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Conservation | Appliances | Frequently Asked Questions
Seattle City Light, Appliances, Frequently Asked Questions
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Q.1 How do I know if the water heater I want to buy is energy efficient?

A.1 Look for the Energy Factor (EF) number on the yellow Energy Guide label. It should be .91 or greater. The EF represents the percentage of the energy you put into a water heater that you get back in the form of useable hot water. For example, a water heater with a .94 EF gives you about 94% of the electric energy it uses back in hot water. Water heaters are now available with an EF of .95 or higher. The higher the Energy Factor, the lower the operating costs.

Q.2 Do "tankless" water heaters save a lot of energy?

A.2 Tankless hot water heaters, which are also sometimes called demand or on-demand water heaters, heat water instantly as a household needs it when they turn on a tap. As a consequence this type of water heater doesn't need a storage tank. This can save energy because it doesn't have to keep 50 gallons of water hot all the time. Unfortunately, this energy saving comes at the cost of a much greater power demand meaning that new wiring from the service panel is almost always needed. Both electric storage tank and on-demand heaters turn 100% of the electricity used into heat, the savings from a demand system come entirely from having lower standby loss.

  The total standby loss (the energy lost from the storage tank to the environment) of a modern energy–efficient electric water heater is generally less than 300 kWh or about $27 per year. This is different from the savings achievable when converting from a gas storage tank to a gas demand water heater. Stand-by losses on typical gas water tanks are higher since the internal stack is un-insulated and sends heat up the flue even when the flame is off. The heating efficiencies on gas fired tanks are in the neighborhood of 60–65% while gas demand heaters typically have heating efficiencies in the neighborhood of 80%. This means converting from a gas storage heater to a gas on–demand heater pays back in both heating efficiency and lower standby loss.

Q.3 Why doesn’t City Light offer rebates on new water heaters?

A.3 Current Federal standards assure that all electric water heaters are well insulated and about as efficient as is practical for a device based on electric resistance heating. New technologies, such as heat pump water heaters are on the horizon and if they prove to be cost effective and efficient in our climate, Seattle City Light will likely develop a program around them.

Q.4 Recently I heard about a timer for an electric water heater that turns it off during parts of the day and only turns it on when one expects to use hot water like in the morning when I’m showering and getting ready to leave for work. Do these timers save much energy?

A.4 Adding a timer to your water heater might save some energy, but it won’t generally save very much energy. We figure the economic bottom line is as follows: you can expect to save about $6 per year for a device that will cost you about $35 to buy and about $65 to have installed by a professional.

Q.5 How much does it cost to take a shower compared to a bath?
(assuming a 2.0 gallons-per-minute showerhead)

  Shower with 2.0 gpm showerhead Bath
time (minutes) 5 10 15 20 N/A
water use (gallons) 10 20 30 40 60
heat used (BTU's) 4,400 8,800 13,200 17,600 26,400
heat used (kWh's) 1.29 2.58 3.86 5.15 7.73
electricity cost ($0.0914/kWh) $0.12 $0.24 $0.36 $0.48 $0.71
water cost
$0.05 $0.10 $0.15 $0.20 $0.30
wastewater cost ($12.58/ccf) $0.17 $0.34 $0.50 $0.67 $1.01
total cost $0.34 $0.68 $1.01 $1.35 $2.02

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or call (206) 684-3800

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