same level of natural gasduring most of the year, except during the summer season when they use lessnatural gas, compared to households with conventional gas water heatingequipment~ However, many factors which influence energy use, other than thetype of water heating system a household has, led to these resultse We turnin the next section to an analysis of the energy savings associated th arwater heating systems6

ANALYSIS OF ENERGY SAVINGS

This section of the paper presents the results of an analysis performedto estimate the average net energy savings attributable to the use of solarwater heating equipment@ The findings of this analysis, as presented inTable 3, show that, o~ average, a solar water heating system installed asa replacement for a conventional electric heater saves 1,433 kWh of

ectri year, and that a sol water n9 system install as aa conventional 1 water heater saves 132 therms

These savings represent a 33 on inene consumption on an annual d

i pment a convent i ona1ar equi

3

e ly 1i )

ectricity~~rket Segment

4,337 kWh

Natura1 GasMarket Seg~!TI:

285 Therms

2,904 kWh Therms

1,433 kWh 132

AverageSavings 33% 46%

BARNES AND GULIASI

Estimating Net Energy Savings

To estimate the net energy savings attributable to the use of solar waterheating systems in single family households, a multivariate statistical tech~

nique called conditional demand analysis was used. Conditional demandanalysis enables one to account for the effects of weather conditions 9

appliance holdings, household demographic characteristics, and other majori nfl uences on energy consumpti on, and thus estimate how much energy, onaverage, households with solar water heating systems use for water heatingcompared to how much energy they would use had they not installed solar waterheaterse

Households used ,in this analysis were obtained from two sourceSePGandE's 1981 Residential Appliance Saturation Survey provi d information ona stratified random sample of households in the service areae In addition,PGandE's Solar Water Heater Survey provided information on households thsolar water heating systems~ Recorded electricity and natural gas consumptiondata covering a 12-month riod of for each household were combined withthe survey data~ Furthermore, information on a number of other variableswhi ; luence household energy consumption -- such as outside temperature,the number of da i hours, the date the solar system was installed, and theaverage amount sunlight availab for the solar system -- was added to thedata se for each household observation@ Using ese data, two separateequations -- one for electricity and ana er for natural gas -- were specifiedand esti or purpose of modeling hou d energy consumption@ The

functional form of the equations is as follows:

:::::

k J'*jk

E is i-th household in the k-th me period~

ablek-th

j-th end-use, in the i

es affecting the energy used byin the k-th time period~

estimated@

,because the pe speci cation used, it was also possiblesaggregate ese equations into smaller, subequations which estimate the

Qnl~~nl\l consumption of pa i ar end-usese Since the chi aim of this studyestimate househol d energy consumpti on for water heating, only theons for water ating -- both for solar and conventional water

ng equipment -- were of importance*

BARNES AND GULIASI-

After the conditional demand equations were estimated, the next step inthe procedure was to solve the electricity and natural gas water heatingsubequations using average household characteristics for households with solarwater heating systemse As part of this process, the variable that indicatesthe amount of sunlight available for solar water heating was set to representa IS-year average rather than actual sky coverage conditions for anyparticular year~ The results of this step were estimates of: (1) the amountof electricity and natural gas used by households with solar water heatingsystems for water heating under average sky coverage conditions, and (2) thethe amount of water heating energy these same households would have used hadthey not installed solar water heaters. Using these two estimates,average net electricity and natural gas savings associated with solar waterheating were calculated~

Discussion

According to these estimates, as shown in Table 3, households withnatural gas backup water heaters realize approximately 13 percent more energysavings for water heating than households with electric backup equipment0This difference in average annual ene savings is due in part to the fact

the two household' groups are generally locat in different geographicons state, and thus receive fferent amounts of sun exposure for

ar ng@ Households with natural gas backup systems are typicallylocated San SCQ Bay Area and in the Central Valley region of the

~ In contrast, households wi electric backup water heaters arecal located in coastal zone and in the erra Nevada foothil1s~

were made explain the difference in the percentage of~~~'~&'~8~n households with electricity and households th natural

heating equipment by examining characteristics other than sunmay affect the performance of solar water heaters& A list of

cs i uded the~ of solar water heating system used andcost of the solar water heating system, as these characteristics vari

significantly between the two household groups~

BARNES AND GULIASI

Type of System. Solar systems may be classified into two broad categories -­passive and active. Passive systems have no automatic controls or pumps. Ingeneral, passive systems are not as efficient as active systems, but if theircost is lower, they may be more cost-effective to operate.

