Key takeaways
- Electrical clearance: Leave the manufacturer’s specified working space in front of the unit; a practical planning allowance is roughly 30 inches wide by 36 inches deep, although local electrical code and the manual control the final requirement.
- Gas vent clearance: Exhaust terminals must be positioned away from windows, doors, soffits, and air intakes according to the installation manual and local code. This can determine whether an exterior wall is usable.
- Water connections: Include isolation valves and service ports so the heat exchanger can be flushed. A sediment filter may protect the unit where the water supply contains grit.
- Flow activation: Many tankless heaters need a minimum flow, often around 0.4–0.5 GPM, before they fire. Very low-flow faucets or fixtures may not activate an older or poorly matched unit.
- Pipe length: Tankless heating does not eliminate wait time in long plumbing runs. A bathroom far from the heater still contains cooled water in the pipe unless a properly designed recirculation system is installed.
- Freeze protection: Exterior or garage installations need protection from freezing, including power for built-in safeguards. A powered-off tankless unit may not protect itself during an outage.
The best tankless water heaters are chosen by required flow rate and temperature rise first, then by fuel, electrical capacity, venting, and installation cost—not by the heater’s maximum gallons-per-minute number alone.
For a small apartment or one low-flow shower, a compact electric model is often the simplest choice. For a larger household using multiple fixtures at once, a gas-condensing unit usually provides more usable hot water with less electrical upgrading. The right comparison is the heater’s output at your incoming-water temperature and target temperature.
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
Included for 'Best Tankless Water Heaters for Reliable On-Demand Hot Water' because the listing specifies Gray, details this guide uses to compare options.
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CAMPLUX Tankless Water Heater Electric 120V, 4.5kW Point of Use Instant Hot Water Heater, Save Space for Sink Wall Mounted LED Digital Display CSA Certified
Included for 'Best Tankless Water Heaters for Reliable On-Demand Hot Water' because the listing specifies 4.5kW Point of Use Instant Hot Water Heater, details this guide uses to compare options.
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Rheem 11kW Multi Fixture Tankless Electric Water Heater - 240V
Included for 'Best Tankless Water Heaters for Reliable On-Demand Hot Water' because the listing specifies 240V, details this guide uses to compare options.
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Rheem RTG Series 7.0 GPM Natural Gas Tankless Water Heater Indoor | 160,000 BTUs High Efficiency On Demand, Continuous Hot Water | Non-Condensing
Included for 'Best Tankless Water Heaters for Reliable On-Demand Hot Water' because the listing specifies 160 and 000 BTUs High Efficiency On Demand, details this guide uses to compare options.
View on AmazonQuick picks by household situation
| Household situation | Best-fit heater type | Typical target capacity | Why it fits |
|---|---|---|---|
| One-person apartment, one shower at a time | Electric, approximately 11–13 kW | 1.5–2.5 GPM at a 45°F rise | Compact installation and no gas vent; usually adequate when fixtures are not used together |
| Two-person home with one shower and occasional sink use | Electric, approximately 18–24 kW | 2.5–4 GPM at a 45°F rise | Can handle a normal shower, but may require a 100–125 amp electrical circuit allocation |
| Family using two showers or a shower plus laundry | Gas-condensing tankless | 5–8 GPM at a 45°F rise | Better recovery and higher simultaneous flow than most residential electric units |
| Cold-climate home with several bathrooms | High-output gas unit or two-unit system | 7–11 GPM at a 60°F rise | Preserves useful flow when incoming water is near 40°F |
| Renter or homeowner avoiding major alterations | Small electric unit only if the panel and plumbing permit it | Usually one fixture at a time | No combustion venting, but landlord approval and electrical capacity remain necessary |
How to size a tankless water heater realistically
1. Add the fixtures that may run together
Estimate simultaneous demand rather than adding every fixture in the house. A standard shower commonly uses 1.5–2.5 gallons per minute (GPM), a bathroom faucet about 0.5–1.0 GPM, a kitchen faucet roughly 1.0–1.5 GPM, and a clothes washer around 1.5–2.0 GPM while filling. A bathtub faucet can demand 4–7 GPM, depending on its design.
For example, a 2.0-GPM shower, 1.5-GPM kitchen faucet, and 1.5-GPM washing machine create approximately 5 GPM of possible demand. A heater rated at 5 GPM under warm-water conditions may not deliver that same flow in winter.
2. Calculate temperature rise
Temperature rise is the difference between incoming cold water and the temperature set on the heater. A 120°F outlet target with 55°F incoming water requires a 65°F rise. If winter water enters at 40°F, the same target requires an 80°F rise.
Higher temperature rise reduces the flow a heater can produce. Manufacturer charts are more useful than headline maximum flow ratings because they show output at specific rises, such as 35°F, 45°F, or 60°F. For most showers, design around approximately 105–110°F water at the fixture, but account for a 120°F heater setting where local plumbing practice and anti-scald controls call for it.
A useful approximation is:
Required heating capacity in kilowatts = GPM × temperature rise × 0.1465.
At 4 GPM and a 60°F rise, the calculation is about 35 kW. That explains why electric heaters become demanding at higher flow: a practical residential electric unit may not support this load without limiting flow or using multiple units.
