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#1
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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 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, |
#2
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Power cost of idle electric water heater
Phil Sherrod wrote:
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) Thanks, Phil. Interesting. Depending on ease of access, you might consider a low temp setting when not in use, turning it up when needed. Some new models have settings for vacation, energy save, peak demand, and normal. Don't know if it's true but I've heard recommendations against turning them off completely unless for very extended periods. Marv |
#3
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Power cost of idle electric water heater
Did you consider a demand heater? Whould seem to make sense
for occational type use. Doug s/v Callista "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) 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, |
#4
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Power cost of idle electric water heater
On 30-Mar-2004, "Doug Dotson" wrote: Did you consider a demand heater? Whould seem to make sense for occational type use. Yes, but there was no way to justify it. Since the maximum savings with my power rate would be $3/month it would take a LONG time to offset the hundreds of dollars in extra cost. Plus, I can easily switch off the circuit breaker for the weeks or months when we don't need the heater. |
#5
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Power cost of idle electric water heater
On Wed, 31 Mar 2004 00:53:00 GMT, "Phil Sherrod"
wrote: On 30-Mar-2004, "Doug Dotson" wrote: Did you consider a demand heater? Whould seem to make sense for occational type use. Yes, but there was no way to justify it. Since the maximum savings with my power rate would be $3/month it would take a LONG time to offset the hundreds of dollars in extra cost. Plus, I can easily switch off the circuit breaker for the weeks or months when we don't need the heater. If you live in a humid area I'd recommend not switching the heater off. The outside of the tank is still just steel with the fiberglass insulation in contact. When the humidity is high humidity will condense on the tank surface and set up rust. The fiberglass will help hold the water in there. BTDT at my summer cabin in the Smokey mountains. Lost 3 water heaters in short order to outside-the-tank rust before I gave in and left it energized all the time. Current heater's been there for 10+ years. John --- John De Armond http://bellsouthpwp.net/j/o/johngd/ Cleveland, Occupied TN |
#7
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Power cost of idle electric water heater
On 31-Mar-2004, "Roland Paterson-Jones" wrote: Mmm, hot water delivered is 114F, that's 45C, hardly luke-warm! We have to take the rest of the measurements with a pinch of salt then. As you know, heat loss is proportional to the temperature delta. How about a realistic scenario. OK, let's bump the temperature up to 125F. That's a temperature differential of 64F. That means the heat loss and cost increases by a factor of 64/51. The average heat loss cost at 114F is $4/month, so at 125F it will be $5.02. (4*64/51) But, of course, don't forget that many people keep their water heaters in areas that are warmer than 61F, so the temperature differential may be lower than mine. The main issue with electric water heaters, as far as I'm concerned, is that the Carnot-limited power source wastes at least 60% of the fuel source heat to the local rivers and dams, for you to take the (optimistic) 40% and convert it back to heat. I have absolutely no control over that nor was my study directed at that issue. Your case study is flawed, and you lack appreciation of the big picture. I never claimed to be presenting either a study or a solution to the "big picture" of energy usage. My study was simply to measure the actual heat loss energy for one water heater in a known environment. As shown above, you can adapt the figures to other situations. |
#8
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Power cost of idle electric water heater
