Best Tankless Water Heaters

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There is nothing quite like the rude awakening of a freezing cold shower when your home’s traditional hot water tank runs dry midway through a lather. Traditional storage tanks are incredibly inefficient. They spend 24 hours a day burning electricity or gas to keep 40 to 50 gallons of water hot, even when you are asleep or at work—a phenomenon known as standby heat loss. But when you actually need back-to-back showers, a load of laundry, and a running dishwasher, that finite tank empties rapidly, leaving you shivering in the cold.

The transition to a tankless water heater seems like a no-brainer, but the mechanical and physical realities of “on-demand” heating often catch homeowners off-guard. On-demand heaters do not store hot water; instead, they use high-powered heating elements or gas burners to heat cold water instantly as it flows through the unit.

Below, we review the eight best tankless water heaters on the market, analyzing their flow rates, power draws, heating consistency, and real-world utility based on professional plumbing standards and hands-on homeowner experiences.

1. Rheem 18kW 240V Tankless Electric Water Heater

The Rheem 18kW 240V Tankless Electric Water Heater (frequently labeled as the RTEX-18) is a highly reliable mid-sized workhorse designed for small to medium-sized homes. Operating on a 240-volt electrical supply, this unit features dual copper immersion heating elements that modulate their power draw dynamically based on your actual water flow. Its digital temperature display allows you to adjust output settings in precise 1-degree increments from 80°F up to 140°F, providing exceptional control over your household water temperature.

In real-world residential setups, the Rheem 18kW model shines in warmer climates where the incoming groundwater temperature is relatively mild. Under these conditions, it can comfortably deliver a continuous flow rate of up to 4.4 gallons per minute (GPM), which is more than enough to run two showers simultaneously. However, if you live in a northern climate where winter groundwater temperatures drop below 40°F, this unit’s capacity will drop to around 1.8 to 2.0 GPM—meaning it will only support a single shower head at a time. Installation is relatively straightforward for an experienced DIYer, but it does require two separate 40-amp double-pole breakers and a minimum 150-amp electrical panel service, which is a critical detail to verify before purchasing.

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Pros:

Modulating technology: Automatically adjusts power consumption to match water flow, minimizing energy waste.
External temperature control: LED display adjusts in 1°F increments for perfect warmth customization.
Durable brass heat exchangers: Highly resistant to corrosion and thermal stress for a longer lifespan.
Compact footprint: Fits easily into tight closets or utility rooms, reclaiming valuable floor space.

Cons:

Heavy electrical requirements: Demands 75 amps of power, requiring a substantial home electrical panel.
Flow rate drops in cold climates: Winter groundwater limits its output to a single shower in northern states.

2. EcoSmart ECO 11 Electric Tankless Water Heater

If you are looking to supply hot water to a small apartment, a tiny home, or a dedicated secondary suite, the EcoSmart ECO 11 Electric Tankless Water Heater is one of the most cost-effective and compact options available. Rated at 13kW at 240 volts (running slightly lower at 11.8kW on a 220V system), this ultra-compact unit is designed to heat up to 2 gallons of water per minute. It utilizes patented self-modulating sensor technology to monitor incoming water temperature and flow rate, adjusting its energy usage to ensure a steady output without spikes.

For point-of-use applications or single-bathroom living, the ECO 11 is incredibly efficient. In southern climates, it can easily handle a shower and a sink running at the same time. In cold northern areas, however, it is strictly a single-point-of-use unit, meaning it should be dedicated to a single bathroom faucet or low-flow shower head. Plumbers appreciate the ECO 11’s easy-to-service design, which features a standard screw-in copper heating element that can be swapped out quickly if hard water scale causes a burnout. To install it safely, you will need a minimum 125-amp electrical panel with a single 60-amp double-pole breaker.

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Pros:

Budget-friendly: Excellent entry-level price point for reliable on-demand heating.
Point-of-use efficiency: Eliminates standby heat loss for secondary bathrooms, cabins, or wet bars.
User-friendly element access: Standard copper elements are inexpensive and easy to replace.
Space-saving design: Extremely small chassis can be tucked inside a standard bathroom vanity.

Cons:

Not for whole-house use: Too small to support multiple appliances or multi-bathroom households.
Low flow rate in cold climates: Cold winter groundwater limits output to roughly 1.3 GPM.

3. Rheem RTEX-13 Residential Tankless Water Heater

The Rheem RTEX-13 is an exceptionally engineered 13kW, 240-volt electric tankless heater that occupies the sweet spot between point-of-use performance and light residential utility. Designed with a single, heavy-duty heating chamber, this model is built for targeted installations like small condominiums, office breakrooms, or detached guest houses. Like its larger siblings, the RTEX-13 features a rugged, brass-top heat exchanger and a digital thermostat on the faceplate for simple, intuitive temperature adjustments.

