Power cost of idle electric water heater

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 Smartwater 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,

(snip)

105 is not hot enough to discourage bacteria that thrive in hot water. My sister complained about the hydrogen sulfide smell from her hot water. I suggested she turn the temperature up to exceed the tolerance of heat loving bacteria. At about 120 to 130 degrees, the smell disappeared.

Perhaps, but this particular batch quite partying, anyway.

The minimum water temperature to keep harmfull bacteria out is 65degC. You're running it on a far too low temperature. And yes, a decent isolation layer helps a lot to preserve power.

The energy used is equal to the energy lost.

How much heat is it losing? That's the same ammount of heat you have to pay for by way of the electric elements.

130 degrees will give you second degree burns in under four seconds 120 - 125 max on residential. Also bacteria will really grown if heater is left off, so much so that after a couple months you will have a nasty black cesspool on your hands.

Thanks for that info, John Have you actually tried it out (the bacteria growing part or do you rely on tests and if so, which tests were you referring to? - this is not to doubt what you are claiming obviously but out of pure curiosity to read more about it)

Phil;

Great study. Your work has made me change my plans from "on demand" to regular hot wtr heater. You saved me alot of calculating.

Lee
Mechanical Engineer