There are two types of passive systems: thermosyphons and integralsystems. Thermosyphons have separate solar collectors and an insulatedstorage tank which is mounted above the collectors. They operate withoutmoving parts as water circulates from the collectors to the tank due to thedi fference in the density between hot and col d water. Integra1 systemscombine the collector and storage tanks in one unit. A water tank, paintedblack, is placed inside an insulated container with a window .facing the sun.The operating principle of these systems is quite simple: The sun shines inthe tank and heats the watere

Active systems have two major components in addition to the solarcollectors and storage tank: a control unit and a pump. Whenever thesystem's sensors tell the unit that the water in the collector panels ishotter than the water in the storage tank, the unit automatically turns on apump@ The pump circulates the solar-heated water from the collector panelsinto the storage tank, and continues to do so as long as there is heat to begained@

Table 4 presents the distribution of the three types of solar equipmentthe ectri ty and natural gas market segments@

TABLE 4

Type of Solar Water Heating Equipmentand Water Heating Market Segment

ActiveThermosyphonIntegralTotal

Electricity

24.6%8@O%IG2%~%

Natural Gas

39.6%8.8%

1708%~%

Total

64.2%16._& 8%19QO%

100.0%

A comparison of households with active, thermosyphon, or integral systemssome differences in the average level of solar water heating energy

consumption among these three groups. On the surface, this suggests~that dif­types of solar water heating systems produce varying amounts of energy

ngs@ However, it was also discovered that there were differences in waterng energy consumption among households with different types of solar

equipment prior to the installation of the solar water heating equipment.Therefore, differences in household water heating energy consumption cannot beattributed to differences in the type of solar water heating system used, but

to other unexplained differences in household characteristicse

[-15

BARNES AND GULIASI

Cost of System. Cost is a major determinant of a consumer's choice of a solarwater heating system. Figure 3 depicts the cost distribution of the solarwater heating equipment for households in the electricity and natural gasmarket segments.

~ 81.-.,. 82,000- 82,M- A,OO@.. P.-'" 34,..... $4,__ .....81,_ ~ a._ $3'- 83,_ ..... SM.. $I" ~

Cooill3f8~(DctMars)

Di bution of Solar Water Heating Systems

ar water heating systems was not found to affect the averageheating energy consumption sign; cantly@ This suggests that

unrel .energy eff; ency affect the cost of solar water heatingTherefore, even though the cost of solar water heating systems

between househol th ectric and households with natural gaswater , it concluded that system cost is responsible

percentage of energy savings@

stems~ An analysis was also performed to estimate~---""""--~-~-_"""""""~--or water pumping in active solar water heating

tha average water he ing energy consumption forpment and ectric backup water heaters r includes the

pu e ng @ (The average gas consumption for householand natural gas backup water heaters will not include theused for water pumpinge) Therefore, an attempt was madecity use for water pumping by determining if households

with ve ar water heating systems and a natural gas backup water heaterhad some sign; cant additional electricity use. No significant electricityconsumption was found after controlling for other differences in the charac­

sties of households as specified in the conditional demand equation~

water pumping equipment does consume some electricity, the averageconsumed was smaller than what could be explained in the model $

BARNES AND GULIASI

In sum, an examination of the characteristics associated with solar waterheating equipment revealed no statistical evidence to account for thedifference in the percentage of annual energy savings between households withsolar water heating equipment and electric backup water heaters and householdswith solar water heating equipment and natural gas backup water heaters&

Avoided Costso Although households with natural gas backup water heatersrealized a greater percent.age of energy savings than households with electricbackup equipment, it would be inappropriate to claim that it is more advan­tageous from a utility's perspective for households with gas water heatingsystems, rather than households with electric water heating systems, toreplace their conventional water heaters with solar systems~ In fact, if theenergy savings were calculated from the point of view of the average avoidedcost* associated with producing a kilowatt of electricity or a therm ofnatural gas, one would find that the average dollar savings is greater when ahousehold replaces a conventional electric water heater with solar equipmentthan when it replaces a gas water heatero The example shown in Table 5illustrates this point using assumed avoided costs of $O~06 per kWh and$0040 per therm~

TABLE 5

An e of Average Annual Avoided Cost Associatedth Solar Water Heating Equipment

(ngle ly Dwellings)Average Avoided Average Annual

Average Net Energy Cost to Produce Avoided CostSavings with Solar One Unit of for Solar

a~t~r Heating Eguipme~! Water Heating

$O~06 per kWh$O@ per Therm

CONCLUSION

resul a performed to estimate theuse solar water heating equipment in

~ study was successful both in demonstrating thesavings associated with residential solar water heating

in fying the demographic characteristics of single familyhave installed solar systems~ As such, the study offers a

contribution by demonstrating the successful application of conditionalanalysis as a tool both in forecasting residential energy demand and in

~lInlMt::S"''''''''8 the accuracy energy savings estimates for long-term demandng~

*This is the cost that a utility would incur to produce or purchase an amountof power equivalent to that purchased from a qualifying facility (i@e~, acogenerator or a small power producer) as defined under the Public Utili esRegulatory Policies Act (PURPA) of 19780

[-11