The best tankless water heater depends on your fuel source, household demand, incoming water temperature, and available electrical capacity. For whole-house demand, compare the MIZUDO 199,000-BTU gas model with the Westinghouse 180,000-BTU gas model. For electric options, the 18kW WINTEMP offers higher stated output than the 8kW Rheem, while the 14kW Westinghouse adds Wi-Fi controls. The comparison below highlights the listed capacity, power requirements, and key trade-offs for each unit.
| Product | Type | Listed output | Best suited for |
|---|---|---|---|
| MIZUDO 199,000 BTU | Indoor natural gas | Up to 10.3 GPM | Higher-demand households |
| Westinghouse 180,000 BTU | Indoor gas | Up to 7.9 GPM | Multiple simultaneous uses |
| WINTEMP 18kW | Electric, 240V | Up to 4.3 GPM | Electric homes with suitable service |
| Westinghouse 14kW | Electric, 240V | Up to 3.4 GPM under stated conditions | Smart-home features and moderate demand |
| Rheem 8kW | Electric, 240V | Up to 1.95 GPM | Lower-flow, multiple-fixture use |
MIZUDO 199,000-BTU Indoor Natural Gas Heater
Best for: Households that need substantial hot-water capacity across several bathrooms and appliances. MIZUDO lists a maximum output of 10.3 GPM and 199,000 BTU of heating input, with support for simultaneous demand across two to three bathrooms, kitchen fixtures, laundry, and a dishwasher.
The unit has a 0.53 GPM activation threshold and listed temperature control within ±1°F. Its default comfort setting is 107°F, and the adjustable range reaches 149°F. These specifications make it the highest-capacity option in this comparison.
Limitation: It is an indoor natural gas model, so it is not the right choice for homes without a compatible gas supply and suitable installation setup. Actual flow depends on conditions such as incoming water temperature.
Westinghouse 180,000-BTU Indoor Gas Heater
Best for: Buyers seeking a gas-fired whole-home option with remote controls and capacity for multiple fixtures. The listed output is up to 7.9 GPM at 180,000 BTU, and the product description says it can supply up to three showers at once.
Built-in Wi-Fi supports app and voice control, temperature adjustment, operating-status monitoring, and fault alerts. The manufacturer also lists temperature stability within ±1°F through automatic adjustment of gas and water flow. Choose this model over a basic unit if connected controls are a priority.
Limitation: Its stated maximum flow is lower than the MIZUDO model’s 10.3 GPM. The listing does not provide the same detailed installation requirements as some other products, so verify venting, gas, and local code needs before purchase.
WINTEMP 18kW Electric Tankless Water Heater
Best for: Electric-heater shoppers who want more listed flow than the smaller electric models here and whose home can meet the electrical requirements. The manufacturer states up to 4.3 GPM and positions it for showers and everyday use in homes with two to three bathrooms in mild to moderate climates.
At 240V, the unit is rated at 18kW and a maximum of 75A. The listed installation requirements are two 40A double-pole breakers and 2 × 8/2 AWG wiring; the product recommends 150A household service. Self-modulating operation adjusts power use to flow and set temperature.
Limitation: The substantial electrical requirements may rule it out for some homes. Consult a qualified electrician before purchase, as the manufacturer advises.
Westinghouse 14kW Wi-Fi Electric Heater
Best for: Buyers who want an electric unit with app and voice controls and moderate simultaneous hot-water demand. The listed output reaches 3.4 GPM when inlet water is 62°F and the outlet is set to 105°F; the product says this can support two water points under those conditions.
The compact wall-mountable unit measures 12.59 × 9.56 × 3.28 inches and uses 1/2-inch NPT connections. Features include a 0.53 GPM minimum startup flow, temperature memory, and compatibility with Alexa and Google Home.
Limitation: It requires a dedicated 60A double-pole breaker and 6 AWG wire at 240V. Its stated 3.4 GPM output is tied to specific inlet and outlet temperatures, so do not treat it as a guaranteed flow rate in every home.
Rheem 8kW Multi-Fixture Electric Heater
Best for: Buyers whose hot-water needs are lower than those suited to the higher-output units in this comparison. Rheem lists up to 1.95 GPM for this 8kW, 240V model and describes it as a multi-fixture option.
The compact, on-demand format can save space compared with a storage tank. Rheem advises choosing the correct size for the home and checking electrical requirements before installation. Consider it when the expected flow is within its stated capacity, rather than assuming it can supply several high-demand fixtures at once.
Limitation: Its maximum listed flow is the lowest of these five products. The provided product information does not specify breaker or wire requirements, so confirm them before buying.
Buying Guide: How To Compare Tankless Water Heaters
Should you choose gas or electric?
Start with the fuel and utilities already available at the installation location. The two gas models list higher maximum flow rates than the electric options here, while electric models require adequate household electrical capacity. Do not choose based on flow alone: installation needs, local codes, and the home’s existing connections matter.
How much flow does your household need?
List the hot-water fixtures likely to run at the same time, such as showers, a kitchen faucet, or laundry. Compare that combined demand with the product’s stated GPM. Maximum flow figures can depend on incoming water temperature and desired outlet temperature; the Westinghouse 14kW listing, for example, specifies the conditions for its 3.4 GPM figure.
What should you check before installation?
Confirm voltage, breaker capacity, wire size, connection type, fuel supply, and indoor installation requirements for the exact model. Have a qualified professional assess the site and applicable local codes. For electric models, the WINTEMP and Westinghouse 14kW listings give specific electrical requirements, while Rheem advises checking requirements and sizing before installation.
Are Wi-Fi and temperature controls important?
Wi-Fi and voice features are available on both Westinghouse models in this comparison. They can be useful for remote adjustments or monitoring, but they do not replace correct sizing. If connected controls are not a priority, compare the unit’s capacity and installation requirements first.
Frequently Asked Questions
Which listed model has the highest maximum flow?
The MIZUDO natural gas model lists up to 10.3 GPM, the highest stated maximum among these five products. Actual output can vary with operating conditions.
Which electric model has the highest stated flow?
The WINTEMP 18kW lists up to 4.3 GPM. It also has significant electrical requirements, including a recommended 150A household service.
Can an electric tankless heater run multiple fixtures?
It depends on the unit’s output, incoming water temperature, and simultaneous demand. The Westinghouse 14kW listing describes two water points at specified temperatures, while the Rheem 8kW lists up to 1.95 GPM.
Do tankless heaters provide unlimited hot water?
They heat water on demand rather than storing a tankful, but their flow capacity is finite. Demand above the unit’s capacity can reduce available hot-water flow or performance.
Should I size a heater by the number of bathrooms?
Bathroom count is only a starting point. Estimate which fixtures may run at the same time and compare their demand with the heater’s output under local water-temperature conditions.