From a practical standpoint, the RTEX-13 is a fantastic upgrade for homes that suffer from a long pipe run between the main water heater and a distant bathroom. Installing this unit closer to the point of use completely eliminates the tedious wait for hot water to travel across the house. In warmer states, it can reliably drive a standard showerhead and a hand sink simultaneously (yielding around 2.5 GPM). In cold-weather climates, it serves as a reliable single-outlet device. It draws 54 amps of electricity, meaning it requires a single 60-amp double-pole breaker, making it highly compatible with standard 100-amp or 150-amp home electrical panels without needing a panel upgrade.

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Pros:

Eliminates hot water lag: Ideal as a booster unit to cure cold water wait times in distant rooms.
Modest electrical footprint: Runs on a single 60-amp breaker, fitting easily into most older home panels.
Rugged construction: Premium copper heating elements and brass threads prevent plumbing leaks.
Highly responsive thermostat: Fast internal sensors adjust to flow fluctuations almost instantly.

Cons:

Strict capacity limits: Will struggle to provide adequate hot water if a shower and washing machine are used together.
Sensitive to hard water: Requires a pre-filter or regular descaling in areas with high mineral content.

4. EcoSmart ECO 18 Electric Tankless Water Heater

The EcoSmart ECO 18 is a highly popular mid-to-large capacity electric tankless water heater designed to handle the hot water demands of a standard two-bathroom home in moderate climates. Operating at 18kW on 240 volts, this unit draws up to 75 amps of electrical current. It features dual copper immersion heating elements housed inside robust stainless steel chambers, which work in tandem to heat water quickly and efficiently as it flows through the manifold.

Homeowners frequently choose the ECO 18 for its excellent balance of price and performance. In regions with groundwater temperatures above 60°F, it effortlessly manages two concurrent showers and a kitchen sink without a drop in temperature, running at a smooth 4.3 GPM. If installed in colder regions, it can still maintain a comfortable shower and a sink at a continuous 2.5 GPM flow rate. The unit’s smart self-modulating board monitors the precise rate of incoming water to draw only the exact wattage required, keeping your monthly electric bills highly manageable. It requires two separate 40-amp double-pole breakers.

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Pros:

Multi-bathroom capacity: Comfortably handles up to two showers simultaneously in warm-weather regions.
Smart modulation: Drastically reduces standby power draw, lowering monthly energy expenses.
Individually replaceable elements: Uses standard, readily available parts for quick maintenance.
Compact, sleek casing: Mounts securely to wall studs, freeing up valuable floor space.

Cons:

Requires a 150A minimum panel: Older homes with 100-amp panels will require a costly service upgrade.
Requires scale maintenance: Mineral scaling in hard water areas can degrade the heating elements if not flushed yearly.

5. ThermoMate 4.5kW Tankless Under Sink Water Heater Electric

For immediate, dedicated hot water at a single sink, the ThermoMate 4.5kW Tankless Under Sink Water Heater is an excellent and highly targeted solution. Rated at 4.5kW at 240V (and compatible with 220V systems), this low-draw, single-phase electric heater is specifically designed for under-sink installation in kitchens, utility rooms, office break rooms, or RVs. It completely bypasses the need for heavy-duty multi-breaker electrical configurations, operating on a single 25-amp double-pole breaker with standard 12-gauge wiring.

The ThermoMate 4.5kW model is engineered for instant convenience. Rather than trying to supply an entire house, it is designed to heat water locally, delivering immediate hot water for hand washing, dish prep, or cleaning. It features a bright LED display showing the real-time outlet temperature and an smart internal thermostat that prevents overheating. Because it operates at a lower wattage, it delivers a maximum of about 1.2 GPM with a modest temperature rise. It is the perfect cure for a distant kitchen sink that takes minutes to get warm, but it should never be bought with the intention of running a shower or a bath.

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Pros:

Instant under-sink hot water: Completely cuts down on water waste while waiting for hot water from a distant central heater.
Very low electrical requirements: Runs on a standard 25-amp breaker, avoiding heavy panel upgrades.
Simple plug-and-play setup: Small chassis mounts directly under the sink cabinet using basic plumbing fittings.
Over-temperature protection: Built-in safety sensors prevent water from reaching scalding levels.

Cons:

Extremely limited GPM: Strictly designed for a single sink; cannot support a shower or bath.
Requires high incoming temps for high heat: In northern winters, it can only provide warm (not steaming hot) sink water.

6. Tankless Water Heater Electric 18kW at 240 Volt

This heavy-duty Tankless Water Heater Electric 18kW at 240 Volt is a robust, multi-chambered heating unit built to provide whole-house hot water coverage for apartments, condos, and medium-sized single-family homes. Equipped with a high-torque electric heating system, this unit utilizes advanced electronic sensors to monitor incoming water flow and temperature. It then modulates its multi-stage heating elements to maintain a highly stable, consistent output, even when hot water taps are turned on and off in different rooms.

