0360-5442/X0/1201-1 IX3SOZ.WO
Energy Vol. 5. pp. 1183-l 192
0 Pergamon Press Ltd 1980. Printed in Great Britain
HOUSEHOLD WATER CONSERVATION: THE ROLE OF
INDIRECT ENERGY SAVINGS
WALLACEJ. HOPP and WILLIAM P. DARBY
Department of Technology and Human Affairs, Box 1106.Washington University,
St. Louis. MO 63130, U.S.A.
(Received 28 March 1980)
Abstract-Recent
regulations specify a cost-effectiveness analysis of implementing household
water conservation measures to reduce the flow of wastewater as a prerequisite to federal funding
of wastewater treatment plant construction. There is a wide variety of devices available to
conserve water: hot water as well as water at ambient temperature. In this analysis we use a
sample of 23 metropolitan areas to evaluate the indirect household energy savings which results
from conservation devices to save hot water.
Devices to conserve hot water are cost-effective in all 23 metropolitan areas up to an equivalent annual cost of SlSO/lOOOgal saved under an assumption of no inflation in energy prices, and
about $7.00/1000 gal saved under an assumption of 20%/yr inflation in energy prices. The impact
of the indirect energy savings can be further illustrated by noting that devices to conserve
ambient temperature water are cost-ineffective in all 23 metropolitan areas at equivalent annual
costs above Sl.OO/lOOOgal saved. Besides increasing the level of cost-effective household water
conservation from 114 gal/day to 146 gal/day for a family of four, while providing significant
economic savings, the indirect energy savings from hot-water conservation devices can provide a
46-62x reduction in energy use for residential water heating.
INTRODUCTION
The Municipal Wastewater Treatment Works Construction Grants Program provides
federal funding for up to 75% of the cost of constructing conventional municipal wastewater treatment facilities and 85% of the cost of constructing facilities based on innovative
technologies. Recent regulations require the municipalities to perform a cost-effectiveness
analysis of flow-reduction techniques, including household water conservation devices,
and to implement those flow-reduction measures found to be cost-effective in order to
qualify for federal construction grants funds.’
This analysis focuses on the effects of installing household water conservation devices,
as viewed from the homeowner’s perspective. In many cities, public water supply is not
metered and homeowners are charged a fixed fee (independent of use).2 Wastewater flow
is hardly ever metered at the individual household level: a fixed fee is levied, or the
charge is sometimes related to water use if public water supply is metered. Even in those
instances where household charges for both wastewater treatment and public water
supply are volume-dependent, widespread conservation measures often result in only
short-term direct savings to homeowners from reduced public water supply and wastewater treatment charges. In the long run, rate restructuring to a higher price per unit
volume is often necessary. 3 The major costs (e.g. debt service, distribution/collection
system upkeep, billing, and support services) of public water supply and wastewater
treatment utilities and the operating revenue required are fairly insensitive to the actual
volume of water or wastewater processed.4* 5
As shown in Table 1, the two largest in-home water uses are toilet flushing and
bathing, which respectively account for about 39 and 31% of the total. The most effective
water conservation efforts are likely to focus on these two uses. While reductions in water
use for toilet flushing save ambient temperature (unheated) water, reductions in water use
for bathing save hot water and provide an indirect savings due to energy conservation.‘-’ ’ The monetary savings from the reduction in energy use may exceed any savings
from reduced public water supply and wastewater treatment charges. l2
1183
WALLACEJ. HOPP and WILLIAMP. DARBY
1184
Table I. Typical daily use of household
I
Gal. per
family
per day
/
j
,
I
Toilet:
(5 flushes/person x 5 gal/flush x 4
persons)
Bathing:
(5 gal/min x 4 min/shower x 1 shower/
person/day x 4 persons)
Laundry:
(50 gal/load x 0.7 load/day)
Dishwaster:
,{;,",~;l;;o,;d
x 1 load/day)
water by a family of four; source: Refs. 6-8.
1
Liters' per
family
per day
j
/ Liters per j
1 capita !
