http://waterheatertimer.org/pdf/300-liter-water-heater.pdf


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 65F and you want to heat tank to 115, 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.
India uses metric system, and celsius
1 gallon = 3.785 liters
79 gallon = 300 liter
1 BTU = to raise 1 gallon 1 degree
79 BTU = to raise 79 gallon 1 degree
79 BTU = to raise 300 liter 1 degree F
4 KVA = 4 Kilowatts electricity
4000 watts
1 gallon of water weighs 8.34 lbs
79 gallons weighs 658 lbs
300 liters weighs 658 lbs
BTU = 2.931 x .0001 = .0002931 Kwh

Wants to heat 300 liters water to following temperatures
70 degree C = 158F
75 degree F = 167F
Coimbatore India: industrial city near Silent Valley National Park
64 rainy days per year
Average high temperature 87.8F and 31 C
Average low 71.76 F and 22C
Lowest temperature 59F and 15C winter
Summer lowest temperature 68F and 20 C
Estimate incoming water temperature.
Water source: Several lakes and tank inside city    
Several waterways going to ocean
Water appears to come from surface, with probable winter lowest 60F 15C      

79 Gallon Water Heater (300 liter) with 4000 Watt (4 KVA) Elements

79 x 8.34 pounds per gallon =  658.86 pounds of water <> it takes .000293 Kwh to raise 1 pound of water 1 F (assuming ideal conditions inside tank*)
658.86 x .000293 x rise in temperature = Kwh
.193046  x rise in temperature = Kwh
Water
Temperature
at start time
Raise temperature to 120/48.8C  79 Gallon/ 4000 Watt Water heater > delivers 4.0 Kilowatts per hour..
Kwh needed to make 79 gallons
of 120 hot water
Set timer to make 40 gallons:
(For gas heater, divide times by half)
Cost per day at 14 per Kwh Cost per
day: @
12
Cost per
day: @
10
60/15.5C 120/48.8C     11.58 Kwh 2 Hour 54 minutes  .82 per day = $24.61 mo  .70  .58
70/21.1C 120/48.8C     9.65 Kwh 2 Hour 24 minutes  .68 per day = $20.50 mo  .58  .48
80/26.6C 120/48.8C     7.72 Kwh 1Hour 55 minutes  .54 per day = $16.42 mo  .47  .39
90/32.2C 120/48.8C     5.79 Kwh 1 Hour 26 minutes  .41 per day = $12.31 mo  .35  .29
60/15.5C168/74C20.85 Kwh5 hours 12 minutes
70/21.1C168/74C18.91 Kwh4 Hours 43 minutes
80/26.6C168/74C16.98 Kwh4 hours 14 minutes
90/32.2C168/74C15.05 Kwh3 Hours 46 minutes