I recently installed an electric water heater to service a guest bedroom
located far from the central water heater. Since water will be drawn from this
heater only when guests are visiting, I plan to leave it turned off to save
power.

But before shutting it down, I decided to take some measurements and calculate
how much it costs to run an idle water heater.

The water heater is an electric GE Smar****er 40 gallon, “lowboy” (squat) unit.
The plate on the unit says it draws 4500 watts, but my measurements show that
it actually draws about 4320 watts (18 amps at 240 volts). The EPA estimated
annual cost of operation is $401.

I used a Supco model DLAC recording clamp-on ammeter to record power (amperage)
over a 3 day interval. During the same period, I used a Supco model DLT
recording thermometer to record the ambient air temperature in the crawl space
where the water heater is located.

Here is a summary of my measurements:
Monitored interval: 3 days
Power draw when heating element is on: 4320 watts (18 amps at 240 volts)
Duty cycle when heater is running: 0.0161 (1.61%)
Average power used (heating watts times duty cycle): 69.55 watts
Temperature of hot water delivered: 114 degrees F.
Average temperature in crawl space during measurement period: 61 degrees F.
Temperature rise for water: 53 degrees F (114 - 61)

When the heater is on, it draws 4320 watts. However, the duty cycle
(proportion of time heating) is only 0.0161 (1.61%), so the average power drawn
is 4320*0.0161=69.55 watts. (On average, the heating element is on 23
minutes/day.)

An average power usage of 69.55 watts over 24 hours works out to 1.669 KWH
(kilo-watt hours) per day.

The EPA average national power rate is 8 cents per KWH. So, using the EPA
power rate, the cost of keeping the idle water heater hot is 13.35 cents/day or
$4.00/month or $48.73/year.

Here in Tennessee, we enjoy relatively cheap TVA power which costs 5.6
cents/KWH. Using that rate, the energy cost is 9.35 cents/day, $2.80/month or
$34.13/year.

The EPA estimated annual cost of operation is $401 (assuming 8 cents/KWH). So
the idle heat-loss cost of $48.73/year is about 12% of the total cost.

If you adapt these figures for another location, remember that the cost is
directly proportional to the temperature difference between the hot water and
the surrounding room temperature, and you must adjust for your KWH power cost.

Phil Sherrod
(phil.sherrod 'at' sandh.com)


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