Tankless Water Heater Sizing Chart

Every tankless listing leads with a maximum GPM figure, and that figure is almost always measured at a small temperature rise. A unit advertised at 9.5 GPM may deliver 5.5 GPM at the rise your house actually needs in February. Both numbers are real. Only one is useful to you.

Step 1: your temperature rise

Rise = target outlet temperature minus inlet temperature. Most households want 120°F at the tap. Inlet temperature is groundwater temperature, and it varies hugely by latitude and by season.

These are planning figures, not measurements of your street. If you want the real number, run the cold tap for two minutes in January and put a thermometer in the stream. That five-minute exercise is more accurate than any chart.

Step 2: your simultaneous flow

Add up the fixtures that will genuinely run at the same time. Not every fixture in the house — the ones that overlap on a real morning. Most households are honest at two showers plus a sink.

Step 3: the chart

The GPM requirement does not change with climate — the shower uses the same water. What changes is which unit can deliver it. That is the whole point, and it is why the bottom row is the row that matters.

Real published rise-versus-flow data

Stiebel Eltron publishes a full rise-versus-flow table for the Tempra Plus, which makes it the easiest unit in this category to size honestly. Here it is at both supply voltages:

On the gas side, Rheem publishes 5.5 GPM at a 70°F rise for the RTGH-95DVLN against a 9.5 GPM maximum at a 35°F rise. Same unit, same day, nearly double the flow — because the second figure is measured at half the rise. When you compare tankless units, compare them at the same rise or you are not comparing anything.

Quick picks

Tap any row to read the full assessment below, or go straight to Amazon from the right-hand column. We earn a commission on qualifying purchases at no extra cost to you.

Ranked picks with what each one is best for, our score and a link to check the current price on Amazon.

#	Product	Best for	Price
1	Rheem Prestige RTGH-95DVLNThe condensing gas tankless most houses should start with: 5.5 GPM at a 70°F rise covers two showers in a cold-inlet climate, and the 12-year residential heat exchanger warranty is the longest on this list.	Most whole-house gas jobs	Check price on Amazon No live price feed on this site, so we show none. #ad
2	Stiebel Eltron Tempra 24 PlusThe electric tankless to buy if you are going to buy one at all: it publishes its full rise-versus-flow curve, and the Advanced Flow Control throttles flow instead of letting the temperature sag.	Whole-house electric, warm inlet	Check price on Amazon No live price feed on this site, so we show none. #ad
3	EcoSmart ECO 27The biggest electric unit most homes can actually feed: 27 kW across three 40 A breakers, 6.6 GPM maximum, and a lifetime limited warranty on the unit.	Maximum electric output	Check price on Amazon No live price feed on this site, so we show none. #ad

Units with honest published figures

Pick 1 — Most whole-house gas jobs

Rheem Prestige RTGH-95DVLN

The condensing gas tankless most houses should start with: 5.5 GPM at a 70°F rise covers two showers in a cold-inlet climate, and the 12-year residential heat exchanger warranty is the longest on this list.

Published specifications for the Rheem Prestige RTGH-95DVLN

Fuel	Natural gas (LP variant sold separately)
Gas input	Up to 199,900 BTU/h
Flow at a 70°F rise	5.5 GPM
Maximum flow	9.5 GPM at a 35°F rise
Efficiency	Up to 0.93 UEF, stainless condensing heat exchanger
Minimum activation flow	0.4 GPM
Venting	Up to 150 ft of 3-inch PVC; 60 ft of 2-inch PVC
Dimensions	18-1/2 in W x 9-3/4 in D x 27-1/2 in H
Warranty	12-year heat exchanger (residential), 5-year parts, 1-year labor

What is good

• +The 70°F-rise figure is published, so you can size it honestly instead of guessing from the headline number
• +Condensing design vents in PVC, which is far cheaper to run than stainless category III pipe
• +0.4 GPM activation means a trickle at a bathroom faucet still fires the burner
• +Long PVC vent runs let you put it where the gas line already is

What is not

• −Needs a gas line sized for nearly 200,000 BTU/h — many existing 1/2-inch runs are not
• −Condensate has to go somewhere, which means a drain or a condensate pump
• −Annual descaling is not optional on hard water

Skip it if: Skip it if your gas meter and line cannot carry 199,900 BTU/h. Upsizing the gas line is often the single largest cost in a tankless conversion, and no amount of unit quality fixes a starved burner.