This 18kW model is praised by plumbers for its structural durability and clean, internal engineering. The dual-chamber layout is constructed from high-grade copper heating elements and heavy-walled plumbing loops, which helps reduce mineral deposit accumulation and minimizes thermal shock. Under moderate climate conditions, this unit delivers up to 4.5 GPM, which easily translates to running two standard showers simultaneously without a drop in temperature. It draws 75 amps of power and requires two 40-amp double-pole breakers. It is an excellent, reliable whole-house heating solution for households seeking to eliminate cold-water dips during peak morning routines.

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Pros:

Multi-chamber heating system: Provides stable, uninterrupted hot water for mid-sized homes.
High-grade copper components: Designed to resist thermal stress and scale buildup.
Smart temperature stability: Prevents sudden cold-water dips when multiple faucets are opened.
Wall-mountable chassis: Clears up floor space, giving utility closets a clean, organized look.

Cons:

High peak power consumption: Requires 75 amps of capacity from your household service.
Professional installation recommended: Advanced wiring and dual double-pole breakers demand careful setup.

7. Westinghouse Wifi Tankless Water Heater Electric

The Westinghouse Wifi Tankless Water Heater represents the modern evolution of home water heating, integrating smart home connectivity with powerful electric heating performance. Rated at a highly robust capacity, this premium unit features built-in Wi-Fi and connects directly to a smartphone app, allowing you to monitor real-time water temperature, track your household’s hot water usage patterns, adjust outlet temperatures remotely, and receive diagnostic error codes immediately if maintenance is needed.

Beyond its smart features, the Westinghouse heater is built for serious heavy-duty performance. It features a corrosion-resistant stainless steel heating element array that is far more durable than traditional copper elements when exposed to aggressive or hard water conditions. The digital control board is exceptionally responsive, adjusting its power output on the fly to eliminate the “cold water sandwich” effect during back-to-back showers. Under normal operation, it delivers a highly consistent GPM flow, making it a stellar whole-house option for tech-savvy homeowners who want complete transparency over their energy consumption.

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Pros:

Smart Wi-Fi connectivity: Monitor energy consumption, adjust temperatures, and run diagnostics from your phone.
Stainless steel heating elements: Superior resistance to mineral scale and hard water corrosion compared to copper.
No cold water sandwich: High-speed processing chip prevents sudden dips in temperature during water cycle starts.
Clear digital touch controls: Modern, easy-to-read user interface on the front panel.

Cons:

Premium price tag: Smart features and stainless steel construction carry a higher upfront cost.
App setup curve: Setting up the Wi-Fi connection can occasionally be finicky depending on your router placement.

8. Rinnai V53DeP Propane Tankless Water Heater

For homeowners who want to completely bypass the steep electrical requirements of high-powered electric heaters, the Rinnai V53DeP Propane Tankless Water Heater is an elite, highly reliable outdoor gas solution. Operating on liquid propane, this outdoor-only unit uses a powerful gas burner to deliver a massive, continuous flow rate of up to 5.3 GPM. Because it is powered by gas, it does not put any load on your home’s electrical panel, requiring only a tiny amount of electricity (less than 1 amp) to power its internal digital ignition and control board.

The Rinnai V53DeP is designed to be mounted on the exterior of your home, which completely eliminates the need to run expensive indoor venting pipes through your roof. This makes it an exceptionally popular choice for southern states, cabins, and homes with limited indoor space. The propane burner is incredibly efficient, activating only when water flow is detected, and it can comfortably support up to two showers and a sink simultaneously, even in colder winter temperatures. Because it is an outdoor-mounted unit, it features integrated freeze protection to safeguard the internal copper piping, though it is best suited for climates where sustained sub-zero temperatures are rare.

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Pros:

Incredible 5.3 GPM flow rate: Comfortably runs multiple high-demand fixtures simultaneously.
Bypasses electrical panel limits: Runs on propane gas, eliminating the need for expensive 200A panel upgrades.
Outdoor mounting: Saves indoor space and eliminates the cost of complex indoor venting pipes.
Commercial-grade reliability: Rinnai’s copper heat exchanger is built to last up to 20 years with proper care.

Cons:

Propane supply required: Requires a dedicated propane tank line and professional gas plumbing hookup.
Outdoor-only installation: Not suitable for indoor utilities or extremely harsh sub-zero northern climates.

