Electric vs Gas Tankless Water Heater: Which to Buy

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Quick answer from a licensed plumber: Choose gas if you have it and the incoming water is cold: the Rheem RTGH-84 lists 4.6 GPM at a 67 °F rise, while the largest electric unit here, the 36 kW Stiebel Tempra 36, lists 3.78 GPM at a similar rise and Stiebel recommends at least a 300 A service. Choose electric for an all-electric house with warm incoming water and room in the panel: the Stiebel Tempra 24 lists 3.63 GPM from 59 °F water on two 50 A breakers. In most houses, stay with the fuel you already have.

Our picks: for gas, the Rheem RTGH-84DVLN-3 (check it on Amazon); for electric, the Stiebel Eltron Tempra 24 Trend (check it on Amazon).

Electric and gas tankless water heaters do the same job, but they ask very different things of the house. A gas unit needs a gas line, a vent and a drain. An electric unit needs a lot of panel. And the one fact that decides how much hot water either one makes is the temperature of the water coming in. This page puts the two side by side from the makers’ own charts so you can decide which fuel fits your house. If you already know, go straight to our gas tankless picks or electric tankless picks.

Electric vs Gas Tankless: Hot Water at Your Water Temperature

Gas figures are Rheem’s rated flows at a 67 °F and 45 °F temperature rise. Electric figures are Stiebel’s flow to a 105 °F outlet at 240 V, and EcoSmart’s flow to 105 °F in its coldest zone and in zone 6. The rises are close but not identical (our arithmetic: 39 to 105 °F is a 66 °F rise; 59 to 105 °F is 46 °F). Every linked heater was sold by Amazon.com and in stock on October 3, 2026.

Find your incoming water temperature first

A tankless heater has a fixed amount of power, so colder water in means less hot water out. EcoSmart’s sizing sheet splits the U.S. into nine ground water zones, from zone 1 at an average 37 °F to zone 9 at 77 °F, and its ECOS 27 table runs from 2.7 GPM in zone 1 to 6.6 GPM in zone 9. Rheem points buyers to the same kind of nine-zone map. Use the coldest month, not the average: ground water varies by season, as EcoSmart notes.

Then add up what runs at the same time

EcoSmart’s example is one 2.0 GPM shower and one 1.5 GPM kitchen faucet, which is 3.5 GPM. In a zone 6 house (62 °F water), the ECOS 27 lists 4.3 GPM, so it keeps up. In zone 1 it lists 2.7 GPM and does not. In the same zone 1 winter, the gas RTGH-84 lists 4.6 GPM at a 67 °F rise, which takes 37 °F water to about 104 °F (our arithmetic). Rheem’s own zone table is figured at a 120 °F setting, so it lists less: 3.6 GPM for the RTGH-84 in zone 1 and 5.2 GPM in zone 6. That gap is why the Department of Energy says gas-fired tankless heaters produce higher flow rates than electric ones. Our tankless size calculator does this math for you.

What Each One Needs From the House

Electric: count the panel before you buy

Stiebel’s Tempra table is the clearest. The 24 kW Tempra 24 draws 2 x 50 A and wants two 50 A double-pole breakers, two runs of 8/2 copper and at least a 150 A main service. The 28.8 kW Tempra 29 wants three 40 A breakers and 200 A. The 36 kW Tempra 36 draws 3 x 50 A and wants three 50 A breakers, three 8/2 runs and 300 A. Stiebel says those service figures depend on the total electric load at the house, that the copper must be rated 75 °C or higher, and to check local codes. EcoSmart says panel upgrades are often required when you go from an electric tank to an electric tankless. Plumber’s note: if your house is on 208 V, as many apartments and condos are, Stiebel’s table drops the Tempra 24 to 18 kW and 1.89 GPM from 39 °F water. See 240 V vs 208 V for why.

Gas: the line, the meter and the vent

Rheem’s manual gives the method. Add up the BTU/h of every gas appliance and divide by the heating value of the gas, 1,024 BTU per cubic foot for natural gas or 2,546 for propane, to get cubic feet per hour. If that is more than the meter’s rating, the gas company has to upgrade it. For the RTGH-84 alone, 157,000 ÷ 1,024 is about 153 cubic feet per hour (our arithmetic). In Rheem’s Table 2 (0.3 in. pressure drop), 1/2 in. black iron carries 131 cubic feet per hour at 10 feet, and 3/4 in. carries 188 at 20 feet but 151 at 30 feet. So the heater by itself outgrows a 1/2 in. line and a 30-foot run of 3/4 in. on that table, before the furnace and range are added. Rheem says half-inch lines can work only when the gas pressure is 8.0 in. of water column or more and its high-pressure-drop table is followed. It also lists an allowed inlet pressure of 4.0 to 10.5 in. for natural gas, and says flexible connectors must be at least 3/4 in. inside diameter, rated for the heater, and preferably no longer than 36 in.

The vent on the condensing RTGH is plastic: Schedule 40 PVC, CPVC or polypropylene, up to 60 feet in 2 in. pipe or 150 feet in 3 in., minus 6 feet for each 90° elbow. A condensing heater also needs a drain for its condensate. Rheem says never to convert a heater to a gas type other than the one on its rating plate, so buy the natural gas or propane model you need. If your gas line is being run new, CSST vs black iron compares the two pipes.

Switching Fuels

The simplest tankless install is almost always the fuel you already have at the water heater, because the supply is already there. Going from a gas tank to electric tankless means checking the panel and service, and often upgrading them. Going from an electric tank to gas tankless means a new gas line from the meter, a new vent through the wall or roof, a condensate drain and 120 V power. Both are real projects. Our tankless install cost guide breaks down where the money goes.

