What Size Electric Tankless Water Heater You Need
An electric tankless heater is sized by flow, not gallons. How to find your incoming water temperature, add up the showers that run at once and read the size off Stiebel Eltron's Tempra chart.
In this guide
A tank water heater is sized in gallons. An electric tankless heater has no tank, so the question is how much hot water it can make per minute, and that depends on how cold the water is when it comes in. Stiebel Eltron says the right Tempra size depends mostly on the temperature of the groundwater and the number of bathrooms in the house.
This guide walks through Stiebel Eltron's sizing chart for the Tempra Trend and Tempra Plus: find your incoming water temperature, add up what runs at the same time, then read off the size.
Two numbers decide the size
The first is peak flow: the gallons per minute (gpm) of all the hot taps and showers that run at the same time. The second is temperature rise, which is how far the heater has to lift the water from its incoming temperature to the temperature you use.
The same heater gives less flow when the rise is bigger. A Tempra 29 at 240 V delivers 4.36 gpm at 105°F from 59°F water, but 3.03 gpm from 39°F water, according to the flow table in Stiebel Eltron's manual.
Stiebel Eltron's chart counts a shower at 105°F, a kitchen sink at 105°F and a bathroom faucet at 90°F. Its note explains that a heater set to 120°F gets mixed with cold water at the tap to reach a safe, comfortable temperature.
Step 1: Find your incoming water temperature
The Tempra sizing guide has a U.S. map split into zones by average incoming water temperature, from 37°F in the coldest zone to 77°F in the warmest. The guide notes that these are averages and that the water temperature changes with the season and with the exact location.
Step 2: Add up what runs at the same time
Count the showers and taps that may run together at the busiest time of day. The ranges Stiebel Eltron gives are:
| Fixture | Flow range | Chart counts |
|---|---|---|
| Shower | 1.0–2.5 gpm | 1.5 gpm at 105°F |
| Bathroom faucet | 0.5–1.5 gpm | 0.5 gpm at 90°F |
| Kitchen sink | 1.0–2.2 gpm | 1 gpm at 105°F |
Two showers on the chart's numbers come to 3 gpm. With two 2.5 gpm showerheads, the same two showers can need up to 5 gpm. If your showers flow more than the chart assumes, Stiebel Eltron asks you to call or email them for sizing advice.
Step 3: Read the size off the chart
This table shows the hot water flow Stiebel Eltron's chart gives each size at 240 V, for four of the nine zones. The Trend and Plus of one size share the same row.
| Size | 37°F | 47°F | 57°F | 67°F |
|---|---|---|---|---|
| 12 | Sink only | 1.4 gpm | 1.7 gpm | 2.2 gpm |
| 15 | 1.45 gpm | 1.7 gpm | 2 gpm | 2.6 gpm |
| 20 | 1.9 gpm | 2.3 gpm | 2.7 gpm | 3.45 gpm |
| 24 | 2.4 gpm | 2.8 gpm | 3.4 gpm | 4.3 gpm |
| 29 | 2.9 gpm | 3.4 gpm | 4.1 gpm | 5.2 gpm |
| 36 | 3.6 gpm | 4.2 gpm | 5.1 gpm | 6.5 gpm |
The chart also uses icons to show how many showers and sinks each size can supply at once in each zone. The diagram below copies those counts for the same four zones.
In the 47°F zone the chart shows the 12 for one low-flow shower. In the 37°F zone it shows the 12 for a sink only. We left out the two warmest zones (72°F and 77°F): there the chart lists 6 to 8 gpm for the 29 and 36, while the highest flow in the manual's flow table is 6.61 gpm for any size.
Warning The chart is for 240 V. Stiebel Eltron says to go up one model on 208 V, where each size puts out less power: a Tempra 29 drops from 28.8 kW to 21.6 kW.
Check the panel before you pick
A bigger heater needs more electrical capacity. These are the manual's minimum recommended breakers and wire at 240 V for the three sizes we carry:
- Tempra 12: 12 kW, one 50 A double-pole breaker, one run of 8/2 AWG copper. Recommended main service: 100 A.
- Tempra 29: 28.8 kW, three 40 A double-pole breakers on three separate circuits, three runs of 8/2 AWG. Recommended main service: 200 A.
- Tempra 36: 36 kW, three 50 A double-pole breakers on three separate circuits, three runs of 8/2 AWG. Recommended main service: 300 A.
Stiebel Eltron gives the main service figures as minimums that depend on the total electric load at the site, and says to check local codes. The manual says the heater must be installed by a licensed electrician and plumber, so the electrician decides what your panel and wiring can carry. The full breaker table for all six sizes is in our Tempra Plus vs. Trend guide.
A larger heater also needs more flow to switch on: 0.37 gpm for the 12, 0.5 gpm for the 15, 20 and 24, and 0.77 gpm for the 29 and 36.
Which of our three sizes fits
- Tempra 12 Trend: one shower at a time in the 52°F through 72°F zones of the chart, and two in the 77°F zone. In colder zones the chart drops it to one low-flow shower or a sink.
- Tempra 29 Plus: two showers at once in every zone, including 37°F, two showers and a sink at 52°F and 57°F, and three showers from 62°F.
- Tempra 36 Plus: two showers in the 37°F zone, two showers and a sink at 42°F and 47°F, three showers from 52°F.
Both of our Plus models have Advanced Flow Control: when more hot water is drawn than the heater can supply, it reduces the flow slightly so the water stays at the set temperature. A Trend doesn't have it. Our Tempra Plus vs. Trend guide covers the other differences. More plumbing and water heating products are in HVAC, Plumbing & Water Heating.
FAQ
How many gpm do you need for two showers?
On Stiebel Eltron's numbers, 3 gpm: 1.5 gpm per shower at 105°F. Showerheads range from 1.0 to 2.5 gpm, so two high-flow heads can need up to 5 gpm. On the Tempra chart, the 29 and 36 supply two showers at once in every zone, the 24 from the 47°F zone, the 20 from 62°F, the 15 from 72°F and the 12 only in the 77°F zone.
Can you run two showers at once with a tankless water heater?
Yes, if the heater is big enough for your incoming water. Stiebel Eltron's chart shows two showers for the Tempra 29 and 36 even with 37°F water. A Tempra Plus that gets more demand than it can heat reduces the flow slightly to keep the temperature; a Trend doesn't, and the manual lists too much flow as one cause of water that isn't hot enough.
What size tankless do I need to replace a 50 gallon water heater?
Stiebel Eltron's sizing guide doesn't convert tank gallons to a tankless size. A tank stores hot water and may deliver more at once until it runs low; a tankless heater makes hot water as it flows, with no tank to run out. Size it by the showers and taps that run together and by your incoming water temperature, as in the steps above, and have an electrician check the panel first: for a Tempra 29 the manual recommends three 40 A double-pole breakers and a main service of at least 200 A.
Can an electric tankless water heater run a whole house?
Stiebel Eltron sells the Tempra line as whole-house heaters, and whether one size covers your house depends on how many fixtures run at once and how cold your water is. On its chart, a Tempra 36 at 240 V supplies two showers at once with 37°F water and three from 52°F. A Tempra 12 supplies one shower at a time in the 52°F through 72°F zones.
Figures come from Stiebel Eltron's Tempra sizing guide, the Tempra Trend & Tempra Plus installation and operating manual and the Tempra Plus and Tempra Trend product pages. Stiebel Eltron notes that actual inlet temperature and fixture flow change the results. For unusual cases, such as high-flow showers, Stiebel Eltron offers sizing help at 800-582-8423.