: per day I
Percent
of
total
Use
temperature (OF)
39
ambient2
I
100
’
/
1
80
Lavatory:
Utility sink:
j
Totals
i
I
Gal. per
capita
per day
25
378. 5
’I
20
/
94.6
302.8
75.7
4
31
107
1003
35
0.75
132.5
33.1
14
;:
3.75
45.4
56.8
11.4
14.2
6
I
140
a
23
1.25
30.3
18.9
7.6
4.7
100
/
--
5
255
’
63.75
j
965.2
,
241.3
l
The liter is not strictly an SI unit, but is frequently used where a smaller measure is necessary. To
convert liters to the standard unit for volume (m3), one must divide by 1000.
'Ambient temperature water is water which is used at whatever temperature it is delivered to the home.
It therefore requires no heating.
3
The average temperature is estimated by assuming 30% wash loads done in hot (13O'F) water, 50% loads in
warm (lOOoF) water, and 20% in cold (ambient) temperature water.
Table 2. Water-saving devices that conserve ambient-temperature water;’ source: Ref. 13.
Device
Plastic bottle in the
toilet tank
Water savings
(gal/cap-day=
3.785 liters/cap.
day)
Unit
price
(S)
Installation
cost
(b)
Power
requirements
(Kwhelyr)
Ex;;;;ed
(Vrs)
2.5
0
0
0
15
Toilet tank dam
10.0
5
0
0
15
Dual flush toilet
(incremental)3
17.5
10
0
0
20
Shallow trap toilet
(incremental)
Dual flush toilet (new)4
7.5
10
0
0
20
17.5
100
50
0
20
0
20
Shallow trap toilet (new)
7.5
98
Washwater recycling system
25.0
450
Vacuum toilet (incremental)
22.5
475
Vacuum toilet (new)
22.5
S;;:;;ntained composting
50
655
350
15
200
15
20
575
325
15
20
0
1200 *
20
123
20
5000
20
25.0
795
Composting waste disposal
system
25.0
1700
02
Incinerationtoilet (new)
25.0
1000
0
'All cost and price data are in June 1978 dollars.
2We assume do-it-yourselfinstallation. These devices are generally implementedby
"enthusiasts"who install them themselves. Thus, no installationcost figures are
available. If an "average" homeowner were to make use of one of these devices, it is
likely that he/she would hire someone to install it, thus increasingthe total Cost Of
the device over the estimates given here.
3"Incremental"is the marginal cost of purchasing water-saving equipment over conventional
equipment. This cost is applicable in new housing or in existing housing where the conventional water-using equipment is in need of replacement.
4"New" is the cost of discarding functional.water-usingequipment and replacing it with a
water-saving model for the sole purpose of conservation.
5This device also requires annual maintenance costing about $12.OO/year.
106
140
2.5
1.0
100
107
107
2.5
17.5
2.0
100
10
25
140
104
:::
1.0
4
3
105
105
1.0
0.5
04
04
02
0
150
10
1:
0
0
0
0
Installation
cost
(b)
3.6
6.0
4.5
20
15
6.0
63.96
5.6
4.5
7.9
:::
13.9
27.8
Electric hot
water heating
(electricity,
l+whe)
22.2
29.4
17.7
29.4
306.56
27.1
22.2
38.4
13.0
6.5
67.8
135.7
Natural gas.
fuel oil, hot
water heating
(106 Btu-thermal)5
Expected lifetime energy savings
15
::
15
15
15
15
15
1':
Expected
life
(yrs)
hot water.’ source: Ref. 13.
Unit price includes installation for these devices.
of $1.00 per foot of pipe and 50 feet of hot water pipe.
'This device requires about 49 Kwh,/yr to operate.
5
We assume 70 percent conversion efficiency.
4
3We assume a cost
2we assume do-it-yourself installation. These devices are generally implemented by "enthusiasts" who install them themselves. Thus, no
installation cost figures are available. If an "average" homeowner were to make use of one of these devices, it is likely that he/she
would hire saxeone to install it. thus increasing the total cost of the device over the estimates given here.
250
200
503
3::
100
1
15
107
107
1;::
'All cost and price data are in June 1978 dollars.
