# Turning off electric water heater when not in use?



## skipjack

I have a 40 gal electric water heater.... 

If I go away for say, three days.. would it be more efficient for me to turn off the water heater during that time or to leave it on?

What about four days?

I guess what I want to know is.. what's the cut-off point where it becomes more efficient to turn it off rather than leaving it on to maintain a tank full of hot water when not in use?


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## Grampa Bud

For two days leave it on. For longer periods turn it off unless you have money to burn, in which case please send some my way I surely could use a little extra. An electric water heater should be able to reheat the tank from cold to preset temperature in about 1 to 1-1/2 hours.


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## Sandra20

even for more than one day turn it off its just a waste of money if you leave it on


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## Jackofall1

Agreed, more than a day, turn it off. 

In fact its only a convience to have hot water all the time. If you leave the house in the morning, and return in the afternoon, it would save money to shut it down. 

The down fall is waiting for it to reheat the water, the net loss is your time.

Heres the math if you want to figure it out.

1 KWh = 3,412 BTU's
1 BTU = the amount of energy to raise 1 LB of water 1 Deg F
1 gal = 8.34 Lbs

You can play with how much heat loss from the tank, say tank losses 2 deg F per hour

Set point for your tank 125 Deg F?


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## Jackofall1

Allright, you got my curiousity going, so here is what I come up with

Say

Heat off = 125
Heat on = 110
Tank heat loss @ 2 deg/hr
Tank size = 50 gallons
Abient temperature (basement Temp) = 61 degF

Btu's per KWh = 3412

By turning the tank off it would take 33 hrs to reach an ambient temperature of 61 degF assuming heat loss rate of 2 degF/hr.

It would take 3050 btu's or .89 KWh to heat the tank from 61 degF to 125 degF.

Leaving the tank on, the heater would cycle 3 times a day assuming the above heat on/heat off scenario and a 2 degF loss/hr you would use 2100 btu's or .62 KWh to maintain the tank at 125 DegF for 24 hrs.

With this example there is no difference after 8 hrs but after that its all savings. 

So if you are gone for more than 8 hrs there is savings to be had, you would have to weigh this against the wait time to take that shower after a long day.


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## Mrdippy

This is like the 'it costs less to leave my furnace on all day than have it do all the work to climb 15 degrees once I get home if I turn it off/down during the day. The math looks right to show there is a tipping point of savings if more than so much time but I think a physics person would debunk us as wrong.( I think this has been debunked for home heating) The amount of energy to keep a liquid above a temperature over a time will always be more than not using any energy till you reach the point of actually needing hot water at that temp. You don't get more degrees for the btu when heating a liquid that's already at 120 degrees vs heating ice cold water... I should have paid more attention in science class because I don't know what is amiss to explain it. Generally speaking turning it off I believe will always save money/energy - but I'm no expert.


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## Jackofall1

Mrdippy said:


> Generally speaking turning it off I believe will always save money/energy - but I'm no expert.


There is one truth to the statements made, and won't take a physicists to figure it out either.


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## pyper

Mrdippy said:


> This is like the 'it costs less to leave my furnace on all day than have it do all the work to climb 15 degrees once I get home if I turn it off/down during the day.


But that _can _be true if it's a heat pump. With mine, any time there's more than a 1 degree differential between the room temperature and the set point, the resistance heat strips kick in, and they use more than twice as much electric as the compressor. Depends on how good your insulation is.

I tell my wife to move the thermostat up in 1F increments. Yeah.


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## ehoez

Jackofall1 said:


> Allright, you got my curiousity going, so here is what I come up with
> 
> Say
> 
> Heat off = 125
> Heat on = 110
> Tank heat loss @ 2 deg/hr
> Tank size = 50 gallons
> Abient temperature (basement Temp) = 61 degF
> 
> Btu's per KWh = 3412
> 
> By turning the tank off it would take 33 hrs to reach an ambient temperature of 61 degF assuming heat loss rate of 2 degF/hr.
> 
> It would take 3050 btu's or .89 KWh to heat the tank from 61 degF to 125 degF.
> 
> Leaving the tank on, the heater would cycle 3 times a day assuming the above heat on/heat off scenario and a 2 degF loss/hr you would use 2100 btu's or .62 KWh to maintain the tank at 125 DegF for 24 hrs.
> 
> With this example there is no difference after 8 hrs but after that its all savings.
> 
> So if you are gone for more than 8 hrs there is savings to be had, you would have to weigh this against the wait time to take that shower after a long day.



:thumbsup:

i like your math, but i think its more than .62 KWh per 24hrs

based on the EPA sticker of $500 a year,(on my 2009 hot water heater) thats about $1.36 you save per day ($500 divided by 365 days), not using the water heater. 
(epa sticker assums 10cents per KWh, so thats about 13KWh saved per day)

so my answer, is if you leave the house more than 24hrs, turn it off.


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## Jackofall1

ehoez said:


> :thumbsup:
> 
> i like your math, but i think its more than .62 KWh per 24hrs
> 
> based on the EPA sticker of $500 a year,(on my 2009 hot water heater) thats about $1.36 you save per day ($500 divided by 365 days), not using the water heater.
> (epa sticker assums 10cents per KWh, so thats about 13KWh saved per day)
> 
> so my answer, is if you leave the house more than 24hrs, turn it off.


Read the 3rd last paragraph:yes: of my original post, and the savings start after 8 hrs, not 24, but that would be a pain waiting for that hot shower every day to save a couple of cents.

Mark


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## YerDugliness

All this is great reason to replace that 40 gallon electric water heater with an electric HWOD unit.

I installed one in my home in SW Kansas, where it gets very cold. At times I leave the house for months without heat. No matter how cold it is, when I walk in the door I can turn on the shower and have hot water within a few seconds, and not have enriched the PoCo even one red cent to keep a tank of water hot for all those days, weeks, even months :thumbsup: .

The only issue is that an HWOD capable of supporting multiple use zones requires WAAAAY more amps than used by your current tank-type heater. In my case, that's OK, b/c the house sits vacant for up to 6 months and then gets used for a week to a month, then sits for months on end without use again.

Sure feels good, though, to walk in after a 16 hour drive and be able to have a hot shower within seconds :yes: !

If your electric tank-type water heater is over 5 years old, you're probably due for a replacement soon, if not already. Give the HWOD unit some thought.....:thumbup:

Cheers from Dugly


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