Whirlpool temp control on electric water tank

Wattage is printed on end of each element.
If water heater has 2 elements, then it is 240volts
Only 1 element is ON at a time. Both elements are not ON at same time.
Example element is 4500 watts
So when water is ON, it is consuming 4500 watts
When water heater runs for 1 hour, it has consumed 4500 watt hours.
1000 watt hours = 1 kilowatt hour
So water heater running for 1 hour consumes 4.5 Kilowatt hours
http://waterheatertimer.org/How-to-install-electric-meter-on-240Volt-water-heater.html

How many hours per day does water heater run?
It depends on thermostat temperature, temperature of incoming cold water, amount of hot water consumed, if hot water is dripping at faucets, etc.
Modest-consuming 2 person household will consume 7.6 Kilowatt hours per day. Or 228.75 Kilowatt hours per month x $.14 per Kilowatt hour = $32 per month.
4 person household can consume twice as much hot water.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

Most electric water heaters have 2 heaters - an upper and a lower, each with it's own thermostat. These are readily accessible by removing the screw covers on the side of the tank. Careful - the thermostats have 240VAC on the terminal screws (unless you have a small 120VAC heater) if the circuit breaker or fuses feeding the heater are still on - turn them off before proceeding. After removing the two screw covers and pulling back the insulation you will see temperature (screw) adjustments for the upper and lower thermostats, with a temperature indicating pointer attached to each screw. Set both thermostats to 125 degrees or so. Push the insulation back in place, replace the covers and turn the circuit breaker back on that feeds the water heater. You should have warm water within 15 minutes and hot water within a hour or so.

If the water still does not heat, then you have to troubleshoot to find the problem. With a voltmeter determine if 240VAC is actually getting to the water heater. Remember, the upper element has priority over the lower element, and if the water is cold, the upper element will try to turn on and this locks out the lower element (only one element is allowed to heat at any given time). The lower element comes on ONLY after the upper thermostat is satisfied. Therefore if the upper heating element is burned out you will never get any hot water. If you suspect this, TURN OFF THE POWER TO THE HEATER and take a resistance check of the upper element.