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[an error occurred while processing this directive]Water Heater energy use figures

 Last update: June 2015

This page is simply figures and calculations, and is of limited interest.  You'll probably prefer my page on Water Heaters and How to Save Energy on Water Heating instead.


Hot water used (gallons)

Washing Machine (typical model; hot/hot) 40
Washing Machine (efficiency model; hot/hot) 13-28
Bath 15-25
Shower 5-10
Dishwasher 12
Cooking 5
Washing Dishes 4
Figures from the Dept. of Energy. and CA Energy Commission

Water use figures

  • The average hot water use per day was 44 gallons, in a study of 30 Canadian households.  The DOE test assumes 64 gallons. (Natural Resources Canada PDF, 2008)
  • Households used 69.3 gallons of total water (hot+cold) per capita in a large study. (AWWA PDF, 1999)
  • After installing water-saving toilets, washers, showerheads, and faucets, total (hot+cold) household use dropped 39% from 175 g/d (gallons per day) to 107 g/d, and hot water use dropped 10.8 g/d from 55 g/d to 44.2 g/d, a 20% reduction. (EPA PDF, 2005)
  • The design spec for hot water use per person is 20-35 g/d(Engineering Toolbox)

By the way, you can get a thermostatic shower valve to keep the water temperature constant even when people flush toilets or turn on sinks in other parts of the house.


Energy required to heat one gallon of water, and one tank of water

Definitions & Conversion Factors

  • A Btu, or British thermal unit, is the amount of energy needed to raise one pound of water from 60F to 61F at sea level. (Wikipedia)
  • A gallon of water weights 8.33 lbs.
  • Heating a gallon of water by 1F with no losses thus takes 8.33 lbs x 1 Btu/lb = 8.33 Btu's.
  • One therm is 100,000 btu's. So one Btu is 0.00001 therms. (U.C. Irvine)
  • Heating a gallon of water by 1F with no losses thus takes 8.33 100,000 = 0.00008.33 therms.
  • One kWh is 3413 Btu's, so one Btu is 1/3413 = 0.000293 kWh.
  • Heating a gallon of water by 1F with no losses thus takes 8.33 Btu x 1/3413 kWh/Btu = 0.00244 kWh.
  • Temperature of groundwater.  Varies from 35-77F in the U.S., and eyeballing a temperature map, 47-67 seems like a good range, ad 57 seeming like a good average.

Energy required to heat a 40-gallon tank of water

  1. Groundwater 47F, Heater set to 110F:  63 rise x 8.33 Btu x 40 gallons = 20,992 Btus
  2. Groundwater 47F, Heater set to 120F:  73 rise x 8.33 Btu x 40 gallons = 24,324 Btus
  3. Groundwater 47F, Heater set to 140F:  93 rise x 8.33 Btu x 40 gallons = 30,988 Btus

  4. Groundwater 57F, Heater set to 110F:  53 rise x 8.33 Btu x 40 gallons = 17,660 Btus
  5. Groundwater 57F, Heater set to 120F:  63 rise (see #1 above)
  6. Groundwater 57F, Heater set to 140F:  83 rise x 8.33 Btu x 40 gallons = 27,656 Btus
  7. Groundwater 67F, Heater set to 110F:  43 rise x 8.33 Btu x 40 gallons = 14,328 Btus
  8. Groundwater 67F, Heater set to 120F:  53 rise (see #4 above)
  9. Groundwater 67F, Heater set to 140F:  73 rise (see #2 above)
    Above assumes 100% efficiency.

Cost to heat water a 40-gallon tank with gas

  • The federal standard for gas water heaters is only 55.6-59.4% efficiency (higher for smaller tanks), and actual models are rarely much better, except for Energy Star models which are at least 67% efficient. (DoE 2010, p. 9)
  • Gas price figured at $1.42/therm.  1 therm = 100,000 BTU, so cost is $1.42/therm x 1 therm/100,000 BTU = $0.0000142/BTU
  • 43 rise: 14,328 Btus 67% & 55.6% efficiency = 21,385 & 25,770 BTU, x $0.0000142/BTU = $0.30 to $0.37/tank
  • 53 rise: 17,660 Btus 67% & 55.6% efficiency = 26,358 & 31,763 BTU, x $0.0000142/BTU = $0.37 to $0.45/tank
  • 63 rise: 20,992 Btus 67% & 55.6% efficiency = 31,331 & 37,755 BTU, x $0.0000142/BTU = $0.44 to $0.54/tank
  • 73 rise: 24,324 Btus 67% & 55.6% efficiency = 36,304 & 43,748 BTU, x $0.0000142/BTU = $0.52 to $0.62/tank
  • 83 rise: 27,656 Btus 67% & 55.6% efficiency = 41,278 & 49,741 BTU, x $0.0000142/BTU = $0.59 to $0.71/tank
  • 93 rise: 30,998 Btus 67% & 55.6% efficiency = 46,266 & 55,752 BTU, x $0.0000142/BTU = $0.66 to $0.79/tank
  • Another source comes up with a similar figure: 0.40 therms for the tank, based on 0.11 therms to heat 11 gallons of water. (Multi-housing Laundry Association)
  • MHLA also says it takes 3.3 therms to keep 11 gallons hot for one month.