On 31-Mar-2004, "Roland Paterson-Jones" wrote: Mmm, hot water delivered is 114F, that's 45C, hardly luke-warm! We have to take the rest of the measurements with a pinch of salt then. As you know, heat loss is proportional to the temperature delta. How about a realistic scenario. OK, let's bump the temperature up to 125F. That's a temperature differential of 64F. That means the heat loss and cost increases by a factor of 64/51. The average heat loss cost at 114F is $4/month, so at 125F it will be $5.02. (4*64/51) But, of course, don't forget that many people keep their water heaters in areas that are warmer than 61F, so the temperature differential may be lower than mine. The main issue with electric water heaters, as far as I'm concerned, is that the Carnot-limited power source wastes at least 60% of the fuel source heat to the local rivers and dams, for you to take the (optimistic) 40% and convert it back to heat. I have absolutely no control over that nor was my study directed at that issue. Your case study is flawed, and you lack appreciation of the big picture. I never claimed to be presenting either a study or a solution to the "big picture" of energy usage. My study was simply to measure the actual heat loss energy for one water heater in a known environment. As shown above, you can adapt the figures to other situations. |
#9
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Power cost of idle electric water heater
On Thu, 1 Apr 2004 01:04:20 GMT, "Phil Sherrod"
wrote: The main issue with electric water heaters, as far as I'm concerned, is that the Carnot-limited power source wastes at least 60% of the fuel source heat to the local rivers and dams, for you to take the (optimistic) 40% and convert it back to heat. I have absolutely no control over that nor was my study directed at that issue. Your case study is flawed, and you lack appreciation of the big picture. I never claimed to be presenting either a study or a solution to the "big picture" of energy usage. My study was simply to measure the actual heat loss energy for one water heater in a known environment. As shown above, you can adapt the figures to other situations. Phil, Just ignore Roland. He just likes to hear himself type and throw out fancy terms. Doesn't have a clue. Your measurements were right on target. Even the temperature selected. The water heater I recently installed in one of my cabins came from the Home Depot with the 'stats set at 115 deg F. Probably the lawyers at work again. I bet just about nobody changes that setting. Even Roland's appreciation of the "big picture" is incorrect. Modern supercritcal fossil plants achieve overall efficiencies in the 50% range. Even the good old nuke are in the high 30s. The Sequoyah Nuclear Plant where I worked for about 10 years recently replaced the steam generators. The improved design plus some plant upgrades pushed the efficiency to 38% and a fraction, something that spawned a small staff party. Oh, and BTW, Roland, the waste heat goes up the cooling tower stacks and not into the lake, at least not to any significant extent. Much to the chagrin of local fishermen who discovered that the warm water (pre-cooling towers) spawned great fish growth and fishing conditions. John --- John De Armond http://bellsouthpwp.net/j/o/johngd/ Cleveland, Occupied TN |
#10
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Power cost of idle electric water heater
On Thu, 1 Apr 2004 01:04:20 GMT, "Phil Sherrod"
wrote: The main issue with electric water heaters, as far as I'm concerned, is that the Carnot-limited power source wastes at least 60% of the fuel source heat to the local rivers and dams, for you to take the (optimistic) 40% and convert it back to heat. I have absolutely no control over that nor was my study directed at that issue. Your case study is flawed, and you lack appreciation of the big picture. I never claimed to be presenting either a study or a solution to the "big picture" of energy usage. My study was simply to measure the actual heat loss energy for one water heater in a known environment. As shown above, you can adapt the figures to other situations. Phil, Just ignore Roland. He just likes to hear himself type and throw out fancy terms. Doesn't have a clue. Your measurements were right on target. Even the temperature selected. The water heater I recently installed in one of my cabins came from the Home Depot with the 'stats set at 115 deg F. Probably the lawyers at work again. I bet just about nobody changes that setting. Even Roland's appreciation of the "big picture" is incorrect. Modern supercritcal fossil plants achieve overall efficiencies in the 50% range. Even the good old nuke are in the high 30s. The Sequoyah Nuclear Plant where I worked for about 10 years recently replaced the steam generators. The improved design plus some plant upgrades pushed the efficiency to 38% and a fraction, something that spawned a small staff party. Oh, and BTW, Roland, the waste heat goes up the cooling tower stacks and not into the lake, at least not to any significant extent. Much to the chagrin of local fishermen who discovered that the warm water (pre-cooling towers) spawned great fish growth and fishing conditions. John --- John De Armond http://bellsouthpwp.net/j/o/johngd/ Cleveland, Occupied TN |
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