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TWHs can cause voltage problems in two ways – steady-state ow/undervoltage and voltage flicker. A large unit (something over 10 kW) is a significant load compared to a standard 25 kVA residential transformer, and the largest heaters are over the transformer rating all by  themselves.

At high flow rates (e.g. one or more showers, clothes washer), the heater is likely to be drawing a continuous high current. A voltage problem here would be steadystate low voltage. Depending on the size of the service drop and distribution transformer, an extra 20+ kW of load could cause a significant voltage drop. Any other loads that come on during this time (e.g. electric stove) will add to the problem.

At low to moderate flow rates (e.g. faucet at a sink) a high power heater will cycle the heating element on and off quickly to modulate the amount of heat, matching it to the smaller water demand. This power off/on cycling creates voltage flicker the entire time the water is flowing.

The only ways to mitigate these problems involve reducing the source impedance so that the voltage drop becomes acceptable. The service entrance cable may be upsized, and/or the distribution transformer, depending on which is the weakest link. If the TWH is fed from a subpanel which also feeds other loads, some of the drop may be on internal wiring which can also be upsized.

Utilities are usually obligated to maintain a voltage within +/- 5% of nominal, 120 volt, for 5 minute intervals for residential customers in urban areas. Tankless water heaters can operate longer than this 5 minute interval. A 50kVA transformer can provide enough power for one large TWH, but if that transformer feeds several residential customers, each with TWHs, it could also be overloaded.

The pros and cons of each type are shown in Figure 5, from a utility perspective.

Figure 1. Example Tankless Water Heater   

Figure 4. RMS voltage (min, ave, max) during same times periods as power and current graphs

Figure 3. RMS current (min, ave, max), with large current step changes and rapid fluctuations

Traditional Electric Water Heater

In Figure 5, several comparisons between traditional electric water heaters and tankless water heaters are shown.


Tankless water heaters are now being sold in most every home improvement store, online, and by many local plumbing companies. As more are making their way into residential and commercial customers, if power quality issues don’t exist now due to them, they will in the near future. Being aware of this especially troublesome load, and having the correct tools to evaluate the severity of the power quality impact is key to mitigating them.

In Figure 4, the RMS voltage min/ave/max stripchart is shown, again with the same A through I series of events. Looking at the spread between the min and max values in the voltage stripchart of Figure 4 in the “C” period, the variations are around 1.5%, and happening at l east once  every s tripchart interval. T his indicates a likely very noticeable, if not objectionable level of light flicker.

This model is available in sizes from 12 kW up to 36 kW from vendors such as Home Depot and even Amazon.com. They state for the 36 kW model: “A minimum of 300 amp total service to the residence is necessary, as the Tempra 36 has a power draw of 150 amps via 3 separate 60 amp breakers with 6 gauge copper wiring.” Obviously this will cause voltage problems with a 25 kVA transformer, and will even tax a 50 kVA transformer, especially if it’s shared among several other houses. The more “reasonable” 28.8 kW and 24 kW sizes are also likely to cause problems with a 25 kVA distribution transformer. These models range from $700-$850, and could be installed by an electrician without any prior notice to the utility.

In Figure 2, the power is at a minimum at A, when all hot water flow is shut off. The first step, B, shows when the sink’s faucet only is wide open. This is about 50 % of total capacity. The second major step, C, is when the sink and shower are fully opened as the real power reaches the 28 kW level. Then the shower is turned off in D, and then the sink is turned off in E, and power drops to its base level. The next response around 10 kW of draw, in F, is when the sink only is turned up until the water just get hot at around 25% flow. Then in G, the sink is turned to where it is barely open, followed to where the sink is turned up for hot water again in H, then everything is turned off in I.


Contributed by Cowles Andrus III, July 2015


In this whitepaper the basics of electric tankless water heaters (TWHs) are presented, along with their potential impact on voltage quality. Real-world graphs are discussed to illustrate the significant impact a TWH often has on residential power quality, and suggestions for mitigating these problems are given.

Introduction To Tankless Water Heaters

​Tankless water heaters (also known as instantaneous, continuous flow, on-demand, instant-on, flash, or inline water heaters) are characterized by the lack of any significant hot water storage. Instead, when presented with a demand for hot water, a TWH heats the incoming cold water in real time to the desired hot water temperature. The power required to raise a significant water stream (e.g. 2 or 3 gallons per minute) to a typical hot water temperature is very high. There two major types of tankless water heaters – electric and gas powered. The focus of this paper is the electric type.

A traditional electric water heater spreads the energy required to heat water over a longer time period. This increased time to heat the water results in lower peak power. With a TWH, this is not an option – significant amounts of water must be heated instantly, with a correspondingly large increase in peak power required. Intermediate water flows require continuous cycling of the large heater elements to maintain a correct average power. Both types of heating patterns can create significant voltage problems for any customer on the secondary of the distribution transformer.

These TWHs are marketed as instant hot water that never runs out, with consumer advantages in space savings (no large tank), less maintenance, and lower operating costs due to high efficiency. They’re readily available in home improvement stores, and even online at Amazon.com and eBay. Unfortunately their PQ disadvantages will be borne at least in part by electric utilities.

It goes without saying that instantaneous hot water come with the price of high instantaneous current. To determine the size of heater and the amount of power required to have hot water for a particular location several factors need to be considered. The first is the temperature of the incoming water. Depending on the location mainly associated with the location’s latitude, the shallow ground water temperature will vary from about 40 degrees F in the far north United States to over 70 degrees F in the far south states. Other factors include the desired final hot water temperature, and
the water flow rate. The unit that produced the data in this paper has several power levels, with a max of 28 kW, designed for several different flow rates. The lowest level was at a flow rate of 0 to 0.4 gallons per minute, GPM. The second level was from 0.4 GPM to 2.5 GPM and the highest level was from 2.5 GPM to 4 GPM. At 2.5 GPM the manufacturer claimed it could heat water up to 76 degrees over the incoming water temperature. It takes 2.42 Watt-hours to raise 1 gallon of water 1 degree F. To heat 2.5 gallons 76 degrees in one minute would require 2.42 Wh x 2.5 gallons x 76 degrees = 459 Wh X 60 =27,540 Watts. At 220 Volts this is over 125 amps of current. The load resistance of this particular tankless water heater would be about 1.76 ohms, as shown:

R = (E^2) / P = (220^2) / 27,540 = 1.76Ω

Some of these instantaneous water heaters can pull much more than the 28 kW used here. On a normal residential service, connecting a low impedance load of 1.76 ohms will usually cause a significant voltage drop, and if repeated, flicker and other issues associated with voltage sags on the transformer low voltage side. If more than one customer is on the same distribution transformer, all may see the flicker, not only the customer with the offending TWH. A typical TWH is shown in Figure 1.

Figure 2. Abrupt step power changes (B, C), and rapid power fluctuations (F, H)

Figure 3 shows the RMS current min/ave/maxstripchart graph, with the same A through I events as described for Figure 2. Th e large spread between the min, average, and max current traces indicates large amounts of essentially continuous current fluctuations, from 20 to over 130A.

Figure 5. Pros/Cons of each electric water heater type, from a utility perspective