Electric Tankless Hot Water Heater: Complete Guide for Homeowners

The average storage tank keeps 40 to 50 gallons of water hot around the clock, even at 3 a.m. when nobody is awake to use it. An electric tankless hot water heater flips that logic on its head by heating water only when you open a faucet. That single change can trim your water-heating bill and free up a closet, but it also demands serious electrical capacity that trips up a lot of first-time buyers. This guide walks you through how these units work, what they cost, and how to size one correctly.

How an Electric Tankless Unit Works

When you turn on a hot tap, a flow sensor inside the unit detects moving water and switches on a bank of heating elements. Cold water passes over those elements and comes out hot, usually within two to three seconds. There is no tank, no standby loss, and no reheating cycle.

Capacity is measured in gallons per minute (GPM) at a given temperature rise. Temperature rise is the difference between your incoming groundwater temperature and your target outlet temperature. In a warm climate like Florida, groundwater might arrive at 70°F, so reaching a 105°F shower needs only a 35°F rise. In Minnesota, that same water can enter at 40°F, forcing the unit to work far harder for the same shower.

The Electrical Reality Most People Miss

Here is where projects go sideways. A whole-house electric tankless model like the Stiebel Eltron Tempra 24 Plus or the Rheem RTEX-24 pulls 24 kilowatts. At 240 volts, that is 100 amps of demand spread across two or three double-pole breakers. Many homes have a 100- or 150-amp main service panel, and adding a 100-amp appliance load can require a panel upgrade.

Before you fall in love with a model, do this: check your main breaker rating and count your available slots. A licensed electrician should run a load calculation. Budget $500 to $2,000 for new wiring, breakers, and possibly a subpanel. In older homes, the panel upgrade can cost more than the heater itself.

Sizing the Unit Correctly

Undersizing is the number one complaint with electric tankless heaters. Add up the fixtures you expect to run at once, then match a unit that hits that GPM at your local temperature rise.

• A standard low-flow showerhead uses about 2.0 to 2.5 GPM
• A kitchen faucet uses roughly 1.5 GPM
• A bathroom sink uses about 1.0 GPM

If you want two showers running together in a cold climate, you likely need 8 GPM at a 60°F rise, which pushes you toward a 27 to 36 kW unit. Point-of-use models rated at 3 to 5 kW work well for a single sink or a small cabin, but they will never keep up with a full bathroom.

What It Costs to Buy and Install

Unit prices vary widely by capacity. Small point-of-use heaters run $150 to $300. A capable whole-house model such as the Rheem RTEX-27 or EcoSmart ECO 27 sells for $500 to $750. Installation labor typically adds $300 to $800 for a straightforward swap, and electrical upgrades can add another $500 to $2,000.

Total installed cost for a whole-house electric tankless commonly lands between $1,200 and $3,500. Compare that to $900 to $1,800 for a standard electric tank, and you can see the tankless premium is mostly electrical work, not the appliance.

Running Costs and Energy Savings

Because there is no standby loss, an electric tankless model is roughly 98 to 99 percent efficient. The U.S. Department of Energy estimates tankless units can be 24 to 34 percent more efficient than tanks for homes using less than 41 gallons a day. In dollar terms, that is often $75 to $150 a year for a typical household.

Keep expectations realistic. Electricity costs more per BTU than natural gas in most of the country, so the operating savings come from eliminated standby loss, not from cheap fuel. If you already have gas service, a gas tankless will usually cost less to run.

Installation Considerations Beyond Wiring

Electric tankless heaters are small, often no bigger than a briefcase, so they mount on a wall inside a closet, garage, or utility room. They need no venting, which is a major advantage over gas models that require stainless steel exhaust.

Water quality matters more than most owners expect. Hard water above about 7 grains per gallon will scale the heating elements and copper channels. Install a whole-house softener or a scale filter, and flush the unit annually with white vinegar or a descaling pump kit. Many manufacturers require this maintenance to keep the warranty valid.

Is It Right for Your Home?

An electric tankless heater shines in a few clear situations. It is ideal for homes without gas service, for additions and remote bathrooms where running a gas line is impractical, and for anyone who wants to reclaim closet space. It also suits vacation properties that sit empty for weeks, since there is no tank losing heat while you are away.

It is a weaker choice if your electrical panel is already maxed out, if you live in a very cold climate with high simultaneous demand, or if you have cheap natural gas available. In those cases, either a gas tankless or a high-efficiency heat pump water heater may deliver a better return.

Comparing Popular Models

A few units come up again and again for good reason. The Stiebel Eltron Tempra Plus series uses advanced flow control to automatically throttle water flow when demand outpaces capacity, so it never delivers lukewarm water, only slightly slower flow during peak use. It is the premium choice and carries a strong reputation for longevity.

The Rheem RTEX line offers excellent value with external adjustable digital thermostats and easy element replacement, making service simple down the road. EcoSmart’s ECO series is a budget-friendly self-modulating option that fits many mid-size homes. For a single bathroom or a wet bar, the small Bosch Tronic 3000 point-of-use unit mounts under a sink and delivers near-instant hot water without a recirculation loop. Whichever you choose, verify the amperage draw against your panel capacity before ordering.

Maintenance to Extend Its Life

A well-maintained electric tankless heater routinely lasts 15 to 20 years, well beyond a tank’s typical decade. The single most important task is descaling, especially in hard-water regions. Mineral scale coats the heating elements, forcing them to work harder and eventually burning them out prematurely.

Once a year, shut off the power and water, connect a small submersible pump and two hoses to the service valves, and circulate several gallons of white vinegar or a commercial descaler through the unit for 45 minutes to an hour. Flush with clean water afterward. Also clean the inlet screen filter every few months, since sediment can restrict flow and trigger error codes. Keep the area around the unit dry and accessible, and check the electrical connections periodically for any signs of heat discoloration, which signals a loose terminal that needs attention.

Common Mistakes to Avoid

The three errors that cause the most regret are all preventable. First, undersizing for your coldest month leaves you disappointed every winter, so always size for peak simultaneous demand at your lowest groundwater temperature. Second, ignoring the electrical load leads to nuisance breaker trips or an expensive surprise panel upgrade discovered mid-project. Third, skipping the descaling maintenance in hard water voids many warranties and shortens the unit’s life dramatically.

Avoid these, install a scale filter or softener where water is hard, and confirm your wire gauge matches the amperage before you power anything up. A little planning on the front end prevents almost every headache owners report.

Final Advice Before You Buy

Match the unit to your coldest-month groundwater temperature, not the annual average, or you will be disappointed every winter. Have an electrician confirm your service can handle the load before you order anything. Choose a model with a self-modulating design and a 7- to 10-year element warranty, and plan on annual descaling. Do those four things and an electric tankless hot water heater will give you endless hot water and a smaller utility bill for well over a decade.