Electric Tankless Water Heaters: Amps, Breakers, and Real Flow Rates
An electric tankless water heater for a whole house draws 100 to 150 amps on two or three double-pole breakers and delivers about 2.6 to 3.9 gpm in cold-groundwater regions.
Key Takeaways
- Flow follows one formula: gpm = kW x 3,412 / (500 x temperature rise). It matches Stiebel Eltron’s published Tempra chart within 0.1 gpm.
- Stiebel Eltron’s 36 kW Tempra 36 Plus draws 150 A on three 50 A double-pole breakers and requires 300 A electric service.
- EcoSmart’s 27 kW ECO 27 draws 113 A on three 40 A double-pole breakers with three 8 AWG copper runs, and EcoSmart recommends a 200 A panel.
- Groundwater sets the ceiling: the Tempra 36 Plus gives 3.9 gpm of 105°F water from 42°F groundwater and 7.5 gpm from 72°F groundwater (Stiebel Eltron sizing guide).
- On 208 V service a 240 V heater loses 25 percent of its output; Stiebel Eltron’s 24 kW model drops to 18 kW.
- At EIA’s August 2026 residential average of 18.16 cents per kWh, a 10 minute, 2.0 gpm shower costs about 56 cents to heat in a cold-water region.
How does an electric tankless water heater turn kilowatts into gallons per minute?
An electric tankless water heater makes hot water at a fixed rate set by its kilowatts and the temperature rise you ask for. One kilowatt equals 3,412 Btu per hour, and heating one gallon per minute by 1°F takes 500 Btu per hour (8.33 lb per gallon x 60 minutes). So gpm = kW x 3,412 / (500 x rise). Double the rise and the flow halves.
We checked the formula against the maker’s own numbers. Stiebel Eltron’s Tempra sizing guide rates the 36 kW unit at 3.9 gpm when it heats 42°F groundwater to 105°F, a 63°F rise. The formula gives 3.90 gpm. At a 33°F rise the guide says 7.5 gpm and the formula says 7.44.
How many amps and breakers does an electric tankless water heater need?
A whole-house electric tankless water heater needs 100 to 150 amps split across two or three double-pole breakers, each feeding its own copper cable to the unit. A 24 kW model takes two 50 A breakers, a 27 to 29 kW model takes three 40 A breakers, and a 36 kW model takes three 50 A breakers.
| Model | kW at 240 V (208 V) | Breakers, double-pole | Total draw | Copper wire runs | Minimum service |
|---|---|---|---|---|---|
| Stiebel Eltron Tempra 12 Plus | 12 (9) | 1 x 50 A | 50 A | 1 x 6/2 AWG | 100 A |
| Stiebel Eltron Tempra 15 Plus | 14.4 (10.8) | 2 x 30 A | 60 A | 2 x 10/2 AWG | 100 A |
| Stiebel Eltron Tempra 20 Plus | 19.2 (14.4) | 2 x 40 A | 80 A | 2 x 8/2 AWG | 125 A |
| Stiebel Eltron Tempra 24 Plus | 24 (18) | 2 x 50 A | 100 A | 2 x 6/2 AWG | 150 A |
| EcoSmart ECO 27 | 27 | 3 x 40 A | 113 A | 3 x 8 AWG (75°C) | 200 A recommended |
| Stiebel Eltron Tempra 29 Plus | 28.8 (21.6) | 3 x 40 A | 120 A | 3 x 8/2 AWG | 200 A |
| Stiebel Eltron Tempra 36 Plus | 36 (27) | 3 x 50 A | 150 A | 3 x 6/2 AWG | 300 A |
Sources: Stiebel Eltron Tempra technical specifications and the EcoSmart ECO 27 product page. Stiebel Eltron requires copper sized for under 3 percent voltage drop.
Three double-pole breakers fill six panel slots, which a crowded panel may not have.
Many online guides multiply the draw by 1.25 to pick a breaker. Stiebel Eltron sizes its breakers at 100 percent of load because it treats tankless heaters as a non-continuous load, and tells installers to check local code. Confirm with your inspector before the rough-in.
What electrical service does an electric tankless water heater require?
Stiebel Eltron sets minimum main service by model: 100 A for the 12 and 15 kW units, 125 A for 19.2 kW, 150 A for 24 kW, 200 A for 28.8 kW and 300 A for 36 kW. EcoSmart recommends a 200 A panel for its 27 kW ECO 27. Those minimums assume the rest of the house leaves room.
The trap is the 36 kW class: the size northern homes need is the one that demands 300 A service.
Our tankless water heater cost guide puts a whole-home electric unit at about $1,000 to $2,500 installed and a service upgrade to 200 A at roughly $1,300 to $3,000 in many areas.
- Read the main breaker rating at the top of the panel (100, 150, 200 A).
- Count open full-size spaces: you need two for every double-pole breaker.
- Match the model’s minimum service from the table above.
- Get an electrician’s load calculation covering range, dryer, HVAC and EV charger.
