Best Tankless Water Heaters (2026): 6 Gas and Electric Picks

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The best tankless water heater is the one sized to your peak hot water demand, matched to the fuel and electrical service you actually have, and approved for your altitude. That last point matters on the Front Range: two of the gas listings we compared say outright that they are not meant for use above 2,000 or 3,000 feet. Tankless units heat water only when a tap opens, so they skip the standby losses of a tank, but they trade that for big gas lines, dedicated venting or very large electrical circuits. Below are six picks across gas and electric, then a buying guide on flow rate, venting, wiring and the questions to ask an installer.

Part of our Plumbing & Water guide — start there for the full picture →

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How we compared: we compared 13 current Amazon listings for tankless water heaters, five gas, five whole-house electric and three small point-of-use electric units, on what each listing states: maximum flow in gallons per minute, fuel and input rating, efficiency, venting and indoor or outdoor installation, electrical requirements, certifications, altitude limits and warranty. We did not lab-test or install any of them. Prices and availability change often, so check the current listing before buying.

Quick picks: the best tankless water heaters by use case

Product	Best for	Key features	Price	Buy
Rinnai RX199iN	Best overall whole-home gas	Up to 11.1 GPM Condensing, UEF 0.98 Natural gas or propane	$1,532.79as of October 18, 2026	Buy now →Check current price
Rheem RTG 8.4 Outdoor	Outdoor install, no vent run	8.4 GPM at 35°F rise 180,000 BTU CSA certified	$953.00as of October 18, 2026	Buy now →Check current price
Rheem RTEX-18	Electric for a mid-size home	Up to 4.4 GPM Two serviceable copper elements	$419.53as of October 18, 2026	Buy now →Check current price
Rheem RTEX-11	Electric on a smaller panel	Up to 2.68 GPM One 50 A breaker 100 A panel minimum	$269.58as of October 18, 2026	Buy now →Check current price
Orbek 14kW	ETL-certified mid-range electric	Up to 3.38 GPM ETL certified One 60 A breaker	$209.99as of October 18, 2026	Buy now →Check current price
xwdzswgs 3000W 110V	Single sink point of use	110 V plug-level power 0.3 to 0.5 GPM Not for showers	$59.98as of October 18, 2026	Buy now →Check current price

Prices are Amazon’s listed prices as of the time shown and change often; check Amazon for today’s price. As an Amazon Associate we earn from qualifying purchases.

The six picks, compared listing by listing

1. Rinnai RX199iN — Best Overall Whole-Home Gas Tankless

Best overall

Why we like it

• Highest stated flow on the shelf, 11.1 GPM
• Only condensing unit, UEF 0.98
• Runs on natural gas or propane
• Indoor or outdoor with one model

Rinnai RX199iN — 199k BTU, NG or LP, up to 11.1 GPM, condensing UEF 0.98, indoor/outdoor, Quick Flush, 15/5/5-yr warranty

The Only Condensing Unit, With the Highest Stated Flow

Five of the 13 listings are gas tankless units. Of those, the RX199iN is the only one described as condensing, and its UEF of 0.98 is the highest stated. The two Rheem RTG units list 0.82. Condensing units capture heat from exhaust gases that a non-condensing unit sends up the vent, which is where the efficiency gain comes from. They also produce acidic condensate that needs a drain path, a detail to plan with the installer.

At up to 11.1 GPM it also leads on flow, ahead of the MIZUDO at 10.3 GPM and the Rheem RTG at 8.4. ENERGY STAR gives typical draws of about 2.5 GPM for a shower, 3.3 for a clothes washer, 2.2 for a sink and 1.3 for a dishwasher, so 11.1 GPM leaves headroom for simultaneous showers even when winter water arrives cold.

It is also the most flexible install: one model for natural gas or propane, indoors or out, which helps in foothills homes on propane. The listing recommends professional installation and does not state an altitude rating or adjustment; ask the installer to confirm the high-altitude setup in the manual for your elevation. Wi-Fi needs a separate module.

