Do electric tankless water heaters work in Arizona homes?

A tankless water heater heats water only while hot water is flowing. Maricopa County's water heater guideline describes it as "an on-demand, instantaneous water heater" and explains that when a hot water faucet opens, cold water passes through the unit, where electric elements (or a gas burner) heat it on the way through. There is no tank to keep hot, so nothing is lost sitting between uses.

That design brings one rule that everything else follows from: a tankless unit trades flow for temperature. The county puts it plainly: "tankless water heaters will generate a temperature rise based on the flow rate demanded." If you ask for more water than the unit can heat, the water doesn't run out. It just gets cooler, or the unit cuts the flow to hold the temperature. The county warns that when a tankless heater is undersized, "The temperature at a hot water fixture may remain the same, but the flow could slow to a trickle."

It depends on the unit's power and how much you need to warm the water. Maricopa County's guideline gives the conversion: "1,000 Watts (1kW) will heat 4.1 gallons of water at a 100°F rise in temperature" each hour. Using that figure, here is what two common whole-house sizes can deliver:

These are our calculations from the county's conversion factor, not a manufacturer's rating. Real units lose a little to efficiency. Always check the flow chart for the exact model.

Now compare that with what fixtures use. Under the plumbing code Phoenix uses, a new shower head can flow up to 2.0 gallons per minute and a private bathroom faucet or kitchen sink up to 1.5. A shower mixes in some cold water, so it draws less than its full flow from the heater. Still, the math is tight. A 36 kW unit comfortably runs one shower plus a sink. Two showers at once push it to its limit.

For comparison, the county says "Typical tankless water heaters can provide hot water at a rate of 3 to 6 gallons per minute, with a 60°F water temperature rise," and that "Gas-fired tankless water heaters usually produce significantly higher flow rates than electric tankless water heaters."

Yes, warm incoming water helps a tankless heater deliver more hot water. The warmer the water coming in, the less the unit has to warm it, and the more flow it can give you at the same power. Stiebel Eltron, a major electric tankless maker, says its larger units are "sized according to the maximum hot water temperature desired, the temperature of the incoming ground water, and the number of bathrooms or appliances." It sells some compact models specifically "for smaller home water heating in warm climates."

For planning, Maricopa County has water heaters sized assuming "an incoming water temperature of 60°F" and a 60°F rise, "unless specific temperature data are available." In practice, your own water varies by season, and it's coldest in winter. Size for the coldest month you'll use it in, not for July. If you want a real number, run the cold tap for a minute on a winter morning and check it with a kitchen thermometer.

It needs heavy wiring and enough spare capacity in your electrical service. Stiebel Eltron's installation manual for its Tempra whole-house line shows the scale:

The manual says these main service minimums depend "on total electric load at the installation site," and requires copper wire sized to keep voltage drop under 3%. Many homes have a 200 amp service or less, which already carries the air conditioner, range, and dryer. Adding a large electric tankless unit can mean a service upgrade, and that work belongs to a licensed electrician and the utility.

The plumbing code Phoenix uses also says electric water heaters "shall conform to the requirements of this code and provisions of NFPA 70," the National Electrical Code, so the electrical side has its own code to meet.

### Can a tankless heater miss a low-flow faucet?

It can. An electric tankless unit turns on only above a minimum flow. Stiebel Eltron's manual lists the minimum flow to activate its Tempra units at 0.37 gallons per minute for the smallest model, 0.5 for the mid sizes, and 0.77 for the 29 and 36 kW models. A bathroom faucet opened just a crack may not reach that, so the water never warms. Opening the tap a little further fixes it.

Choose by what your home already has and how much hot water you use at once.

• Choose electric tankless for a single sink or a far bathroom, a small home or casita, or a house with no gas service and plenty of spare electrical capacity.
• Choose gas tankless for a busy household that runs two showers at once, when the home already has gas service. The county notes gas units "usually produce significantly higher flow rates," and they don't need a large electrical service.
• Consider a tank when you need lots of hot water for a short time and don't want a service upgrade. See our page on gas vs electric water heaters.

A common middle ground is a small electric tankless unit at one fixture, paired with an existing tank for the rest of the house. That is a point-of-use setup. It cuts the wait for hot water at a sink far from the main heater. The plumbing code Phoenix uses limits the hot water pipe run from the source to a fixture to 50 feet, and a point-of-use heater is one way to meet it.

### Does hard water matter for electric tankless?

Yes. Scale from hard water builds up on the heating elements and inside the unit. Descale on the maker's schedule. Our page on how often to descale a tankless water heater in Phoenix covers it, and what size water heater you need walks through adding up your fixtures.

HQ Plumbing & Air installs tank, tankless, and point-of-use water heaters across the Phoenix area, with free estimates and upfront pricing. See our water heater service.

Electric tankless works here. Check the panel before you buy, and size it for January.