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How many
kilowatts needed to
heat water in water heater Home |
 Electric water heater thermostats Thermostats stick, melt and come out of calibration... replace thermostat Symptoms: Cold water, uneven temperatures, reset tripping, TP releasing water Buy: Water heater thermostats at Amazon Resources: How to replace electric water heater thermostat Add another thermostat to water heater How to wire thermostats Marathon water heater thermostats Two water heaters Troubleshoot electric water heater |
  Gas water heater thermostats If tank is overheating or not heating correctly, replace thermostat Symptoms: Cold water, uneven temperatures, TP releasing water Buy: Gas water heater thermostats at Amazon Resources: How to replace gas water heater thermostat Add another thermostat to water heater Troubleshoot gas water heater |
| Kilowatt hours of electricity
needed to heat water ... the math formula: 1.) Basic formula > it takes .0002931 Kwh to raise 1 pound of water 1°F 2.) Basic number >A gallon of water weighs 8.34 lbs (pounds) 3.) "A BTU is amount of heat needed to raise 1 pound of liquid water by 1° from 60° to 61° F at constant pressure of one atmosphere. Other definitions of BTU exist which are based on different water temperatures and cause the results to vary by .5%." (For analysis of water heaters, .5% is ignored. Variation in atmospheric pressure is also ignored. Water heater 'science' is an approximation since calcium carbonate in tank will vary electrical efficiency and tank capacity.) Formula for heating water inside tank: Gallon Per Hour (Electric) = (KW x 3413) divided by (temp. rise x 8.25) or (KW x 414) divided by (temp rise.) Let's say you have 5500 watt elements. let's say incoming water temperature is 65°F and you want to heat tank to 105°, or 40° rise in temperature. 5500 watt or 5.5Kw elements x 3413 = 18771.5 40 degree temperature rise x 8.25 = 330 So in this specific situation, the element would heat 56.88 gallons per hour This number is for EACH element, but does NOT consider heat loss from tank, room temperature, or thickness of insulation etc. Remember, with residential simultaneous tank, only ONE element is ON at one time. Upper element is ON, or lower element is ON, or BOTH elements are off. At no time are both elements on. 4.) BTU formula > 1 BTU = 2.931 x 10–4 Kwh (kilowatt hours). BTU = 2.931 x .0001 = .0002931 Kwh Therefore it takes .000293 Kwh to raise 1 pound of water 1° F 5.) 30 Gallons of water x 8.34 = 250.2 lbs 40 Gallons of water x 8.34 = 333.6 lbs Calculation using 40 Gallon water heater: > Ordinary 40 gallon water heater in attic. Temperature in attic = 50° F. Temperature of water in tank = 50° F. > Water in tank weighs 333.6 lbs. > How much electricity is needed to raise temperature of full tank to 120° F. > 120° minus 50° = 70° > so the temperature needs to go up 70° > Multiply 333.6 x 70 x .0002931 = 6.84 Kwh Calculation using 30 Gallon water heater: > Same problem as above > Water in tank weighs 220.2 lbs > Multiply 250.2 x 70 x .0002931 = 5.13 Kwh |
| Formula for heating water inside
tank: The number of gallons of water that are heated Per Hour (for Electric heater) = (Kw rating of element x 3413) divided by (temperature rise x 8.34) 8.34 is weight of 1 gallon water. Resource: Water heater formulas and terminology .pdf  Calculation #1 Let's say you have 5500 watt elements which are 55 Kw. Let's say incoming water temperature is 65°F and you want to heat tank to 105°, or 40° rise in temperature. (40 temp rise not shown on the AO Smith chart) 5500 watt or 5.5Kw elements x 3413 = 18771.5 40 degree temperature rise x 8.25 = 333.6 So 18771.5 divided by 330 = So in this specific situation, the element would heat 56.26 gallons per hour. Calculation #2 If you have 4500 watt elements and incoming water temperature is 55°, and you want to heat to 120°, then the temperature rise is 65° (65 temp rise is not shown on chart) 4.5 KW x 3413 = 15358.5 65 degree temperature rise x 8.34 = 542.1 15358.5 divided by 536 = 28. So one element would heat 28.33 gallons per hour. Note: the chart shows 30.8 for 60° rise, so you would expect 65 rise in temperature would result in lower output than 60. This says that reasonable estimates can be made using the AO Smith chart.  Measuring water temperatures can be accomplished using a PID sensor taped to pipe and covered with insulation, or by installing temperature gauge on pipe, or bleed water out of TP valve and use cooking thermometer. Buy: Winters Temperature gauge Temperature-pressure gauge for boiler Pocket thermometer Kitchen Thermometers PID proportional integral device Resource: Control water heater with PID  ConsiderationsCalculations do NOT consider heat loss from tank, ambient room temperature, thickness of insulation, low voltage, approximate calibration of thermostats, and inefficiencies such as elements coated with sediment etc. There are too many variables to make any calculation exact, so all number for heating water are approximations which help determine actual performance vs potential problems with heater. The number of gallons per hour is per EACH element, but remember, with residential simultaneous tank, only ONE element is ON at one time. Upper element is ON, or lower element is ON, or BOTH elements are off. At no time are both elements on, unless heater is wired for simultaneous operation. Resource: How to wire simultaneous water heater thermostats |