Plumber’s note: in an all-electric house in a cold climate, a whole-house electric tankless may need a service upgrade and still fall short in January. A heat pump water heater is worth a look there; see heat pump vs tankless.

I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.

Which One Should You Buy?

• Gas is already at the water heater and you can vent it: gas. The Rheem RTGH-84 for most houses, the RTGH-95 for big households or cold water.
• All-electric house, warm incoming water (about 59 °F or warmer, where Stiebel lists 3.63 GPM), 150 A or larger service with room: the Stiebel Tempra 24.
• All-electric house, mild climate, family-sized demand, 300 A service: the Stiebel Tempra 36.
• Cold incoming water and no gas: run the numbers first. From 39 °F water, even the Tempra 36 lists 3.78 GPM. A tank or heat pump water heater may serve you better; see tank vs tankless.

Before You Buy

1. Find your coldest incoming water temperature. Use a ground water zone map or measure the cold tap in winter.
2. Add up the GPM you use at once. Showers about 2.0, kitchen faucet 1.5, washer 3.0, per EcoSmart’s fixture list.
3. For electric, read the panel label. Main service size, free breaker spaces, and 240 V or 208 V. Have an electrician do the load calculation.
4. For gas, list every gas appliance. Total the BTU/h and have the installer check the meter, pipe size and gas pressure.
5. Plan the vent and drain. A condensing gas unit needs a PVC vent route and a condensate drain.

Installation and Safety

Both types are licensed work. Stiebel says the Tempra must be installed by a licensed electrician and plumber and must meet national, state and local plumbing and electric codes. Rheem says installation and service of the RTGH must be done by a qualified installer, service agency or the gas supplier, and the Department of Energy says it is best to have a qualified plumbing and heating contractor install a demand water heater. A homeowner can measure, choose the unit and the fuel. The breakers, wire, gas pipe and vent belong to licensed trades, with a permit where your town requires one.

FAQ

Is gas or electric tankless better?

Gas gives more hot water at once, especially from cold incoming water: Rheem lists 4.6 GPM at a 67 °F rise for the RTGH-84, and Stiebel lists 3.78 GPM at a 66 °F rise for its largest electric unit. Electric is simpler to install, with no gas line or vent, and works well where the incoming water is warm and the panel has room. The Department of Energy also says gas-fired tankless heaters produce higher flow rates than electric ones.

Which is harder to install, gas or electric tankless?

It depends on what the house already has. Electric needs no gas line, vent or drain, but a whole-house unit needs two or three large double-pole breakers and enough main service: Stiebel recommends at least 150 A for the Tempra 24 and 300 A for the Tempra 36. Gas needs a line and meter sized for the heater plus every other gas appliance, a PVC vent and a condensate drain on a condensing unit like the RTGH, and 120 V power. Either way, the breakers, wire, gas pipe and vent are work for licensed trades, with a permit where your town requires one.

Can a gas tankless water heater run on a 1/2-inch gas line?

Sometimes. Rheem says half-inch lines are permissible only if the gas pressure is 8.0 inches of water column or more and the pipe capacity in its high-pressure-drop table is met. In Rheem’s standard 0.3 in. pressure-drop table, 1/2 in. black iron carries 131 cubic feet per hour at 10 feet, less than the about 153 an RTGH-84 needs at full fire (our arithmetic). A licensed installer has to check it.

Does an electric tankless water heater work in cold climates?

It works, but it makes less hot water. From 39 °F incoming water, Stiebel lists 2.52 GPM at 105 °F for the Tempra 24 and 3.78 GPM for the Tempra 36. EcoSmart lists 2.7 GPM for the ECOS 27 in its coldest zone, at 37 °F. That is one or two showers, not a busy morning in a big house.

Which tankless water heater works in a power outage?

Neither of the types on this page. Electric units need power to heat, and Rheem’s RTGH runs its controls and fan on 120 V. Rheem also says that if the power goes out in freezing weather, you should drain the heater to protect it.

Why Trust Plumbing Picks

Plumbing Picks is written by a licensed Georgia journeyman plumber. Every spec here comes from the maker documents linked below, or from the Amazon listings we link; nothing comes from hands-on testing. On October 3, 2026, every linked listing was sold by Amazon.com and in stock. If you buy through a link we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.

Sources

• Stiebel Eltron: Tempra Trend and Plus technical specifications (rev. 6.2025), Tempra flow rates at temperature rise (technical service bulletin)
• Stiebel Eltron: Tempra Trend and Plus operation and installation manual, tankless electric limited warranty
• EcoSmart: ECOS 18 and ECOS 27 spec sheet with ground water zone table (04/24), ECOS 27 electrical requirements, use and care manual
• Rheem: RTGH series condensing tankless gas water heaters specification sheet (TK-RTGH Series Rev. 2, 10/24)
• Rheem: RTGH series use and care manual (gas supply, pipe sizing tables, venting, freeze protection), residential tankless warranty
• U.S. Department of Energy: Tankless or Demand-Type Water Heaters (Energy Saver, archived June 30, 2026)
• Amazon listings for the Stiebel Eltron Tempra 24 Trend and 36 Trend, the EcoSmart ECOS 27 and the Rheem RTGH-84DVLN-3 and RTGH-95DVLN-3 (linked above)

Bottom Line

Gas tankless makes more hot water from cold water and needs a gas line, vent and drain; electric tankless needs no vent but a large share of your electrical service. Start with your coldest incoming water temperature and the flow you use at once, then check the panel or the gas supply. For gas, buy the Rheem RTGH-84 or the RTGH-95 for a big, cold house. For electric, buy the Stiebel Tempra 24, or the Tempra 36 if your service can carry it.