How cold is your groundwater, and how much flow does that leave?
Groundwater temperature decides how hard an electric tankless water heater has to work, and it varies more than most buyers expect. Stiebel Eltron’s sizing map splits the US into four bands: 42°F, 52°F, 62°F and 72°F. The warmest band nearly doubles the flow of the coldest.
On Stiebel Eltron’s map, the 42°F band covers the northern Plains, the upper Midwest, the Rockies, northern New England and Alaska. The 52°F band runs through the Great Basin, the central Plains and the Ohio Valley. The 62°F band covers Texas, Oklahoma and most of the Southeast and coastal California. The 72°F band hugs the Gulf Coast, South Florida, the southern California desert and Hawaii.
| Groundwater band | Rise to 105°F | Tempra 24 Plus | Tempra 29 Plus | Tempra 36 Plus |
|---|---|---|---|---|
| 42°F | 63°F | 2.6 gpm | 3.1 gpm | 3.9 gpm |
| 52°F | 53°F | 3.1 gpm | 3.7 gpm | 4.6 gpm |
| 62°F | 43°F | 3.8 gpm | 4.6 gpm | 5.7 gpm |
| 72°F | 33°F | 5.0 gpm | 6.0 gpm | 7.5 gpm |
Flows are Stiebel Eltron’s published maximums at 240 V for 105°F outlet water, the temperature you actually shower at. Size for February groundwater, not August.
What flow do electric tankless water heaters deliver at a 35, 50 and 70 degree rise?
At a 70°F rise, which a northern home sees when it heats water toward 110 to 120°F, the largest residential electric tankless water heaters make only 2.3 to 3.5 gpm. At a 35°F rise the same units make 4.7 to 7.0 gpm. The table below converts each model’s rated kW with the formula verified against Stiebel Eltron’s chart.
| Model (kW at 240 V) | 35°F rise | 50°F rise | 70°F rise |
|---|---|---|---|
| Tempra 15 Plus (14.4) | 2.81 gpm | 1.97 gpm | 1.40 gpm |
| Tempra 20 Plus (19.2) | 3.74 gpm | 2.62 gpm | 1.87 gpm |
| Tempra 24 Plus (24) | 4.68 gpm | 3.28 gpm | 2.34 gpm |
| EcoSmart ECO 27 (27) | 5.26 gpm | 3.68 gpm | 2.63 gpm |
| Tempra 29 Plus (28.8) | 5.62 gpm | 3.93 gpm | 2.81 gpm |
| Tempra 36 Plus (36) | 7.02 gpm | 4.91 gpm | 3.51 gpm |
EcoSmart’s own sizing chart, built for 105°F outlet water, lists about 2.8 gpm for the ECO 27 at 40°F inlet and 7.4 gpm at 80°F inlet. The formula gives 2.83 and 7.37.
Two other limits sit at the low end of the flow range. The Tempra 29 and 36 need 0.77 gpm to switch on and the smaller Tempras need 0.37 to 0.50 gpm, so a 0.5 gpm bathroom aerator may not fire the big units. Stiebel Eltron caps inlet water at 131°F, which limits solar preheat setups.
How do you size an electric tankless water heater for your home?
Size an electric tankless water heater by adding the gpm of every hot fixture that runs at once, then finding the model that delivers that flow at your winter rise. Required kW = gpm x 500 x rise / 3,412. Our tankless sizing by fixture count guide covers gas units.
- Read the flow stamped on each showerhead and aerator. EPA WaterSense showerheads flow no more than 2.0 gpm.
- Add the fixtures you run together. Two WaterSense showers equal 4.0 gpm.
- Find your groundwater band on the maker’s map and subtract it from 105°F for showers.
- Compute kW, then pick the next model up.
- Confirm breakers, wire and service against the electrical table.
Worked example, Minneapolis (42°F band): two 2.0 gpm showers need 4.0 gpm at a 63°F rise, or 36.9 kW. Even the 36 kW Tempra 36 Plus falls short at 3.9 gpm. Swap to 1.5 gpm heads (3.0 gpm total) and 27.7 kW is enough, so the 28.8 kW Tempra 29 Plus at 3.1 gpm works.
Worked example, Atlanta (62°F band): the same 4.0 gpm at a 43°F rise needs 25.2 kW. The Tempra 29 Plus delivers 4.6 gpm there on 200 A service, while the Tempra 24 Plus tops out at 3.8 gpm.
What does a 208-volt supply do to an electric tankless water heater?
A 240 V electric tankless water heater wired to 208 V loses about 25 percent of its heating power, because resistance-element output falls with the square of voltage. Stiebel Eltron’s spec sheet shows the 24 kW Tempra producing 18 kW at 208 V and the 36 kW unit producing 27 kW. Many apartments and condos run 208 V.
Stiebel Eltron’s fix is simple: step up one model size on 208 V service. In the 42°F band that turns a Tempra 24 Plus (2.6 gpm at 240 V) into roughly a 1.95 gpm heater at 208 V, which is less than one WaterSense shower and a sink.
