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 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°/15.5C  168°/74C  20.85 Kwh  5 hours 12 minutes  
70°/21.1C  168°/74C  18.91 Kwh  4 Hours 43 minutes  
80°/26.6C  168°/74C  16.98 Kwh  4 hours 14 minutes  
90°/32.2C  168°/74C  15.05 Kwh  3 Hours 46 minutes 