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Mr Kilowatt
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How 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 1F

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.)

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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