Ai!%%"%?er
Thermostatic mixing valve
Insultation of hot water
pipes
Water-saving clothes washer
(new)
Water-saving dishwasher
(new)
Shower flow restrictor
Low-flow shower head
Flow-limiting faucets
(kitchen/bath)
Aerators (kitchen/bath)
Water-saving dishwasher
(incremental)
Pressure-reducing valve
Water-saving clothes washer
Device
Unit
price
(S)
devices that conserve
Use
temperature
(OF)
Water savings
(1 gal/cap.day =
3.785 liters/
cap-day)
Table 3. Water-saving
1186
WALLACEJ. HOPP and WILLIAMP. DARBY
Table 4. Marginal water and energy prices’ for a family of four; source: Refs. 2, 14-17
_
IMetropolitan area
-
(B/Kwhe)
1
Atlanta, GA
Baltimore, MD
Boston, MA
Chicago, IL
Cincinnati, OH
Cleveland, OH
Dallas, TX
Denver, CO
Detroit, MI
Honolulu, HA
Kansas City, MO
Long Beach, CA
Los Angeles, CA
Milwaukee, WI
New York, NY
Newark, NJ
Oakland, CA
Philadelphia, PA
Pittsburgh, PA
St. Louis, NO
St. Paul. MN
San Francisco, C'A
Seattle, WA
Monthly use on
which the marginal price is
based
Electricity
Water
($/lo00 gal)
1.12
0.66
0.26
0.52
0.002
0.32
0.67
0.77
0.26
0.54
0.65
0.48
0.44
0.47
0.00
0.77
0.50
1.55
1.10
0.000
0.58
0.47
0.53
0.039
0.051
0.042
0.025
0.035
0.053
0.037
0.039
0.050
0.050
0.048
0.049
0.040
0.030
o.oa7
0.061
0.029
0.045
0.049
0.040
0.049
0.029
0.010
I
;
,
I
!
I
I
I
I
I
I
I
+
7,650 gal
1
j
!
I
Fuel oil I
(S.gal) I
Natural gas
(2/100 cf)
I
0.22
0.38
0.25
0.25
0.27
0.20
0.27
0.20
0.26
0.88
0.17
0.17
0.17
0.29
0.40
0.40
0.21
0.31
0.24
0.25
0.26
0.21
0.32
j
/
0.500
0.505
:
!
/
j
0.486
0.504
0.487
1
/
0.450
0.497
0.450
0.489
0.492
0.484
0.470
0.470
0.482
0.515
0.515
0.470
0.500
0.500
0.484
0.483
0.470
0.513
I
,I
j
;
,
i
!
!
I
850-1600 K!Whi 100,000 cf
---
,
I
'All prices are in June 1978 dollars.
2
A marginal price of 0.00 indicates fixed-fee pricing.
ANALYTICAL
METHOD
In this analysis, we compare the equivalent annual savings from water conservation
to the equivalent annual cost for a family of four with water use patterns as shown in
Table 1. If the equivalent annual savings exceeds the equivalent annual cost (resulting in
a positive equivalent cash inflow), the conservation measures is called cost-effective.
Tables 2 and 3 describe the expected water savings, cost of implementation and operation, and temperature of the water saved, for each conservation device. Because there is
considerable regional variation in the marginal prices for public water supply and energy,
23 geographically-representative
metropolitan areas were chosen for analysis. These are
shown in Table 4, along with the marginal prices for public water supply, electricity,
natural gas and fuel oil. The assumption of a fixed charge (independent of volume) for
households was made for wastewater treatment in all 23 cases.
NET
ANNUAL
SAVINGS
TO
HOMEOWNERS
FROM
WATER
CONSERVATION
To calculate the total annual cost of achieving a reduction in water use through
implementation of a water-conservation
device, annual maintenance and energy costs
necessary to operate the device must be added to the annualized cost of purchasing and
installing the device. Purchase and installation costs are annualized over the useful life
using the standard capital recovery factor”
r(1 + r)
A=P(l+r)“-l’
where
A = annualized cost,
P = present cost,
Household water conservation
1187
r = discount rate representing homeowners’ cost of capital,
n = useful life of the device.
A rate of 180/,/yr was chosen to represent the homeowners’ cost of capital. This rate is
equivalent to the interest charged by many bank credit cards.
