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)


Index: power, energy, cost, water heater, waterheater, KWH, energy use, cost of
hot water, hot water cost, efficiency, power rate, electric water heater,

Phil Sherrod wrote:

Temperature of hot water delivered: 114 degrees F.

Your numbers are nice and thanks for posting the info,
however it seems to me 114 is a little on the low end,
with 120-125 being more common and I run my domestic
hot water at 140F.

To complete your experiment you really ought to put an
insulating blanket over the unit to see how your numbers
improve. Ideally you should be able to cut "idle"
consumption by half without too much effort by
reducing thermal losses.

On 29-Mar-2004, Bill Vajk wrote:

Your numbers are nice and thanks for posting the info,
however it seems to me 114 is a little on the low end,
with 120-125 being more common and I run my domestic
hot water at 140F.

I agree. If this was my primary water heater, I would probably bump the
temperature up to 125. I may change it later.

To complete your experiment you really ought to put an
insulating blanket over the unit to see how your numbers
improve. Ideally you should be able to cut "idle"
consumption by half without too much effort by
reducing thermal losses.

That would be a good experiment. However, that has the potential of saving
$2/month if I leave the water heater running. With the expected usage, I would
be lucky to save $5/year.

"Phil Sherrod" wrote in message
news

On 29-Mar-2004, Bill Vajk wrote:

Your numbers are nice and thanks for posting the info,
however it seems to me 114 is a little on the low end,
with 120-125 being more common and I run my domestic
hot water at 140F.

I ASSume you are providing hot water for a complete bathroom and that your
guest would be taking a bath or shower.

I would suggest that you just place a switch at the water heater. When you
open the room, you turn on the power to the heater. With 4 kW coming in,
you can get water for hand washing within a few minutes.

You don't really need a two pole switch but you can get heavy duty 2 pole 20
amp switches for less than $20 and it might come in handy if you service the
heater (To set the temperature on most water heats you have to uncover BOTH
thermostats. It's a good idea to have the power completely off when you do
this.

If you only power the heater when you have a guest you just don't have to
worry about standby losses and you reduce that chance of something BAD
happening when no one would be around to turn off the water.


I agree. If this was my primary water heater, I would probably bump the
temperature up to 125. I may change it later.

Well, if you only used when you have a guest, the higher temperature setting
would let your guest take a longer shower without running out of hot water.

On 29-Mar-2004, Bill Vajk wrote:

Your numbers are nice and thanks for posting the info,
however it seems to me 114 is a little on the low end,
with 120-125 being more common and I run my domestic
hot water at 140F.

Since the energy loss is directly proportional to the temperature differential
of the stored water and the air temperature around the tank, it's easy to
calculate how much more it would cost to maintain the water at 140F.

Assuming a room temperature of 61 degrees F around the water heater:

Cost to maintain water at 114F with 8 cents/KWH cost = $4.00/month (53 degree
differential)

Cost to maintain water at 140F with 8 cents/KWH cost = $5.96/month (79 degree
differential)

Phil Sherrod wrote:
On 29-Mar-2004, Bill Vajk wrote:

Your numbers are nice and thanks for posting the info,
however it seems to me 114 is a little on the low end,
with 120-125 being more common and I run my domestic
hot water at 140F.

Since the energy loss is directly proportional to the temperature differential
of the stored water and the air temperature around the tank, it's easy to
calculate how much more it would cost to maintain the water at 140F.

Assuming a room temperature of 61 degrees F around the water heater:

Cost to maintain water at 114F with 8 cents/KWH cost = $4.00/month (53 degree
differential)

Cost to maintain water at 140F with 8 cents/KWH cost = $5.96/month (79 degree
differential)

Ideally, yes. I do wonder, however, what the startup cost of
the electric heating element(s) is. A higher cycling rate
isn't going to present a linear extension based on delta T
alone since the current consumed during the period the
heating element takes while coming up to operating temperature
is higher than at near steady state operation.

The water temperature, as it rises, results in successively
higher operating temperature of the heating element with a
corresponding (albeit small) decrease in the current drawn.
I don't know what the typical slop in the thermostat
temperature is for an electric hot water heater.

Yes, I realize I'm nitpicking, but that's part of the fun
in discussions like this one. :-)

Your numbers are plenty close enough for the average guy.

On 29-Mar-2004, Bill Vajk wrote:

Ideally, yes. I do wonder, however, what the startup cost of
the electric heating element(s) is. A higher cycling rate
isn't going to present a linear extension based on delta T
alone since the current consumed during the period the
heating element takes while coming up to operating temperature
is higher than at near steady state operation.

That doesn't matter at all to the total energy usage.

If the average water temperature is stable over a long period (small
fluctuations don't matter), then the total energy going into the tank during
this time must exactly equal the total heat energy lost from the tank. This is
a consequence of the law of the conservation of energy. If we put more energy
in, the water temperature will rise over time; if we put in less, the
temperature will fall. If it is stable of an extended period, then the total
energy put in during that period must match the energy taken out. (If you find
a tank that violates this law, explain why and immediately apply for a Nobel
prize.)