The Ultimate Buyer’s Guide to Sizing a Tankless Water Heater

Unlike traditional tanks, where you simply choose a 40, 50, or 80-gallon capacity, sizing a tankless water heater requires a bit of math. If you purchase a unit that is too small, you will suffer from weak water pressure or lukewarm showers. Here are the three crucial factors you must analyze before buying:

1. Regional Groundwater Temperature

The temperature of the water coming out of your city line or well changes dramatically depending on where you live and the season. This is the starting point for your calculation:

Southern States (60°F to 75°F groundwater): Your heater only needs to raise the water temperature by roughly 30°F to 40°F to reach a comfortable shower temperature (105°F). Because the “temperature rise” is small, a mid-sized unit (like an 11kW or 18kW) can easily achieve its maximum advertised GPM.
Northern States (35°F to 50°F groundwater): Your heater must raise the water temperature by a staggering 55°F to 70°F. Because the heating elements have to work twice as hard, the flow rate (GPM) is cut in half. In these regions, you must buy a larger unit (like a 27kW electric or a gas unit) to get standard multi-bathroom performance.

2. Household Gallons Per Minute (GPM) Demand

To figure out how much capacity you need, add up the GPM of the fixtures you plan to run at the same time:

Standard Low-Flow Showerhead: 1.5 to 2.0 GPM
Bathroom Faucet: 0.5 to 1.0 GPM
Kitchen Sink: 1.5 GPM
Washing Machine / Dishwasher: 2.0 GPM

If you want to be able to run two showers (4.0 GPM total) and a kitchen sink (1.5 GPM) at the same time, you need a heater that can deliver at least 5.5 GPM at your local groundwater temperature rise.

3. Electrical Panel Capacity vs. Gas Lines

Before buying an electric tankless water heater, you must open your home’s main electrical panel and check your service rating:

100-Amp Service: This is common in older homes. A 100A panel cannot support an 18kW electric tankless heater (which draws 75 amps on its own) without risking a main breaker trip when you turn on your stove or dryer. You will either need to upgrade to a 200-amp panel (costing $2,000 to $4,000) or choose a gas-powered unit.
150-Amp or 200-Amp Service: This can comfortably accommodate an 18kW or 24kW electric heater, provided you have open breaker slots to install the necessary dual double-pole breakers.
Gas/Propane Units: These are highly efficient and deliver the highest GPM, but they require a dedicated 1/2-inch or 3/4-inch gas line and proper ventilation, which requires professional plumbing installation.

Common Tankless Water Heater Mistakes to Avoid

Neglecting the Annual Vinegar Flush: Over time, calcium and magnesium minerals in your water will bake onto the hot heating elements or heat exchangers. This scale acts as an insulating blanket, forcing the elements to work harder, run hotter, and eventually burn out. You must flush your unit with food-grade distilled white vinegar once a year to dissolve this buildup.
Sizing Based on Summer Flow Rates: Many homeowners buy a tankless heater in July, are thrilled with the performance, and then wonder why their winter showers are lukewarm. Always size your water heater based on your region’s coldest winter groundwater temperatures to ensure year-round comfort.
Ignoring the “Cold Water Sandwich” Effect: When you turn a hot water faucet off and on quickly, a pocket of cold water can get trapped in the plumbing line between your hot water cycles. Premium units with smart modulation or tiny internal buffer tanks resolve this, but budget units often suffer from this momentary temperature dip.
Skipping isolation valves during installation: If you do not install service isolation valves (hot and cold bypass valves) on your unit’s intake and outlet, you will have no way to hook up a pump to flush and descale the unit. Always ensure an isolation valve kit is installed with your heater.

Frequently Asked Questions (FAQ)

How often do I need to descale a tankless water heater?

For standard municipal water, you should descale your unit once a year. If you have hard well water, you may need to flush it every six months. Installing a scale inhibitor or a water softener before the water heater intake can dramatically extend the time between flushes and protect your heating elements from premature failure. At the shower itself, a filtered shower head keeps mineral scale and chlorine off your skin between flushes.

Can a tankless water heater run a whole house?

Yes, a tankless water heater can easily run a whole house, provided it is sized correctly. Gas or liquid propane units are typically best for larger, multi-bathroom homes in northern climates due to their superior GPM capacities. In warmer climates, powerful electric units (18kW to 27kW) can comfortably handle whole-house hot water demands.

Why does it still take a few seconds for my water to get hot?

A tankless water heater heats water instantly, but it cannot instantly heat the cold water already sitting inside the pipes between the heater and your faucet. It still takes time for that standing cold water to be pushed out of the pipe. To get truly instant hot water at a distant faucet, you either need a local under-sink booster unit or a dedicated recirculation pump loop.

Does a tankless water heater save money on electricity?

Yes. Because they eliminate standby heat loss (the energy wasted keeping a large tank of water hot all day long), tankless water heaters are typically 24% to 34% more energy-efficient than traditional storage tanks. Most homeowners see a noticeable reduction in their monthly utility bills after making the switch.

About the Author

Andie Baz is a home improvement and appliance writer who focuses on helping homeowners make smarter purchasing decisions. He researches everyday home products and breaks down their features, performance, and practical value into straightforward buying advice.