For devices which save ambient temperature water, the annual savings in public
water supply charges may be calculated directly and compared with the annualized cost.
The results are summarized in Table 5. They show that while inexpensive devices to
conserve ambient temperature water used in toilet flushing are almost universally costeffective (i.e. in any city with a non-zero marginal price for water), devices with equivalent
annual costs in excess of $1 .OO/lOOOgal are almost universally cost-ineffective. Savings
estimates are based upon a constant marginal price for public water at the current level.
The expected savings in energy costs due to a device which conserves hot water
depend on the type of fuel used. We consider separately the savings from a reduction in
hot-water use for households which use each of three water-heating fuels: electricity,
natural gas, and fuel oil. Since the transfer of heat to the water is nearly lOOo/,efficient,
annual savings to homeowners with electric hot-water heaters can be calculated directly. lo Natural gas provides app rox. 1000 Btu per cubic foot and the transfer of heat to
the water in natural gas-fired hot-water heaters is about 70% efficient.g Fuel oil provides
approx. 130,000 Btu per gal, and the transfer of heat to the water in oil-fired hot-water
Table 5. Cost-effectiveness of devices that conserve ambient-temperature
Oevice
Annual
equivalent
cost ($/yr)
T
Net annual
savings (Jlyrj2
--
Maximum
Minimum
--
Average
r
water.
Percent of
cities where
the device is
cost-effective
Equivalent
Annual cost
of conservation
($/I,000 gal)
Plastic bottle in
toilet tank
0.00
5.66
0.00
2.01
87l
0.00
Toilet tank dam
0.98
21.65
-0.98
7.05
a7
0.07
Oual flush toilet
(incremental)
1.87
37.37
-1.87
12.20
87
0.07
Shallow trap toilet
(incremental)
i .a7
15.10
-1.87
4.16
a7
0.17
Dual flush toilet
(new)
28.02
0.08
-28.02
-13.96
13
1.10
Shallow trap toilet
(new)
27.65
-10.68
-21.65
-21.62
0
2.53
Washwater recycling
system
127.15
-70.57
-127.15
-107.06
0
3.48
Vacuum toilet
(incremental)
126.70
-75.79
-126.70
-108.62
0
3.86
168.74
-117.82
-166.74
-150.66
0
5.14
Self-contained composting toilet
196.52
-139.95
-196.52
-176.43
0
5.38
Composting waste
disposal system
322.51
-265.94
-322.51
-302.42
0
8.84
Incineration toilet
386.82
-330.24
-386.83
-366.73
n
10.60
vacuum toilet
(new)
p-p
'Three of the 23 cities in the analysis (13%) effectively have fixed-fee pricing for water sold to
Private households. In these cities. a reduction in water use does not reduce the household water bill.
Since they produce no energy cost savinqs. devices which conserve ambient temperature water cannot be
cost-effective for homeowners in these cities.
,/
2Numbers in these columns represent the maximum, minimum, and average savings to typical families of
,rfour in the 23 cities considered in this analysis.
14.72
7.17
2.95
0.79
0.59
1.96
6.87
5.89
Low-flow shower head
Low-flow faucets
Faucet aerators (2)
Water-saving dishwasher
(incremental)
Pressure-reducing valve
Water-saving clothes
washer (incrmental)
49.10
Water-saving dishwasher
(new)
-22.73
-4.38
11.71
6.48
186.92
29.01
38.13
24.40
158.36
-45.30
-33.33
-15.29
-6.75
-49.38
0.05
0.69
1.84
0.69
1.77
23.33
12.94
-35.29
-20.06
-3.03
-0.73
57.93
13.33
18.03
11.85
3.64
7.67
84.66
43.60
tlinimum Average
Electricity
-35.42
-20.24
-3.20
-0.82
56.48
13.15
17.79
11.72
3.60
7.59
83.83
43.19
Haxlmum
-45.91
-34.38
-16.22
-7.20
-57.46
-1.04
-0.64
1.17
0.46
1.32
18.72
10.64
Minimum
Natural gas
-44.10
-31.72
-13.81
-40.90
-25.76
-8.75
-3.51
7.84
-36.42
-6.02
7.63
10.19
6.24
2.29
4.97
56.03
29.29
Maximum
1.67
2.79
3.03
1.05
2.48
30.74
16.55
Average
-43.35
-31.67
-13.51
-5.89
-33.80
1.72
3.07
3.79
1.10
2.60
32.24
17.40
Minimum
Fuel oil
-42.51
-29.61
-11.86
-5.06
-19.32
3.78
5.55
4.63
1.52
3.42
40.51
21.53
Average
33.63
10.76
7.04
6.40
3.73
1.61
1.57
1.34
0.81
0.54
0.20
0.03
Equivalent
annual cost of
conservation
(S/l,000 gal)
in these columns represent the maximum. minimum. and averaqe savings to typical families of four in the 23 cities considered in this analysis.