So regardless of the voltage, amps, wattage, size or shape of the heating
element, as long as the heater is able to supply enough energy to match the
loss (76 watts in my case), the total energy used over a long period will be
the same; but the duty cycle will change. You could put a 76 watt heater
inside the tank, and it would use the same long-term energy as a 4500 watt
heater. It would just have a longer duty cycle -- 100% rather than 1.6%.

The water temperature, as it rises, results in successively
higher operating temperature of the heating element with a
corresponding (albeit small) decrease in the current drawn.
I don't know what the typical slop in the thermostat
temperature is for an electric hot water heater.

Irrelevant, it just changes the duty cycle -- the long term energy use is
exactly the same.

Conservation of energy -- Not just a good idea, it's the law.

In misc.industry.utilities.electric Phil Sherrod wrote:

| So regardless of the voltage, amps, wattage, size or shape of the heating
| element, as long as the heater is able to supply enough energy to match the
| loss (76 watts in my case), the total energy used over a long period will be
| the same; but the duty cycle will change. You could put a 76 watt heater
| inside the tank, and it would use the same long-term energy as a 4500 watt
| heater. It would just have a longer duty cycle -- 100% rather than 1.6%.

But you're still losing 76 watts of energy. The question is, is there a
way to recover that cheaply. In cold weather, if you could recover 100%
thay would be 76 watts less (or equivalent) energy used for other purposes.

Also, how much would these figures change if you put the water heater on a
timer to ensure that it only heated during night?

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

"Phil Sherrod" concluded with

Irrelevant, it just changes the duty cycle -- the long term energy use is
exactly the same.

Conservation of energy -- Not just a good idea, it's the law.

I wonder if there is any problem with the life span of the water
heater, being cycled from room temperature to operating temperature on
a frequent basis, vs. leaving it at a fixed temperature. I know when
a gas water heater is first installed, the first heating cycle causes
enough sweating that there is sometimes dripping water into the burner
area, causing many do-it-yourselfers to think they bought a leaker.
Do the electric water heaters sweat when first turned on?

Lena
lenagainsterathotmaildotcom

"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.

On 30-Mar-2004, "Lou" wrote:

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

My analysis was to measure the energy loss for an IDLE water heater. No water
was drawn from the heater during the test, so the incoming water temperature is
irrelevant -- there was no incoming water. The 53 degree figure is the
difference in the temperature between the water in the tank and the ambient air
temperature. That figure is significant because the heat loss is directly
proportional to the temperature differential.

And all these years, I thought the energy guide labels were from the
Department of Energy, not the EPA

You may be right about the department. I'll have to go under the house to
check the label again.

"Phil Sherrod" wrote in message
...

On 30-Mar-2004, "Lou" wrote:

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

My analysis was to measure the energy loss for an IDLE water heater.

Right. So you're not measuring the cost of the water temperature rise.

Someone commented on the high cost of the instant hot water heaters...
Well about five years ago I made one out of parts that can be pretty
easily bought for around fifty dollars and it is still going strong...
Essentially it is made out of 1 inch galvanized pipe and T-fittings..
The T fittings allow you to screw in the screw-in type heating
elements into the heater... On mine I put in a flow switch (that I
got from a surplus place but that could be bought from a plumbing
supplier) I wired that to a 240 volt contactor large enough to handle
the flow of electricity... (I got mine out of a junked large air
compressor)... and I screwed in two heating elements in series into
the one inch pipe... All sort of hard to explain... but you get the
picture I hope.. It doesn't take up much room and remarkably it is
still working well years later.. I had commercial bought ones before
and got tired of replacing expensive little parts in them (like their
heating elements that would burn up if some air got in the line) or
their electronics which seemed to be damaged by power surges... etc..
This thing I have now just won't quit and when it does I can just get
the parts easily and cheaply... I don't have the luxury of a really
steaming shower (especially in the winter when the incoming water is
colder) but my power bills for a 3 person household are under
$25/month so I can live with it..

david fraleigh wrote:

Someone commented on the high cost of the instant hot water heaters...
Well about five years ago I made one out of parts that can be pretty
easily bought for around fifty dollars and it is still going strong...
Essentially it is made out of 1 inch galvanized pipe and T-fittings..
The T fittings allow you to screw in the screw-in type heating
elements into the heater...

Sounds interesting. I'm building something like this into a removable
clamp-ring lid of a 55 gallon drum, along with 300' of 1" plastic pipe,
to make a greywater heat exchanger with a little post-heating from
about 100 to 110 F from a $7 1500 W heating element.

On mine I put in a flow switch (that I got from a surplus place but
that could be bought from a plumbing supplier) I wired that to
a 240 volt contactor large enough to handle the flow of electricity...

You might use a $10 motion detector instead of the flow switch, and
add a $12 "single element water heater thermostat" (which is actually
2 in series, for safety.)

and I screwed in two heating elements in series into the one inch pipe...

Why two?

