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them), and the standard temperature conditions for the DOE Energy Factor test. In this example, the
break-even point for the net effect of cooling and standby loss is a daily use of about 8 gallons at a 77°F
rise. At the Energy Factor standard quantity of 64.3 gallons, this example water heater would have a net
cooling effect of 1.76 ton-hours per day, or about the same as running a 1-ton window air conditioner for
1¾ hours. This relationship does not include the heating effect from direct contact of the produced hot
water with the air in the space, such as from a shower or faucet, since this would normally not be in the
same area as the water heater.
Economics
The test results are conclusive that the heat pump water heaters will have an economic advantage over
conventional electric resistance water heaters, by providing the same amount of hot water for as little as
half the electric energy input. The size of the difference will depend on how the system is operated and if
the use of the backup electric resistance elements is minimized. Heat pump water heaters also offer a
significant demand reduction of as much as 3½ kW. However, the cost effectiveness in comparison with
gas water heaters is less clear; and for most PG&E customers, gas water heating is still the most
economical option.
The analysis of cost effectiveness is complicated by PG&E’s tiered residential electric rate, where the
more energy is used, the higher the cost. The tiers are separated by a factor of a baseline quantity, which
varies according to which of ten regions the customer is in, the season (summer or winter), and whether
the customer has only electric service. Figure 22 shows an example of the current E-1 rate structure and
how it varies with the baseline quantity. The customer’s average electric rate may be found as the slope
of a line drawn from the origin until it intersects the appropriate trend.
Comparing the cost of operation for different water heater systems may be done based on the knowledge
that the Energy Factor rating represents a total hot water use of 150 therms or 15 million Btu per year
(64.3 gal/day × 77°F rise × 8.3 lb/gal × 1 Btu/lb-°F × 365 days/year). Using the conversion factor of
3,412 Btu/kWh, this can also be figured as 4,396 kWh per year. The cost to operate either a gas or
electric water heater under the standard conditions is then:
Operating Cost: $/Year = [ 150 therns/year / Energy Factor × $/therm ]gas
= [4,396 kWh/year / Energy Factor × $/kWh ]electric
Assuming a customer is under the baseline amount and being billed at the current minimum of
$0.11877/kWh (as of 3/1/2010), a heat pump water heater with an Energy Factor of 2.0 would have an
equivalent operating cost as a gas water heater with the Title-24 minimum Energy Factor of 0.59 if the
average gas cost is $1.03 per therm. The more electricity the customer uses, the less economically
attractive it becomes, particularly if switching from gas to electricity pushes the user into a higher rate
tier.
The DEER database lists an average annual energy consumption for a 50-gallon standard electric water
heater in a single family residence of 3,579 kWh, based on an average Energy Factor of 0.86. If for
example, the Rheem unit operated always in heat pump mode and achieved an average Energy Factor
close to the measured average 1.98, it would consume about 1,555 kWh/year; representing a reduction of
2,024 kWh/year or 57%.
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