From the article in the current edition (a few typos in it):
**Tank Water Heater **vs. Demand WH
Average Annual Traditional Tank Heater Costs
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Annual hot water demand: 87 gallons / day
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335 days / year = 29,145 gallons/ year (I guess we’re not home for 30 days/year)
Power to heat up 1 gallon of water from 57 to 110F according to the multi-housing laundry association is 0.2 kWh. The annual power requirement of heating the water from 57 to 110F for an American household is: 29,145 gallons/ year x 0.2 kWh/ gallon = 5,900 kWh/ year
Annual cost of 5,900 kWh: 5,900 kWh/ year x $0.11/ kWh = $649
The cost to keep one gallon of water hot is $0.003. So, the cost to keep 21,145 gallons of water hot (standby): 21,945 gallons/ year x $0.003/ gallon = $87
**So the total annual cost of heating water using a traditional tank system is $649 + $85 or **$736
Average annual Tankless Water Heater Costs
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Average energy to heat water to 110F is 6.2 kW
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Average daily usage is 1.5 hours
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Daily power needed to heat the water to 110F is: 1.5 hours x 6.2 kW = 9.3 kWh
Annual costs to heat water using a tankless water heater is: 9.5 kWh/ day x 335 days/ year x $0.11/kWh = $343
The total cost of heating water using a tankless water heater is $343 + $17 or $360.
MY ANALYSIS:
STORAGE TANK:
> 87 gal/day x 8.33 lb/gal** x** 1 btu/lb-deg x 57 deg temp rise = 41,308 btu’s/day
> divided by 3413 btu’s/kWh = 12.1 kWh/day used
> 12.1 kWh/day x 335 days/year x $0.11/kWh= $445.89 + $87 (yearly standby loss)= $532.89…a bit different than $736 in the article
TANKLESS:
The tankless instantaneous unit will still have to heat the same 87 gal/day of 57 degree water to 110 degrees and will use the same $445.89 electricity as the tank**. The only difference should be the** $87/per year standby losses.
SOOOO…Let’s come at it another way:
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The tankless claims to use 9.3 kWh/day = 31,741 btu’s/day
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8.33 lbs/gal X 1 btu/lb-deg X 57 deg temp rise = 474.8 btu’s/gal
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Divide 1 by 2 = 66.9 gallons per day heated!!! A MISTAKE or A MISREPRESENTATION???
And they assumed that the tankless was 100% efficient but figures by better authors/orgs claim energy factors (EF) of from 0.82 to 0.94 for them. so add another 10-15% onto the $445.89 comes to $490.48 to $512.77.
versus… $532.89 for a storage tank
Something’s amiss in the article!!!