Check price on Amazon

No live price feed on this site, so we show none. #ad

Specifications above from Rheem RTGH series specification sheet (PDF); Pace Supply listing, RTGH-95DVLN, 199 kBtu/hr. We have not tested this unit ourselves.

Pick 2 — Whole-house electric, warm inlet

Stiebel Eltron Tempra 24 Plus

The electric tankless to buy if you are going to buy one at all: it publishes its full rise-versus-flow curve, and the Advanced Flow Control throttles flow instead of letting the temperature sag.

Published specifications for the Stiebel Eltron Tempra 24 Plus

Output at 240 V	24 kW
Output at 208 V	18 kW
Breakers	2 x 50 A double-pole
Wire	2 x 6 AWG copper
Current draw	100 A
Minimum electrical service	150 A
Rise at 240 V	54°F at 3.0 GPM; 73°F at 2.25 GPM; 37°F at 4.5 GPM
Rise at 208 V	41°F at 3.0 GPM; 54°F at 2.25 GPM; 27°F at 4.5 GPM
Maximum flow	8 GPM

What is good

• +Publishes rise at four flow rates on both voltages, which is the only honest way to size an electric tankless
• +Advanced Flow Control cuts flow to hold the set temperature rather than delivering lukewarm water
• +No venting, no gas line, no condensate
• +Small enough to go in the cabinet it replaced

What is not

• −100 A of draw and a 150 A minimum service rules out a lot of older houses
• −At 208 V it loses a quarter of its output, and plenty of homes are nearer 208 than 240
• −Running cost follows the electric rate, which is usually worse than gas per BTU

Skip it if: Skip it if your inlet water drops to 45°F in winter and you want two showers at once. At a 73°F rise it delivers 2.25 GPM — one shower, nothing else. Cold-inlet climates are where electric tankless disappoints people.

Check price on Amazon

No live price feed on this site, so we show none. #ad

Specifications above from Stiebel Eltron Tempra Plus technical specifications (PDF); Stiebel Eltron Tempra Plus submittal sheet (PDF). We have not tested this unit ourselves.

Pick 3 — Maximum electric output

EcoSmart ECO 27

The biggest electric unit most homes can actually feed: 27 kW across three 40 A breakers, 6.6 GPM maximum, and a lifetime limited warranty on the unit.

Published specifications for the EcoSmart ECO 27

Output	27 kW at 240 V, single phase
Current draw	113 A
Breakers	3 x 40 A
Wire	3 x 8 AWG
Maximum flow	6.6 GPM
Minimum activation flow	0.3 GPM
Dimensions	17 in x 17 in x 3-5/8 in
Weight	14.7 lb
Connections	3/4 in NPT
Warranty	Lifetime limited

What is good

• +Highest output of any single electric unit here, so it holds temperature at higher flow
• +Lifetime limited warranty on the unit is unusual at any price
• +3-5/8 inches deep — it genuinely disappears on a wall
• +0.3 GPM activation flow is the lowest on this page

What is not

• −113 A of draw needs three circuits and, in practice, a 200 A service
• −The published flow is a maximum, not a rise-qualified figure — you have to do the rise math yourself
• −No buffer, so a sudden flow change is felt at the shower head

Skip it if: Skip it if your panel is 100 A or 125 A. Three 40 A circuits and 113 A of draw is a service upgrade, and a service upgrade usually costs more than the entire water heater.

Check price on Amazon

No live price feed on this site, so we show none. #ad

Specifications above from EcoSmart ECO 27 technical data (BIMobject). We have not tested this unit ourselves.

The electric service problem

Electric tankless sizing runs into the panel before it runs into the water. The real numbers from manufacturer documentation:

• Stiebel Eltron Tempra 24 Plus — two 50 A double-pole breakers, two runs of 6 AWG copper, 100 A of draw, minimum 150 A service.
• EcoSmart ECO 27 — three 40 A breakers, three runs of 8 AWG, 113 A of draw.
• Rheem RTEX-24 — three 40 A breakers and a 200 A service requirement.
• EcoSmart ECO 11 — one 60 A breaker, 6 AWG, 54 A of draw. This is the one that fits an existing panel, and it is a single-fixture unit.

If your house has a 100 A or 125 A service, whole-house electric tankless is a service upgrade before it is a water heater, and a service upgrade usually costs more than the entire appliance. Breaker and wire size has the detail.