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many kilowatts needed to heat water Mr Kilowatt
does water heater math

Kilowatt hours of electricity needed to heat water ... the math formula:1.) Basic formula > it takes .002931 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 .000239 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 .0002391 = 5.58 Kwh Calculation using 30 Gallon water heater: > Same problem as above > Water in tank weighs 220.2 lbs > Multiply 250.2 x 70 x .0002391 = 4.18 Kwh 
Temperature of water 
Cost if you use 40 Gallon per day  Cost if you use 30 Gallon per day 
Raise from 32° to 120°  7.01 Kwh x 14¢ = $1.20  5.26 Kwh x 14¢ = 73¢ 
Raise from 50° to 120°  5.58 Kwh x 14¢ = 78¢  4.18 Kwh x 14¢ = 58¢ 
Raise from 70° to 120°  3.99 Kwh x 14¢ = 56¢  2.99 Kwh x 14¢ = 42¢ 
Raise from 85° to 120°  2.79 Kwh x 14¢ = 39¢  2.09 Kwh x 14¢ = 29¢ 
See Family chart for setting water heater timer 
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