Check Rinnai RX199iN price on Amazon

2. Rheem RTG 8.4 GPM Outdoor — Best for an Outdoor Install

Best outdoor install

Why we like it

• Outdoor mounting avoids an indoor vent run
• States GPM at a specific temperature rise
• Built-in freeze and flame-failure protection
• 15-year heat exchanger warranty

Rheem RTG-84XLN-3 — 180k BTU NG, 8.4 GPM at 35°F rise, UEF 0.82, outdoor, freeze protection, CSA, 15/5/1-yr warranty

The Only Listing That Ties Its Flow Rating to a Stated Temperature Rise

Flow rating without a temperature rise is half a number. The two Rheem RTG listings are the only ones that state the rise: 8.4 GPM (or 7.0 for the smaller unit) at a 35°F rise. That is the right way to compare, because in winter on the Front Range, incoming water is colder than in summer and the heater has to lift it further. Expect less flow than the headline at a larger rise.

This model is the outdoor version, and the listing says outdoor installation eliminates venting. Moving combustion outside also takes combustion gases out of the house, which the CDC flags as a carbon monoxide concern for indoor gas appliances that are not vented properly. The trade-off is weather exposure; the listing describes built-in freeze protection, but a Colorado install still needs a sheltered wall and a plan for power outages during cold snaps.

The listing does not mention altitude, and at UEF 0.82 it is less efficient than the condensing Rinnai. A line in the listing calls it an indoor gas tankless, contradicting the title and specs; it is the outdoor model.

Check Rheem RTG 8.4 Outdoor price on Amazon

3. Rheem RTEX-18 — Best Electric Tankless for a Mid-Size Home

Best whole-home electric

Why we like it

• No venting, no gas line
• Two field-serviceable copper elements
• Self-modulates power to demand
• Digital thermostat with LED display

Rheem RTEX-18 — 18 kW, 240 V, up to 4.4 GPM, 2 serviceable copper elements, self-modulating, 3/4 in NPT, 99.8%

Field-Serviceable Elements From a Brand With Three Sizes on the Shelf

Five listings are whole-house 240 V electric tankless units: three Rheem RTEX sizes (11, 18 and 24 kW), the NOVIEEX 18 kW and the Orbek 14 kW. The RTEX-18 is the middle Rheem, rated up to 4.4 GPM, and its listing is the only one that calls out field-serviceable copper immersion elements. On a unit with no tank to replace, being able to swap an element is the main repair a plumber will ever do.

Electric tankless units need no vent and no gas line, which makes them attractive in homes without gas or where venting a gas unit would be difficult. The cost is current. This listing does not give breaker or wire requirements, but the RTEX-11 and RTEX-24 listings show the pattern: the 11 kW needs a single 50 A double-pole breaker and a 100 A panel minimum, while the 24 kW needs three 40 A breakers and a 200 A panel. The 18 kW sits between, so have an electrician confirm the panel before buying.

The NOVIEEX 18 kW lists a higher 4.9 GPM maximum, but its warranty is one year against Rheem’s 5-year leak and 1-year parts coverage on the sibling listings. At 4.4 GPM the RTEX-18 handles a shower plus a sink in most seasons; winter shower-plus-laundry is a stretch for any electric unit this size.

Check Rheem RTEX-18 price on Amazon

4. Rheem RTEX-11 — Best Electric Tankless on a Smaller Panel

Best for 100 A panels

Why we like it

• Most modest wiring of any whole-house unit
• Lists breaker, wire gauge and panel size
• Digital control from 80 to 140°F
• 5-year leak warranty

Rheem RTEX-11 — 11 kW, 240 V, 50 A DP breaker, #8 AWG, 100 A panel min, up to 2.68 GPM, 80–140°F, 5-yr leak / 1-yr parts

The Lowest Electrical Demand Among the Whole-House Electric Units

Electric tankless units draw enormous current, and the panel is often the limit. Of the five whole-house electric listings, the RTEX-11 asks the least: one 50 A double-pole breaker and a 100 A minimum service. The Orbek 14 kW calls for a 60 A breaker and a 150 A panel, the NOVIEEX 18 kW for two 40 A breakers and up to 75 A of current, and the RTEX-24 for three 40 A breakers and a 200 A panel. In an older Front Range home on 100 A service, the RTEX-11 may be the only whole-house electric option that does not trigger a service upgrade.