Cost to heat water a 40-gallon tank with electricity

  • A typical electric water heater is 90.4-95% efficient. (DoE 2008, p. 2)
  • Conversion factor: 0.000293 kWh/Btu x $0.14/kWh = $0.00004102/BTU
  • 43 rise: 14,328 Btus 95% & 90.4% efficiency = 15,082 & 15,850 BTU, x $0.00004102/BTU = $0.62 to $0.65/tank
  • 53 rise: 17,660 Btus 95% & 90.4% efficiency = 18,589 & 19,535 BTU, x $0.00004102/BTU = $0.76 to $0.80/tank
  • 63 rise: 20,992 Btus 95% & 90.4% efficiency = 22,097 & 23,221 BTU, x $0.00004102/BTU = $0.91 to $0.95/tank
  • 73 rise: 24,324 Btus 95% & 90.4% efficiency = 25,604 & 26,907 BTU, x $0.00004102/BTU = $1.05 to $1.10/tank
  • 83 rise: 27,656 Btus 95% & 90.4% efficiency = 29,112 & 30,603 BTU, x $0.00004102/BTU = $1.19 to $1.25/tank
  • 93 rise: 30,998 Btus 95% & 90.4% efficiency = 32,629 & 34,290 BTU, x $0.00004102/BTU = $1.34 to $1.41/tank
  • WaterHeaterTimer.org gets 6.84 kWh for a 70F rise; for the 63 rise I calculated, that would be 6.16 kWh, so our figures are within the ballpark of each other's.

Yearly energy requirements

   The DoE test procedure assumes an inlet water temperature of 58F, a set point of 135F, and daily hot water demand of 64.3 gallons.  Annual consumption is figured in therms as 41,045 Btu/EF x 365/100,000, or 149.8 therms at 100% efficiency.  A 2008 EPA report showed yearly energy use figures as follows:
  • 2195 kWh for a 50-gallon air-based heat pump with a 2.00 energy factor
  • 4435 kWh for an electric tankless with a 0.99 energy factor
  • 4622 kWh for a 50-gallon electric tank with a 0.95 energy factor
  • 242 therms/yr. for a 50-gallon gas tank with a 0.62 energy factor
  • 183 therms/yr. for a gas tankless with a 0.82 energy factor
Miscellaneous

 

Electric tank wattage


A typical 50-gallon electric tank runs at 4500 watts.

At 3.412 BTUs per watt, 4500 watts = 15,354 BTU.
At 92% efficiency, that's 14,126 BTU.
From above, heating a gallon of water by 1F takes 8.33 BTU.
Heating from 68F to 104F would by a 36F rise, or 36 x 8.33 = 300 BTU to heat 1 gallon of water.
With 14,126 BTU, we could heat 14,126 BTU 300 BTU/gallon = 47 gallons.
So, our typical 4500-watt electric heater can make 47 gallons of hot water per hour — about a full tank.

Like others, I've wondered why specs often read "Top element: 4500 watts,  Bottom element: 4500 watts,  Total wattage: 4500 watts".  Is only one element used at a time, and if so, why?  I found an answer on Yahoo Answers:  "On a hot water tank there is only one of the elements on at a time. The incoming cold water is taken to the bottom of the tank through a pipe inside the tank. When the lower thermostat senses the cold water the lower element turns on. As you draw hot water from the top of the tank the cooler water from the bottom will rise. When the top thermostat senses the cool water it shuts off the bottom element and turns on the top element. When this water is heated to the tank set- point it shuts off and the lower element turns on to heat the rest of the water. When the total tank temperature is at the set point all elements turn off. By using this type of procedure there is always hot water at the top of the tank for use."

Also, 9000 watts with both elements running simultaneously would require a massive electrical circuit, and it's not necessary, since as we see from the calculations above, 4500 watts at a time can heat a whole tank in pretty short order anyway.

Solar water heaters

While solar electricity takes a while to recoup its installation cost, solar water heating works a lot better, and is easier to install and maintain.  A system starts at around $4500 for a family of four, but rebates and tax credits can lower the cost substantially.  You're looking at a payback time of maybe 12 years.  Mr. Electricity's family enjoys solar hot water, and no longer being tethered to the gas company.  See my page on solar water heating.

 


On a separate page I have questions I've received and answered about how saving on water heating costs.


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