39.28
Water-saving clothes
washer (new)
I Numbers
20.55
9.34
Insulation of hot
water pipes
Thermostatic mixing
valve
95.37
Air-blower shower
(2)
Shower flow restrictor
80.46
0.20
Device
Maximum
Annual
equivalent
cost (blyr)
Net annual ravings ($/yr) by water heating fuel1
Table 6. Net annual savings for devices that conserve hot water.
Household water conservation
1189
heaters is only 67% efficient.g The savings in the cost of public water supply may also be
calculated directly. For homeowners using each of the three hot water heating fuels in
each of the 23 metropolitan areas, net annual savings have been calculated as the sum of
the annual water and energy cost savings less the total annualized cost of achieving the
reduction in water use through implementation of the water conservation device.
Table 6 summarizes the results of the analysis. Once again, inexpensive devices are
almost universally cost-effective while expensive devices are almost universally costineffective. However, the cut-off point is considerably higher than for ambient-temperature water. The indirect energy savings makes devices that conserve hot water almost
universally cost-effective up to an equivalent annual cost of about $1.50/1000 gal saved.
In addition, savings are likely to be greater than the estimates in Table 6. During the
useful lives of the devices (typically, 15-20 yr), energy prices are certain to increase
substantially over June 1978 levels. Further analysis considers the effect of increasing
energy prices on the justification of devices which were not universally cost-effective at
mid- 1978 levels.
ENERGY
INFLATION
RATES
NECESSARY
FOR
COST-EFFECTIVENESS
In those cases for which a water-conservation
device is not cost-effective to
homeowners assuming constant prices for energy and water, an analysis of the present
equivalent costs and savings was carried out allowing for inflation of current-dollar
prices for natural gas, electricity, and fuel oil. To do this, discounting was done in terms
of an effective interest rate, given by”
fj = r -
ij
-
ijY,
where
fj
=
effective interest rate for energy source j,
r = homeowners’
ij = current-dollar
cost of capital,
price inflation rate for energy source j.
Separate calculations were performed holding the price of public water supply constant and allowing an inflation rate to affect the price of the energy source: natural gas,
electricity, and fuel oil. Assuming continuous compounding, the present equivalent savings? in any given year (t) can be writtenI
pj = We-”
+ Eje-f~‘,
where
Pj = present equivalent savings for homeowners
using energy source j to heat water,
W = annual savings in public water supply charges,
E j = first year savings in charges for energy source j.
Integrating over the expected life of the water conservation
savings into a total present equivalent savings,lg we find
device (T) to sum the annual
T
TP, =
Pjdt = (W/-r)(eKrT
- 1) + (E,/-fj)(e-fjz
- I),
where
TPj =
total present equivalent savings for homeowners
hot water.
using energy source j to heat
tThis formulation neglects energy used in operating the conservation device. The only device to conserve hot
water that this simplification affects is the air blower shower, which requires 49 kWh,/yr.
