Heat Pump vs Tankless Water Heater

This comparison gets framed as "which is the better modern water heater", which is the wrong question. They are answers to different problems, and the useful first step is deciding which problem you have.

Which complaint are you fixing?

Why electric tankless is not the efficiency answer

A point worth making plainly, because the marketing blurs it. An electric tankless unit eliminates standby loss, which is a real but modest saving. It is still resistance heating, so the energy per gallon of hot water is the same as an electric tank's.

A heat pump water heater moves heat rather than making it, which is why it reaches a UEF above 1.0 at all. If running cost on electricity is the goal, the heat pump is a different order of saving and electric tankless is not competitive with it.

Where the heat pump wins

• Running cost on electricity, by roughly four to one against resistance.
• Modest electrical demand — a 240 V 30 A circuit, or 120 V 15 A on the plug-in version.
• No gas line, no venting, no condensate from combustion (though it does make condensate from dehumidification).
• Rebate eligibility, which is widespread for heat pump units and rare for tankless.
• It still behaves like a tank, so there is stored hot water and no flow-rate ceiling at the shower head.

Where tankless wins

• It cannot run out. Five showers in a row. A heat pump's 67-gallon first hour is good but finite.
• Zero standby loss, where a heat pump still keeps 50 gallons warm.
• Floor space. Wall-hung against a cylinder.
• Up to 20 years of service life against a 10-year heat pump warranty.
• No air volume requirement, no noise, no cooling of the space. A tankless unit can go in the closet a heat pump cannot.
• High simultaneous flow on gas — 5.5 GPM at a 70°F rise serves two showers at once.

One of each, and the electric tankless for contrast

Pick 1 — Most heat pump installs

Rheem ProTerra 50 gal hybrid

The heat pump water heater to start with: a 3.88 UEF, a 67-gallon first-hour rating, and a 37°F lower operating limit that is the widest ambient range in its class — which is the specification that decides whether it works in a cold garage.

Published specifications for the Rheem ProTerra 50 gal hybrid

Capacity	50 gallons
Efficiency	3.88 UEF; up to 4.07 UEF advertised on 30 A models
First-hour rating	67 gallons
Recovery	27 gallons per hour
Heat pump capacity	4,200 BTU/h
Backup element	5,000 W
Voltage	208 - 240 V
Ambient operating range	37 - 145°F
Cold-climate rating	Meets Northern Climate Specification Tier 4
Leak protection	Integrated detection with factory-fitted auto shutoff; leaks limited to no more than 20 oz
Warranty	10-year limited

What is good

• +A 37°F lower limit means the compressor keeps running in an unheated garage through most of a northern winter
• +3.88 UEF is roughly four times a resistance tank, so the running cost difference is not marginal
• +67-gallon first-hour rating is better than most 50-gallon resistance tanks
• +EcoNet control lets you force high-demand mode before guests arrive instead of discovering the limit

What is not

• −It is a refrigeration appliance in your house: there is a compressor, a fan and a condensate drain
• −Needs roughly 700 cubic feet of air, or louvered doors, in a closet
• −It cools and dehumidifies the space it stands in, which is a gift in a Houston garage and a problem in a Minnesota basement

Skip it if: Skip it if the only place it can go is a finished, heated room you occupy. It will steal heat from that room in winter — your furnace then pays for the hot water, and the efficiency you bought disappears.

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Specifications above from Rheem ProTerra PROPH50 T2 RH375-30 product page; ABR Wholesalers listing, PROPH50 T2 RH375-30. We have not tested this unit ourselves.

Pick 2 — 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.

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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 3 — 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.

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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 a heat pump if...

• You are all-electric and the bill is the complaint.
• There is a garage, basement or utility space with air volume and a drain.
• Your household does not routinely empty the tank.
• There are rebates available in your area, which there often are for heat pumps and rarely for tankless.

Pick tankless if...

• The complaint is running out, not the bill.
• You have gas, and the line can carry 199,900 BTU/h.
• Floor space is genuinely valuable.
• The only install location is a closet, which rules out a heat pump outright.
• You travel often and standby loss on an empty house irritates you.