How many kilowatts needed to heat water in water heaterElectric water heater math 

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 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° 4.5 KW x 3413 = 15358.5 65 degree temperature rise x 8.25 = 536 So one element would heat 28 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.  
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°F2.) 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.) 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


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 (60120°F) 5.866689 Kwh to raise 40 gallons of water 60°F (60120°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 
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Bath  [15 gallons hot for bath x 8.34 llb per gallon x 60 degree rise in temperature x .14 cets per kwh] divided by ________________________________________________________________________ ]3413 BTU content of fuel x .96 efficiency rating of heater] = _________________________________________ = 32 cents cost of hot water per bath inside standard 15' x 30" x 60" Princeton bathtub  
My rule of thumb for solar is 5x to 6x the panel rating
during a really sunny day. 1250 watts of PV should make about (1250watts x 5) = 6.250 Kwh 
How much water is heated using PV solar water heaterResource;Read more about solar water heating 

What does watt rating meanWattage is just a rating that does not measure heating power.Wattage over time, or watthours or Kilowatthours (Kwh) is measure of heating power.If you have 1250 watts for 1/2 hour, then you have 625 watthours 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 3phase 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 brownout 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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bath uses 13 15
gallons hot water (20 gal water total) Set water heater timer for 4560 minutes for hot bath Shower uses 6  8 gallons of hot water (10 gal water total) Set water heater timer for 2030 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 
What size water heater for oversize bathtub?Large bathtub / whirlpool tubRecommend 80 gallon first hour delivery Electric: minimum 60to80 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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