1190
WALLACEJ. HOPP and WILLIAMP. DARBY
Holding the annual savings in public water supply charges (W) constant, a price
inflation rate (i,) at which the total present equivalent savings (TP,) will just exceed the
total present equivalent cost of the conservation device (purchase price and installation
cost plus present equivalent of any operation and maintenance costs) can be determined
for a specific energy source, j. If that rate is less than that which can reasonably be
expected, the device is considered cost-effective to the homeowner, based on life-cycle
economics. Past experience places conservative estimates of annual inflation rates for
energy prices at lO-20% for electricity; 15-20x for natural gas; and IO-15% for fuel
oil. ”
Although five devices are universally (all energy sources, all metropolitan areas) costeffective under the constant price assumption, as shown in Table 6, inflation of energy
prices has a substantial effect on the cost-effectiveness of devices to conserve hot water.
At 10, 15, and 20% annual energy price inflation rates, these numbers increase, respectively, to 7, 8, and 10 devices universally cost-effective, as shown in Table 7. This result
illustrates dramatically the effect of indirect energy savings in determining the costeffectiveness of devices to conserve hot water. At 15 and 20% annual inflation rates for
energy prices (not inconceivable), devices to conserve hot water are universally costeffective at equivalent annual costs of $3.73 and $7.04/lOOOgal, respectively. Devices to
conserve ambient temperature water, on the other hand, were almost universally costineffective at equivalent annual costs greater than $l.OO/lOOOgal.
Considerable geographic variation is evident in the electricity price inflation rates
necessary for cost-effectiveness, as shown in Table 8. This variation is not evident in the
required price inflation rates for either natural gas or fuel oil. In Seattle, where prices for
electricity are very low and likely to remain so due to hydropower, the highest required
price inflation rates are observed. This result indicates that devices to conserve hot water
at equivalent annual costs in excess of $1.61/1000 gal are unlikely to be cost-effective for
Table 7. Cost-effectiveness of devices that conserve hot water
Percentage of cities where the devices are cost-effective with
inflation rates of 10, 15, and 20%
in the price of:
Electricity'
flevice
Fuel oil'
Yatural gas'
10
15
2n
10
15
20
10
15
20
qhower flow restrictor
100
100
100
190
100
100
100
100
100
Low-flow shower head
100
100
100
100
100
100
100
130
100
Faucet aerators (2)
100
100
100
100
100
100
100
100
100
Low-flow faucets (2)
100
190
100
100
100
100
100
100
100
Water-savinq dishwasher
(incremental)
100
100
100
100
100
100
100
100
100
pressure-reducing valve
100
100
100
100
100
100
lij0
100
100
Water-savinq clothes
washer (incremental)
100
100
100
100
100
100
100
100
100
Air-blower shower
96
100
100
52
100
100
100
100
100
Insulation of hot
water pioes
91
96
100
4
30
100
4
100
100
Thermostatic mixinq
valve
96
96
100
4
26
100
0
100
100
Water-saving clothes
washer (new)
43
87
96
0
4
4
0
0
13
4
9
78
0
0
4
0
0
0
Water-saving dishwasher
(new)
I
We assume constant water prices.
1191
Household water conservation
Table 8. Minimum annual inflation rates for electricity prices that
are necessary for cost-effectiveness of water-conserving devices.
Percent/year
_..___
--
7-
1
kl
i
zt
Eletropolitanarea
42
LVI
z
12.5
19.0
9.5
16.0
IAtlanta, GA
0.
1.5
3.0
I3altimore. MD
0.
0.
0.
I3oston, MA
0.
2.5
4.0
12.5
18.5
IChicago, IL
0.
4.5
6.0
14.5
20.0
IIincinnati, OH
0.
6.0
7.0
15.0
20.5
ICleveland, OH
0.
0.
0.
9.5
16.0
IDallas, TX
0.
3.5
4.5
13.5
19.5
IDenver,
0.
2.5
4.0
12.5
19.0
IDetroit, MI
0.
0.
1.0
10.0
16.5
IHonolulu, HA
0.
0.
0.5
10.0
16.5
IKansas
CO
0.
0.
1.0
10.0
17.0
Long Beach, CA
0.
3.0
4.5
13.0
19.0
Los Angeles, CA
0.
3.0
4.0
13.0
19.0
IMilwaukee, WI
0.
7.0
8.0
16.0
21.5
New York, NY
0.
0.
0.
2.0
10.0
INewark,
0.
0.
0.
6.5
14.0
City, MD
NJ
Oakland, CA
0.