The catch is flow. At up to 2.68 GPM, it is a single-shower heater, or a heater for a small condo, an accessory dwelling or a basement bathroom. Rheem markets it for multiple fixtures, but the ENERGY STAR typical shower draw of about 2.5 GPM leaves little room for a second tap, and less in winter.

It is also a sensible way to serve a distant bathroom where hot water takes a long time to arrive from the main heater. The listing’s wiring detail makes it easy for an electrician to quote.

Check Rheem RTEX-11 price on Amazon

5. Orbek 14kW — Best ETL-Certified Mid-Range Electric

Best ETL-certified electric

Why we like it

• Only whole-house electric naming ETL certification
• Single breaker and wire set
• Wiring and panel minimum stated
• Compact 14.2-inch case

Orbek 14kW — 240 V, 60 A two-pole, 6 AWG, 150 A panel min, up to 3.38 GPM, 86–140°F, ETL certified, 1/2 in NPT

The Only Whole-House Electric Listing That Names a Safety Certification

Among the five whole-house electric listings, the Orbek is the only one that names a third-party safety certification, ETL. The Rheem RTEX listings do not mention one, and the NOVIEEX describes an 8-in-1 certified security system without naming a lab. For a 240 V appliance pulling up to 60 A, a named listing is worth having; confirm the mark on the unit’s label when it arrives.

At 3.38 GPM it falls between the RTEX-11 and RTEX-18, and it runs on one breaker and one wire set rather than two or three, which can simplify the electrical work. It does need a 150 A panel minimum, more than the RTEX-11.

Two cautions: the 1/2 in NPT water connections are smaller than the 3/4 in on the Rheem and NOVIEEX, which can limit flow on a whole-house line, and the warranty is stated only as at least one year.

Check Orbek 14kW price on Amazon

6. xwdzswgs 3000W 110V — Best Point-of-Use Unit for One Sink

Best single-sink point of use

Why we like it

• Runs on 110 V, not 240 V
• Mounts under a single sink
• States flow and rise together
• Says it qualifies for an ETL standard

xwdzswgs 3000W — 110 V, 0.3–0.5 GPM, 82–131°F rise stated, IPX4, vertical mount, 1/2 in NPT, 24-month warranty

A Sink Heater, Clearly Labeled as Not for Showers

Three listings are 3000 W, 110 V point-of-use units: two from xwdzswgs and one from KZU. They are not whole-house heaters. This one is honest about it: the listing says it is not suitable for showers and gives an output tied to tiny flows, a large rise at 0.3 to 0.5 GPM. That is enough to put hot water at a bar sink, a garage utility sink or a far-off powder room faucet, where waiting for hot water from a distant tank wastes water.

It is the only point-of-use listing on the shelf that claims an ETL standard, and it states a 24-month warranty. It must be mounted vertically, and the listing gives wiring guidance but advises proper installation; a dedicated circuit is the safe assumption for a 3000 W appliance near water.

See our guide to the point-of-use water heater for where these units make sense and where a small tank does better.

Check xwdzswgs 3000W 110V price on Amazon

Two listings that rule themselves out on the Front Range

Two gas units on the shelf state altitude limits that exclude most Colorado Front Range homes. The MIZUDO 199,000 BTU natural gas unit says it is not recommended for use above 3,000 feet, and the Ranein 80,000 BTU propane unit says it is not for elevations above 2,000 feet. Denver sits around 5,280 feet and the foothills go far higher, so neither should be installed here, whatever their flow ratings. The other gas listings do not state an altitude limit or adjustment at all. Gas appliances burn differently in thin air, and manufacturers specify high-altitude settings, derating or model variants in their manuals. Ask any installer to show you the altitude section for your elevation before signing off.

