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Old 04-10-2012, 02:48 PM   #1
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Calculating the cost of hot water


The Energy Guide label on my water heater states that the annual operating cost of my heater is $411.00.
Is this only the cost for keeping 40 gallons hot for a year or does it also include the cost to heat water used for showers, dishwasher, etc? When Iím figuring the cost for hot water do I need to figure the cost to heat how many gallons we use and add it to the $411.00 operating cost?
Thank you for your help,
Grit

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Old 04-10-2012, 02:53 PM   #2
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Calculating the cost of hot water


The $411 is a guestimate, kind of like the mileage estimate on a new car. The more hot water you use, the more it is going to cost per year. So a family of six, with lots of small children, is going to use more hot water (and cost more) than an empty nest family. Similarly, if you have an electric hot water heater, the cost is directly related to the price per kilowatt for electricity. If you have a gas hot water heater, the cost is directly related to the cost per cubic foot of gas.

If you really want to estimate the cost to produce hot water for a year, you need to estimate how many gallons of hot water you are going to use, measure the inflow temperature of the water, find out the efficiency of the heater, and estimate the heat loss from hot water sitting in the tank. This is not so simple, hence the tag on the water heater, which is best regarded as a way to compare two water heaters (the lower the cost on the tag, the more efficient and better insulated the heater). But do not assume that YOUR costs will equal the costs shown on the tag.

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Old 04-10-2012, 03:20 PM   #3
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Calculating the cost of hot water


Thanks for the quick response.

I understand the $411.00 is an estimate, but is it an estimate of only how much it will cost to maintain a hot tank. Or does that estimate also include the cost of heating water for use in the home?

If it is only the cost of constantly reheating a tank of water so that it is always available then I think I need to add to that, the cost to actually heat the gallons of water we use each month.
Should I add the two together to get a more accurate estimate?

Thanks for your help,
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Old 04-10-2012, 04:17 PM   #4
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Calculating the cost of hot water


Go to:
http://www.mge.com/images/PDF/Brochu...nergyCosts.pdf
Data there are for: Based on a four-person household:
Average electric rate of $.14/kWh
Average gas rate of $1.00/therm
Average hot water use 64.3 gallons/day

They explain how to calculate for different appliances and energy costs if you are so inclined.

Last edited by bob22; 04-10-2012 at 04:43 PM.
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Old 04-10-2012, 04:29 PM   #5
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Calculating the cost of hot water


Quote:
Originally Posted by Grit View Post
Thanks for the quick response.

I understand the $411.00 is an estimate, but is it an estimate of only how much it will cost to maintain a hot tank. Or does that estimate also include the cost of heating water for use in the home?

If it is only the cost of constantly reheating a tank of water so that it is always available then I think I need to add to that, the cost to actually heat the gallons of water we use each month.
Should I add the two together to get a more accurate estimate?

Thanks for your help,
The figure is an estimate based on a specific cost per kilowatt hour of electricity for both maintaining the water in the heater hot (heater losses) and for use of hot water. If you want an analogy to the label on cars you would look at the estimated cost to drive for a year (not the miles per gallon)--which as I recall is based on 15,000 miles.

For water heaters the figure is used to compare with other water heaters.

Electric water heaters are, for practical purposes, 100% efficient in converting electric energy to heat, but they vary in their heat loss because of differing insulation as well as other factors.

Gas water heaters vary considerably in the efficiency in converting the energy in gas to heat in the water. Highly efficient units are 90+ efficient while less efficient units may only convert 80% of the energy in gas to useful heat in the water. Gas water heaters also vary in the amount of heat lost because of differences in insulation.

I think the figure you really want to know is how many gallons of annual usage is included in the figure--and I don't know that figure.

But you should be aware there are other factors--for example, all other things equal a water heater will use more energy the hotter you set the thermostat on the water heater.
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Old 04-10-2012, 06:00 PM   #6
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Calculating the cost of hot water


Bob999,
You are exactly right.
I'm looking for how many gallons are included in the estimate.
Thank you for explaining it better than I did.
Maybe someone knows the number.
Thank you
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Old 04-10-2012, 08:17 PM   #7
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Calculating the cost of hot water


Ok This is another way to look at it, how do I figure the cost of a heater maintaining a tank at 123 degrees? No water used, just maintained. Then I can add the cost to heat the water used.
Does that make sense?
Thanks

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