What does an electric tankless water heater cost to run?
Energy per gallon is fixed by physics: kWh = gallons x 8.33 x rise / 3,412. An electric tankless water heater saves only the standby loss a tank wastes, so its bill tracks how much hot water you use and your local power price. A 10 minute, 2.0 gpm shower with a 63°F rise takes about 3.1 kWh.
At EIA’s August 2026 residential average of 18.16 cents per kWh, that shower costs about 56 cents. A household drawing 40 gallons a day at a 68°F rise (52°F groundwater to 120°F) uses about 2,424 kWh a year, or roughly $440 at the national average.
| State (EIA, Aug 2026) | Residential cents per kWh | Annual cost, 2,424 kWh example |
|---|---|---|
| Louisiana | 12.87 | about $312 |
| Florida | 14.99 | about $363 |
| US average | 18.16 | about $440 |
| California | 29.75 | about $721 |
| Massachusetts | 30.48 | about $739 |
The Department of Energy’s Energy Saver page says demand heaters can be 24 to 34 percent more efficient than storage tanks in homes using 41 gallons or less a day.
Is an electric tankless water heater worth it?
An electric tankless water heater is worth it when three things line up: groundwater in the 62°F or 72°F band, a panel that already meets the model’s service minimum, and no more than two showers at once. Miss any one and a tank, heat pump water heater or gas tankless often costs less over its life. Our tankless vs tank comparison runs the full trade-off.
- Good fit: a Florida or Gulf Coast home on 200 A service with a Tempra 24 Plus or ECO 27 class unit.
- Good fit: a single remote bath or sink, where a point-of-use water heater on one 30 to 50 A breaker skips the long pipe run.
- Poor fit: a three-bath house in the 42°F band, where 36 kW still gives 3.9 gpm and needs 300 A service.
- Poor fit: a 100 A panel already feeding an electric range, dryer and central air.
How we sourced this
Specs come from Stiebel Eltron’s Tempra technical specifications and sizing guide and EcoSmart’s ECO 27 product page and sizing chart. Electricity prices come from EIA Electric Power Monthly Table 5.6.A (August 2026). The flow formula was checked against both makers’ published charts. Flows at 35, 50 and 70°F rises are our calculations from rated kW. Rheem RTEX and Bosch Tronic models were left out because we could not open their current spec sheets; retail prices were left out because they change weekly. Reviewed October 2026.
Frequently asked questions
How many amps does an electric tankless water heater use?
A whole-house electric tankless water heater draws 100 to 150 amps at 240 volts. Stiebel Eltron lists 100 A for its 24 kW Tempra 24 Plus, 120 A for the 28.8 kW Tempra 29 Plus and 150 A for the 36 kW Tempra 36 Plus. EcoSmart lists 113 A for the 27 kW ECO 27. Small single-sink units draw 30 to 50 A.
Can a 200 amp service run an electric tankless water heater?
Often yes, up to about 29 kW. Stiebel Eltron requires 150 A service for the Tempra 24 Plus and 200 A for the Tempra 29 Plus, and EcoSmart recommends a 200 A panel for the ECO 27. The 36 kW Tempra 36 Plus needs 300 A service. An electrician should still run a load calculation.
Is an electric tankless water heater worth it?
It is worth it where groundwater runs 62 degrees or warmer, the panel already has the capacity, and the home needs one or two showers at a time. In 42 degree groundwater even a 36 kW unit makes only 3.9 gpm of 105 degree water, and a service upgrade can erase the savings over a tank.
Can an electric tankless water heater supply a whole house?
In warm regions, yes. Stiebel Eltron rates its Tempra 36 Plus at 7.5 gpm of 105 degree water with 72 degree groundwater, enough for three showers. With 42 degree groundwater the same unit gives 3.9 gpm, roughly two low-flow showers.
What size breaker does an electric tankless water heater need?
Use the breaker count and size printed in the manual, not a rule of thumb. The Tempra 24 Plus takes two 50 A double-pole breakers, the Tempra 29 Plus and EcoSmart ECO 27 take three 40 A double-pole breakers, and the Tempra 36 Plus takes three 50 A double-pole breakers. Each breaker feeds its own copper cable run to the heater.
Does an electric tankless water heater use a lot of electricity?
It draws a lot of power for short bursts but uses the same energy to heat each gallon as any electric resistance heater, minus standby loss. A 10 minute shower at 2.0 gpm with a 63 degree rise takes about 3.1 kWh, or 56 cents at the August 2026 US residential average of 18.16 cents per kWh from EIA.
Can I install an electric tankless water heater myself?
The plumbing is simple, two 3/4 inch NPT connections on a Tempra, but the wiring is a 100 to 150 amp job with two or three new breakers and 6 or 8 AWG copper runs. Many areas require a permit and a licensed electrician for that wiring.
