How do I determine the water heater element power rating and which voltage is more suitable?

Question

I am designing a water heater project and need to know the calculations to determine what power rating element should i get. also, does a higher voltage (220V) work better or worse than a lower voltage (120V)?Please explain which would heat faster?

Answer 1

Voltage makes no difference. The power rating is what does the heating.

Water heaters tend to be 220v rather than 120v because the current is lower for the same power, hence smaller wires and switches etc. Also slightly less losses but not really significant in small heater circuits.

Answer 2

Normally, on an existing water heater, the nameplate would give the technical data you require. However since you've taken the challenge to build your own, here's my best.
To the first question: Power (in watts) equals the current (in amps) times the supplied voltage, (120v or 220v) This formula can be transposed mathematically. To the second question: I doubt the availability of 120v heating element. The efficiency of 220v I'm sure are many, one of which is the unnecessarily loading of one leg of the circuit. Adding the second hot leg divides the load across the two, which at least will diminish the heat losses ($) in the supply line: a more effective use of energy and safety (keeping the heat dissipation away from combustibles). I will also recommend matching the circuit breaker (in amps) to the draw of the element. Good Luck!!!

Answer 3

You need to specify the gallons of hot water, the temperature and the recovery time. I work in USA units.

Here is a sample calculation with my selections. Cold water is 40 F, Hot is 140 F for a change of 100 degrees.

Tanks is 20 gallon, which is 166 pounds of water.

1 btu raises 1 pound of water one degree. So the required btu input is 16,600 btu

Recovery time is 1 hour. 1000 btu h is 293 watt h. which gives 4864 watt hr.

Select a 5 kw heating element. at 220 volt supply this will draw 22.7 amp which is the prefered arrangement for a house supplied with 220 - 2 phase power. 120 should be selected only for true 120 single phase house service which is rare.