Gas vs electric tankless

Gas

Gas units deliver far more flow: the gas listings here run from 3.6 to 11.1 GPM, while whole-house electric units top out at 5.9 GPM. They need an adequately sized gas line and venting, unless installed outdoors. ENERGY STAR says the installer should confirm the right GPM size for the home, whether the existing natural gas line is sufficient, whether electricity is available near the heater for forced-ventilation models, and how the combustion gases will be vented (ENERGY STAR whole-home gas tankless water heaters). Replacing a 40-gallon gas tank with a 199,000 BTU tankless often means a larger gas line, because the tankless burner fires at several times the tank’s input. Our tankless water heater guide covers the trade-offs in more depth.

Electric

Electric units need no vent and no gas, but they need very large circuits. On this shelf, requirements run from a single 50 A breaker to three 40 A breakers and a 200 A panel. Many older homes cannot support the largest units without a service upgrade. In cold-water months, electric units lose the most flow, because the same kilowatts must produce a bigger temperature rise.

Sizing by flow rate and temperature rise

Tankless heaters are sized by GPM, not gallons. ENERGY STAR’s typical figures: about 2.5 GPM for a shower or tub, 3.3 for a clothes washer, 2.2 for a kitchen or bathroom sink and 1.3 for a dishwasher. Add up the fixtures likely to run at once, then apply the temperature rise. If incoming water is 45°F in January and you want 120°F at the tap, you need a 75°F rise, roughly twice the 35°F rise the Rheem RTG listings quote, and flow drops accordingly.

A practical approach:

• One shower plus a sink: about 5 GPM at your winter rise.
• Two showers plus a dishwasher: about 6.3 GPM at your winter rise.
• Two showers, a washer and a sink: 10 GPM or more, which only the largest gas units approach.

Our water heater sizing guide walks through the math for both tank and tankless.

Efficiency ratings and what UEF means

ENERGY STAR certifies water heaters on Uniform Energy Factor (UEF) and defines instantaneous, or tankless, units by how little water they hold relative to burner input (ENERGY STAR water heater key product criteria). On this shelf, the condensing Rinnai states UEF 0.98 and the non-condensing Rheem RTG units state 0.82. Electric units here state 98 to 99.8 percent thermal efficiency, which is a different measure, and electric resistance heating is near 100 percent efficient at the unit regardless; the question is what your electricity costs relative to gas.

Venting and carbon monoxide

Every gas tankless unit produces combustion gases that must leave the house. The CDC advises having water heaters and other gas appliances serviced by a qualified technician every year, making sure gas appliances are vented properly, and notes that horizontal vent pipes for appliances such as water heaters should slope slightly upward toward the outdoors to keep CO from leaking at joints (CDC carbon monoxide basics). Indoor tankless units typically use sealed direct-vent or power-vent systems with specific pipe materials; condensing and non-condensing units often call for different vent materials. Follow the manual exactly. Our guides to the direct vent water heater and water heater vent pipe explain the options. Any home with a gas water heater needs working CO alarms.

Maintenance on hard Front Range water

Water hardness varies across the Front Range by utility and by well, and where water is hard, scale builds inside tankless heat exchangers. Most manufacturers call for periodic flushing with a descaling solution through isolation valves; the Rinnai’s Quick Flush feature is designed for exactly that. Ask the installer to fit service valves, and see our tankless water heater maintenance guide for the routine. Outdoor units and units in unheated garages also need a plan for freeze protection during power outages.

When to call a professional

Every gas tankless and every whole-house electric unit on this shelf is a professional install: gas line sizing, venting, condensate drains, 240 V circuits, panel capacity, permits and, on the Front Range, altitude setup. The Rinnai listing itself says not to install a water heater without a technician. ENERGY STAR suggests getting written estimates, asking for references and checking that the contractor will pull a local permit. Our plumbing and water hiring guide covers what to ask. If you are buying a home with a tankless unit, an inspector can check the venting, gas connection and installation quality. Contact us to connect with a vetted Front Range inspector.

References

• Whole-Home Gas Tankless Water Heaters — ENERGY STAR
• Water Heater Key Product Criteria — ENERGY STAR
• Carbon Monoxide Poisoning Basics — Centers for Disease Control and Prevention