Nick

I'm using a new $35 55 gallon lined steel drum with a strong removable lid
(because the drum might end up under 2' of greywater head, with the inlet
and outlet above the lid) and bolt ring and a 3/4" bung and a 2" bung with
a 3/4" threaded knockout, with 100 psi/73.4 F pipe from PT Industries at
(800) 44 ENDOT. Their PBJ10041010001 1"x300'100psi NSF-certified pipe is
actually tested to 500 psi. The price is $59.99 from any True Value hardware
store. Lowes sells the rest of the hardware needed, all of which is
installed through the lid, so the drum itself has no holes:

sales total
# qty price description

25775 1 $5.73 24' of 1.25" sump pump hose (for greywater I/O)
105473 1 1.28 2 SS 1.75" hose clamps (for greywater hose)
54129 2 3.24 1.25" female adapter (greywater barb inlet and outlet)
23859 2 2.36 1.25x1.5" reducing male adapter (bulkhead fittings)
75912 1 0.51 2 1.25" conduit locknuts (bulkhead fittings)
28299 1 1.53 2 1.25" reducing conduit washers (")
22716 1 1.36 1.5" PVC street elbow (horizontal greywater inlet)
23830 1 2.98 10' 1.5" PVC pipe (for 3' greywater outlet dip tube)

The parts above are greywater plumbing ($18.99.)

23766 4 1.28 3/4" CPVC male adapter (for 1" pipe barbs)
23766 2 0.64 3/4" CPVC male adapter (fresh water I/O)
42000 2 3.84 3/4" FIP to 3/4" male hose adapter
23813 1 1.39 10' 3/4" CPVC pipe (for 1"x3/4" fresh water outlet)
23760 2 0.96 3/4" CPVC T (fresh water I/O)
22643 2 0.86 3/4" CPVC street elbow (fresh water I/O)
4 - 1" 3/4" CPVC pipes (fresh water I/0)
1 - 3' 3/4" CPVC pipe (fresh water inlet)
22667 2 2.56 2 SS 1.125" hose clamps (fresh water I/O)
219980 1 4.87 10.1 oz DAP silicone ultra caulk (bulkhead fittings)
150887 1 3.94 4 oz primer and 4 oz PVC cement

Parts above are fresh water plumbing. Subtotal $39.33.

26371 1 6.83 1500 W electric water heater element
22230 1 2.31 1" galvanized T ("nut" for heating element)
61294 1 11.76 single element thermostat with safety
136343 1 0.56 5 10-24x3/4" machine screws (mount thermostat with 3)
33368 1 0.37 5 #10 SS flat washers (mount thermostat with 3)
198806 1 1.38 10 #0 rubber faucet washers (mount thermostat with 3)
8763 1 0.67 5 10-24 SS nuts (mount thermostat with 3)

The above would make a standalone water heater, if needed. Grand total: $63.21.

For 4 10 min showers per day and 20 minutes of dishwashing at 1.25 gpm we
might heat 75 gallons of 55 F water to 110 with 8x75(110-55) = 33K Btu with
about 10 kWh worth about $1/day at 10 cents/kWh. If the "heat capacity flow
rate" Cmin = Cmax = 75gx8/24h = 25 Btu/h-F and the pipe coil has A = 300Pi/12
= 78.5 ft^2 of surface with U = 10 Btu/h-F-ft^2 (for an HDPE pipe wall with
slow-moving warm dirty water outside and 8x300Pi(1/2/12)^2 = 13 gallons of
fresh water inside), the "Number of heat Transfer Units" for this counterflow
heat exchanger NTU = AU/Cmin = 78.5ft^2x10Btu/h-F-ft^2/25Btu/h-F = 31.4, so
the "efficiency" E = NTU/(NTU+1) = 97% for hot water usage in bursts of less
than 13 gallons. This works best with equal greywater and cold water flows,
with either a 110 F water heater setting (preferable), or the heat exchanger
output feeding the cold water shower inlet as well as the water heater.

The Hazen-Williams equation says L' of d" smooth pipe with G gpm flow has a
0.0004227LG^1.852d^-4.871 psi loss. At 1.25 gpm, the pressure drop for 2 150'
coils of 1" pipe might be 0.0004227x150x(1.25/2)^1.852x1^-4.871 = 0.03 psi.

If greywater leaves a shower drain and enters the heat exchanger at 100 F and
fresh water enters at 50 F, the fresh water should leave at 50+0.97(100-50)
= 98.5 F. Warming it further to 110 F would take 8x75(110-98.5) = 6.9K Btu/day
or 2 kWh worth about 20 cents/at 10 cents/kWh, for a yearly savings of about
($1-0.20)365 = $292, or more, with a tighter shower enclosure and higher drain
temperature. The 1500 W heater might operate 2kWh/1.5kW = 1.3 hours per day.
We might wrap the drum with 3.5" of fiberglass and a 4'x8' piece of foil-
foamboard with 7 4' kerfs (knife cuts partially through the board) to make
an octagon and aluminum foil tape to cover the kerfs and hold it closed.