Gas versus electric tankless water heaters
| Factor | Electric tankless | Gas tankless |
|---|---|---|
| Typical residential input | 11–36 kW | 120,000–199,000 BTU per hour |
| Best use | One or two fixtures, modest hot-water demand | Multiple simultaneous fixtures and larger homes |
| Electrical needs | Often two or three 240-volt circuits; large models may require a 150–200 amp service review | Usually a 120-volt circuit, though controls and pumps vary |
| Installation complexity | No combustion vent, but substantial wiring and breaker space | Gas line sizing, combustion-air rules, condensate drainage, and approved venting |
| Cold-weather performance | Flow may drop sharply at high temperature rise | Generally better for high flow, though output still falls with colder inlet water |
| Maintenance | Periodic scale flushing, especially with hard water | Scale flushing plus inspection of burner, venting, condensate, and air intake |
Electric models are physically smaller and often easier to place near a point of use. They can be attractive for an accessory dwelling, a distant bathroom, or a home without gas service. Their limitation is power demand: a 27-kW unit draws about 113 amps at 240 volts before considering other household loads.
Gas models can supply more hot water at once, but they are not simple appliance swaps. Existing tank-heater venting may be incompatible, and a high-output unit may need a larger gas line than the old heater. Condensing models also need a condensate drain and generally use plastic vent materials approved by the manufacturer.
Strong product choices by specification
Best compact electric category: Rheem RTEX-13
The Rheem RTEX-13 is suited to a small apartment, guest bathroom, or single-shower application where demand is limited. Its approximately 13-kW class is more realistic for one fixture than for a full family home. Expect performance to depend heavily on inlet temperature; it is not a two-shower solution in a cold climate.
Best higher-capacity electric category: EcoSmart ECO 27
The EcoSmart ECO 27 is a high-output electric model commonly considered for larger electric-only installations. Its approximately 27-kW input can support several fixtures in favorable conditions, but buyers must verify available service, breaker space, wire size, and local code requirements. In winter, simultaneous showers may still exceed its practical capacity.
Best premium electric category: Stiebel Eltron Tempra series
Stiebel Eltron’s Tempra line is known for electronic modulation and compact wall-mounted designs. Select the model by its published flow chart rather than by the series name: a smaller Tempra model may suit one shower, while a higher-output model can serve more demand if the electrical system supports it. These units remain subject to the same temperature-rise limits as other electric heaters.
Best gas category for high demand: Rinnai RU series
Rinnai’s RU series includes high-output gas tankless heaters intended for substantial residential demand. Depending on the exact model and fuel, units in this category can approach 199,000 BTU per hour and provide much more simultaneous flow than typical electric heaters. Confirm whether the selected model is natural gas or propane, and check venting, gas-line, and condensate requirements before purchasing.
Best gas category for efficiency-focused installations: Noritz NRCB series
Noritz NRCB units are condensing gas heaters designed for high efficiency and strong flow. They can be a good fit where several bathrooms operate at once, but their installation requires correct intake and exhaust routing, condensate management, and adequate gas delivery. The installation may cost more than the heater itself when extensive vent or gas work is needed.
Installation details buyers often underestimate
- Electrical clearance: Leave the manufacturer’s specified working space in front of the unit; a practical planning allowance is roughly 30 inches wide by 36 inches deep, although local electrical code and the manual control the final requirement.
- Gas vent clearance: Exhaust terminals must be positioned away from windows, doors, soffits, and air intakes according to the installation manual and local code. This can determine whether an exterior wall is usable.
- Water connections: Include isolation valves and service ports so the heat exchanger can be flushed. A sediment filter may protect the unit where the water supply contains grit.
- Flow activation: Many tankless heaters need a minimum flow, often around 0.4–0.5 GPM, before they fire. Very low-flow faucets or fixtures may not activate an older or poorly matched unit.
- Pipe length: Tankless heating does not eliminate wait time in long plumbing runs. A bathroom far from the heater still contains cooled water in the pipe unless a properly designed recirculation system is installed.
- Freeze protection: Exterior or garage installations need protection from freezing, including power for built-in safeguards. A powered-off tankless unit may not protect itself during an outage.
What a realistic budget looks like
Compact electric units usually cost about $200–$700 for the equipment, while high-output electric models often fall around $500–$1,200. Gas tankless equipment commonly ranges from $700–$2,500 or more. Installed prices vary widely: a straightforward electric installation may total roughly $800–$2,500, while a gas conversion involving new venting, gas piping, electrical work, and condensate drainage can reach $2,000–$5,000 or more.
Do not compare heater prices without comparing infrastructure. A low-cost electric unit can become expensive if the panel needs an upgrade. A gas model can become expensive when the existing vent and gas line are undersized.
Final buying rule
Choose an electric tankless water heater when your demand is modest and the electrical service has verified capacity. Choose a gas-condensing model when two or more high-flow fixtures may operate together, especially in a cold climate. Before ordering, write down the worst-case inlet temperature, add the simultaneous fixture flow, and read the manufacturer’s output chart at that temperature rise. That calculation is more reliable than choosing by maximum GPM, household size, or marketing label alone.
