"Phil Sherrod" wrote in message
...
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.

That is handy to know. Did you calculate whether more energy will be used
to re-heat from cold when only having the water on when you need it? A
quick recovery water heater may be very efficient as the larger power input
is used for a much shorter time. When you need the hot water it will be
ready quite quickly and no hanging around. I have seen electic water heater
cylinders on push timer switches set for say 2 hours. Then you can't leave
them on 24/7. If you need hot water, push the switch; 15 minutes maximum
later, you have a full cylinder of hot water.

Something like this requires a set like by like test. $48 a year extra for
each household per year is a lot of collective energy. Energy that is
running out and pollutes when used.