Electric tankless heaters seem to cause some confusion in the market place due to their high amperage (current draw) and limited 120 volt heaters. All tankless heaters are not direct drop in replacements for tank heaters. Tankless usually require an infrastructure change to accommodate the requirements for tankless heaters.
First off, water is a very difficult material to heat. It takes twice the amount of heat to heat water than oil and 10 times the amount of heat to heat water over metals. Tank heaters have smaller amount of heat input than tankless as they use a time factor to get the water up to temperature. the first time a tank heater is turned on, it can take up to 2-6 hours to get to temperature depending on tank size and heat input and insulation on the tank. Once installed and turned on the tank is constantly trying to keep the water at temperature. The formula for knowing how to heat water is:
kilowatts/hour = (mass x temperature change) / 3413 (btu's per KW)
If we look at a standard 50 gallon tank for example, the formula is:
10 = (415 x 80)/3413
10 kw will heat the tank in 1 hour.
Now, 10 kw requires 41 amps running on 240 volts.
Common breaker sizes are 30 amps, so a tank heater usually has 6 or 7.5 kw input to stay under the 30 amp breaker size. which will increase the time to heat the tank.
Tankless heaters have to heat the water as it passes through the heater, thus much more heat input is required. Our whole house heater has a heat input rating of 27 kw. At this heat input the heater will raise the temperature 45 degrees at a flow rate of 4.0 gpm. If the flow rate is reduced to 2.5 gpm the heat rise will be about 65 degrees. This heater has three 9 kw heating modules and thus each module requires 40 amps to operate. This means that a total of 120 amps ( 240 volts) is required to operate the heater. This is significant power requirements. the difference is the time factor has been removed from the tank equation.
Amps are defined by the wattage divided by the voltage. Lower amps mean smaller breaker sizes and smaller wire sizes. This reduces costs and panel sizes. This is why electrical appliances such as ovens, dryers and water heaters utilize 220-240 volts in lieu of 120 volts (1/2 the amps) for their power requirements.
Will tankless save on electricity? The answer is Yes.
In electrical terms, a tank heater can use between 60,000 watts to 75,000 watts per day to keep water hot. Our 27,000 watt heater will use about 27,000 watts per day. That is a significant reduction. A tank heater can be on up to 10 hours a day keeping water hot. A 6kw heater times 10 hours is 60kw or 60,000 watts. The tankless heater is NOT on except when hot water is called for. A normal usage for a home is about an hour per day. Of course, when you are not home, there will be NO usage with a tankless heater. Most home owners do not turn off their tank water heater and are thus using power even when no one is home.
I use an electric tankless heater and it works great, we have set the temperature at 108 and it is more than hot enough and the showers can be endless. We did have to install a new panel to handle the breakers and increased amperes required.
In summary, it is simple physics as to why a tankless heater has such large KW and amp requirements, water is just difficult to heat and it needs a "muscle car' to do it with an on-demand or tankless heater. Save money and install an electric tankless heater.
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