7.0
8.0
16.5
22.0
Philadelphia, PA
0.
0.
0.
10.0
17.0
Pittsburgh, PA
0.
0.
0.
9.5
16.5
St. Louis, MD
0.
4.0
5.0
13.5
19.0
St. Paul, MN
0.
0.
0.5
10.0
16.5
San Francisco, CA
0.
7.0
8.0
16.5
22.0
12.0
17.5
18.0
26.5
31.0
Seattle, WA
Seattle residents who use electricity to heat water. On the other hand, eastern cities
(Atlanta, Baltimore, Boston, New York, Newark, Philadelphia, Pittsburgh) tend to have
lower required electricity price inflation rates because electricity is already expensive in
these locations. The average electricity price (June 1978) for these 7 metropolitan areas is
$O.O53/kWh, compared with $O.O38/kWh, for the remaining 16 metropolitan areas. For
these 7 areas, nearly every device to conserve hot water is cost effective when using
electricity to heat water, except for the most expensive: discarding usable equipment to
purchase a water-saving clothes washer or dishwasher for the sole purpose of conservation. However, in New York and Newark, even these extreme measures are justified
(equivalent annual costs up to $33.63/1000 gal) because of the high prices for electricity,
and the associated indirect energy savings.
SUMMARY
AND
CONCLUSIONS
This analysis shows the influence of the indirect energy savings which result from
devices intended to reduce residential hot water use. Using June 1978 prices for energy
and public water supply, devices to conserve hot water are almost universally (all fuels,
all 23 metropolitan areas) cost-effective up to an equivalent annual cost of $1.50/1000 gal.
Allowing for increasing current-dollar prices of energy, devices to conserve hot water are
universally cost-effective for equivalent annual costs of $1.61, $3.73, and $7.04/1000 gal
for annual inflation rates of 10, 15, and 20x, respectively. Conclusions for individual
metropolitan areas reflect even more extreme results. For example, for New York City
residents who heat water with electricity, an inflation rate of lO%/yr will justify use of a
1192
WALLACEJ. HOPP and WILLIAMP. DARBY
hot-water conservation device with an equivalent annual cost of $33.63/1000 gal. Devices
to conserve ambient temperature water are almost universally cost-ineffective at equivalent annual costs in excess of $l.O0/1OOOgal. This contrast illustrates that the indirect
energy savings due to conserving hot water is the determining factor in evaluating the
cost-effectiveness of residential water conservation devices.
Considering only the savings in charges for public water supply, it is cost-effective for
the typical homeowner to conserve ambient temperature water with a toilet tank dam as
a retrofit device and with the purchase of a dual flush toilet for use in new construction
or at ordinary replacement times. Based upon savings in public water supply charges
alone, there is justification for the typical homeowner to conserve hot water with a
low-flow shower head and flow-limiting faucets. Collectively, these devices would save at
most about 114 gal/day for a family of four.
Considering the energy savings and assuming a 15%/yr increase in energy prices, the
typical homeowner would additionally install an air-blower shower, and choose watersaving dishwashers and clothes washers at normal replacement times. Collectively, these
devices (along with the above devices to conserve ambient temperature water and flowlimiting faucets) could save as much as 148 gal/day for a family of four, nearly a 30%
increase. Furthermore, besides a significant economic savings, net energy savings of
4.1 mWh,/yr for a family of four using electricity to heat water, and 19.6 x lo6 Btuthermal/yr for a family of four using natural gas or fuel oil to heat water, result. These
savings amount to approx. 46-62% reduction in energy use for the residential water
heating needs for a family of four.20
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AWWA No. 20112, 2 Park Avenue, New York, NY 10016.
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Labor Statistics, Washington, DC 20212 (June 1978).
18. Analysis ofNo. 2 Heating Oil Pricesfor rhe 1977-78 Heating Season, U.S. Department of Energy, Economic
Regulatory Administration Office of Fuels Regulation, Washington, DC 2046 1.
19. J. K. Gohagan, Quantitative Analysisfor Public Policy. McGraw-Hill, New York (1979).
20. R. P. Wilson, Jr., Energy 3(2), 149 (1978).