Wattages and Ohms Law for Electric Tankless Water Heaters

We sell Bosch Electric Tankless Water Heaters.

Electric Heaters and heating elements become hot due to a voltage being pushed through a wire.  The resistance in the wire measured in Ohms creates the heat.  The more resistance the hotter the wire gets. The more voltage pushed through the wire, the hotter the wire gets. Simply the smaller the wire (or the longer the wire run) the more resistance it gives to the voltage passing through.

So as voltage increases the wattage will increase four-fold.  As the voltage decreases the wattage will decrease by a factor of 4 as well.  The below table shows the percentage of drop or gain with different applied voltages to a heating element.

Rated at 277 volts the expected wattage % is

277 applied 100% (rated Wattage)

240 applied  75%

230 applied  69%

208 applied  56%

120 applied  19%

Rated at 240 volts the expected wattage % is

277 applied 133%

240 applied 100% (rated wattage)

230 applied  92%

208 applied  75%

120 applied  25%  (1/4 wattage)

Rated at 120 volts the expected wattage % is

240 Applied  400%  (4 times wattage - Dangerous) 

208 Applied  300%  (3 times wattage - Dangerous)

120 Applied  100%  (rated Wattage)

110 Applied   84%

This is all based on Ohm's Law.

Simple Ohms Law equations are:

Amps = WATTS / VOLTS

Watts = VOLTS x AMPS

Volts =  AMPS x OHMS

Ohms = VOLTS / AMPS

A Conductor Wire's resistance is constant based on wire size and length.

A heater's watt rating is based on the wire used in the heating element and the voltage applied.

To get a 9.5 kilowatt rated heater run at 240 volts; a wire with a resistance of 6.6 Ohms is used. (If there are two elements in the heater, then each is rated at 4.75 kw and the resistance of the wire would be 12.12.)

Calculating the Volume of Water in a Water Line

Whenever you turn on a gas, propane or electric tankless water heater the hot water starts heating up as soon as the minimum flow rate is met for the hot water demand. It takes a few seconds for the water to get heated up to the requested temperature. But, before the hot water starts to flow out of your shower or sink faucet the cold water that is in the pipe has to be pushed through your faucet. This is often referred to as pushing a "cold water slug".

Customers frequently ask us how much water is contained in this cold water slug and how long it will take to push this slug through their faucet. This can be easily calculated by using the formula for the volume of a cylinder.

The formula for the volume of a cylinder is V = Pi x r x r x h

where Pi = 3.1416; r = the radius of the water line; and h = height (or in this case the length) of the line

If your water line is 1/2" then r = .25" and r x r = .0625" and Pi x r x r = 0.19635 square inches
If your water line is 3/4" then r = .375" and r x r = .1406" and Pi x r x r = 0.4417875 square inches

Now to get all units in inches calculate h (the length of the water line) by taking feet x 12". As an example let's assume we have a 40' water line of 1/2" pipe. 40' = 480" so we calculate the Volume as:

V = 0.19635 square inches x 480" = 94.248 cubic inches

Now there are 1728 cubic inches in a cubic foot and there are 7.48 gallons in a cubit foot. Therefore there are 0.0043 gallons in a cubic inch.

So, 94.248 cubic inches * 0.0043 gallons = 0.4 gallons of water.

If you are running this cold water slug through a 1 GPM sink faucet then it will take .4 x 60 seconds or 24 seconds to push the cold water slug through the 1 GPM faucet. Add a few more seconds for the tankless water heater to bring the water up to temperature and remember that there are about 15' of internal pipe in a gas or propane tankless water heater that also starts out with cold water. In this example it will take just more that 1/2 minute to get the hot water flowing out of the faucet.