| 0.0002931 Kwh to raise 1 pound of
water 1°F 0.0024444 Kwh to raise 1 gallon of water 1°F 0.146666 Kwh to raise 1 gallon of water 60°F (60-120°F) 5.866689 Kwh to raise 40 gallons of water 60°F (60-120°F) 1 Kw = 3413 BTU 2.42 watt required to raise 1 gallon water 1 degree 1 Kwh will raise 6.8 gallon water 60 degrees F 8.33 BTU will raise 1 gallon of water 1 degree at 100% efficiency 11 BTU required to raise 1 gallon of water 1 degree at 75% efficiency (gas) 1 cubic foot natural gas = 1000 BTU 1 therm 100,000 BTU or 100 cubic feet 1 cubic foor propane = 2500 BTU 1 gallon propane = 91,250 BTU |
More
formulas Water heater formulas and terminology/ pdf Electrical formulas /pdf |
| Cost for 1 bath 15 gallons hot for bath x 8.34 llb per gallon x 60 degree rise in temperature x .14 cets per kwh = 1050 3413 BTU content of fuel x .95 efficiency rating of heater = 3139 1050 divided by 3193 = 33¢ cost of hot water per bath inside standard 15' x 30" x 60" Princeton bathtub. 9 gallon shower cost 19.8¢ |
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| Temperature of water |
Cost if you use 40 Gallon per day | Cost if you use 30 Gallon per day |
| Raise from 32° to 120° | 8.60 Kwh x 14¢ = $1.20 | 6.45 Kwh x 14¢ = 90¢ |
| Raise from 50° to 120° | 6.84 Kwh x 14¢ = 96¢ | 5.13 Kwh x 14¢ = 72¢ |
| Raise from 70° to 120°> | 4.88 Kwh x 14¢ = 68¢ | 3.66 Kwh x 14¢ = 51¢ |
| Raise from 85° to 120° | 3.42 Kwh x 14¢ = 48¢ | 2.56 Kwh x 14¢ = 36¢ |
 See Family chart for setting water heater timer |
Resources: Calculate first hour delivery Calculate recovery |
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Batch solar water heater uses no
electricity Resource Batch solar water heater |
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Closed loop system recirculates
solar-heated water Consumes various amounts of electricity depending on conditions, location etc. This type system does extremely well in naturally warm environments such as Hawaii. Resource Ask questions before buying closed loop |
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Do it yourself PV solar panel This is best option for lowering electric bill and supplying hot water Resources Convert AC water heater to DC/ high voltage Convert AC water heater to DC/ low voltage |
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Install meter on electric water heater | |
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What
does watt rating mean
Wattage is just a rating that does not measure heating power Wattage over time, or watt-hours or Kilowatt-hours (Kwh) is measure of heating power. If you have 1250 watts for 1/2 hour, then you have 625 watt-hours or .625 Kwh. If you have 1250 watts for 2 hours of the day, and 750 watts for 3 hours, then you have 4.75 Kwh... which should raise 40 gallons of water from 75° to 120° Resource: Select and understand water heater elements 3-phase water heaters 9000 watt water heater Temperature for killing bacteria |
 Same 4500 watt element @ 240 volts and @ 120 volts |
Wattage
drops when voltage is reduced See water heater label showing difference between 208V - 240V Resistive loads such as light bulbs and elements burn less hot when voltage is reduced. Examples: ordinary household dimmer for incandescent bulbs, or power brown-out when household voltage is reduced and light bulbs dim. Same is true for water heater elements. Lower wattage heats less water per hour See chart Resource: Convert water heater from 240 volts to 120 volts Select and understand water heater elements |
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Install meter on electric water heater |
 Take showers instead of bath. |
Regular
bath uses 13- 15
gallons hot water (20 gal water total) Set water heater timer for 45-60 minutes for hot bath Shower uses 6 - 8 gallons of hot water (10 gal water total) Set water heater timer for 20-30 minutes for hot shower Resources: How much hot water for bath or shower Compare water heater timers Off peak water heater options Buy: Bathtubs and shower stalls at Amazon Shower stalls at Amazon Home energy savings books at Amazon |
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What size water heater for oversize bathtub?Large bathtub / whirlpool tubRecommend 80 gallon first hour delivery Electric: minimum 60-to-80 gallon size just for that one tub Gas: minimum 50 gallon size just for that one tub Electric heat pump water heaters generally cannot produce enough hot water. Tankless promise unlimited hot water, but temperature of water is 120 F and water in tub may be cool by time tub fills. Cost of maintenance and repairs never pay back. All costs are energy. Resource: 80 gallon electric water heater ended in April 2015 over 50 gallon electrci must be hybrid |
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