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Lou
 
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Default Power cost of idle electric water heater


"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)


Um, not quite. The temperature differential your heater is maintaining is
53 degrees. The temperature rise for the water is the difference between
temperature of the cold water coming into the tank and the hot water coming
out. The air temperature in the vicinity of the tank doesn't tell you what
the inlet temperature is - it could be warmer or cooler - unless there's a
long run of pipe before it enters the tank, or possibly your water supply is
from a well and there's a pressure tank in the same vicinity - either would
allow the water to pre-warm (or cool) to the ambient temperature.

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.


And all these years, I thought the energy guide labels were from the
Department of Energy, not the EPA. Either way, your measurements pretty
much agree with the DOE's estimate that standby losses generally run from
10% to 20% of the total water heating bill.

In your case, it appears that you didn't draw hot water from the tank during
your test period. The standby loss figure, as well as the annual bill
estimate, assumes "normal" use of hot water.

Interesting post - it's nice to see that lab-derived government figures
agree with real world installations.