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The best instant water heater for a shower depends less on the brand name and more on sizing. A 13 kW electric unit can work well for one low-flow shower in a warm climate, but the same unit can disappoint in a northern winter. A high-output condensing gas tankless unit can support multiple showers, but only if the gas line, venting, condensate drain, and maintenance plan are handled correctly.
On our job sites, we treat an instant water heater like a system, not an appliance swap. We look at showerhead gallons per minute, incoming water temperature, desired outlet temperature, simultaneous fixtures, available electrical service or gas capacity, water hardness, access for service, and local permit requirements. That is the difference between a shower that stays hot and a unit that technically turns on but throttles flow to a weak stream.
For 2026, our practical short list is simple: use a small point-of-use electric tankless for one efficient shower in a warm region, a mid-size electric tankless where the panel has capacity and demand is modest, a large electric unit only after a load calculation, and a condensing gas tankless for whole-home multi-shower comfort where gas is available and allowed by local code.
Quick Picks by Shower Situation
If you want the fast answer, start here. These are not one-size-fits-all recommendations; they are starting points that still need verification against your home.
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- CONTINOUS FLOW – Hot showers will never be interrupted again. Enjoy continuous, on-demand hot water with an unlimited supply, when you want it without wasting any energy when not needed
- SPACE & ENERGY SAVER – Its small, sleek white design saves space yet still provides endless hot water for your whole house, and no venting is required. Eco friendly unit saves energy with auto-modulation
- SILENT OPERATION – One electronic switch will activate the hot water heater and it’s noise free while in use. Digital temperature display is easy to read, while the interior solid copper heating system does all the power work
- TEMPRA 24 TREND – This specific model is 24kW, 240V, requires a min recommended electric service of 150 A, and offers an output water temperature of 68° to 140°F. Check our guides below to compare features of our various Trend and Plus models, each available in different kW levels
- WARRANTIES INCLUDED – Stiebel Eltron’s top-rated electric tankless water heaters perform with the highest standards for comfort and reliability. Our 7-Year Leakage and 3-Year Parts Warranties are included with purchase
| Shower situation | Best instant water heater type | Example models | Best fit | Watch-outs |
|---|---|---|---|---|
| One low-flow shower, warm climate | Small electric tankless | Rheem RETEX-13 or similar 13 kW unit | Point-of-use or nearby bathroom installation | Needs 240V power and typically a 60A double-pole breaker; poor fit for cold groundwater |
| One stronger shower or light whole-home use | Mid-size electric tankless | Rheem RETEX-18/24, Stiebel Eltron Tempra 20/24 Plus | Homes with panel capacity and moderate hot-water demand | Requires electrician review; output falls sharply as temperature rise increases |
| Large electric whole-home setup | Large electric tankless | Stiebel Eltron Tempra 29/36 Plus, Rheem RETEX-36 | All-electric homes with confirmed service capacity | May require 200A to 300A service depending on the home and model; electrical upgrade can exceed appliance cost |
| Two or more showers at once | Condensing gas tankless | Rinnai RU199iN/RX199iN, Rheem IKONIC condensing models | Whole-home shower comfort, larger families, long showers | Gas-line sizing, venting, condensate neutralization, drain routing, permits, and annual service matter |
| Long wait for hot water at a far bathroom | Recirculation or point-of-use design | Central tankless plus recirculation, or small electric unit near bath | Remodels, additions, distant primary suites | Tankless does not erase pipe-lag unless the plumbing layout addresses it |
What “Instant” Really Means at the Shower
“Instant water heater” usually means a tankless heater that heats water on demand instead of storing 40, 50, or 75 gallons in a tank. Once water is flowing and the unit is firing, it can keep heating continuously as long as the unit is properly sized and supplied.
It does not always mean hot water appears instantly at the showerhead. If the heater is 45 feet of pipe away from the bathroom, the cold water sitting in that pipe still has to clear first. That wait can be 20 seconds, 45 seconds, or more depending on pipe size, pipe length, and flow rate. For true fast delivery, we either place the heater close to the bathroom, use a point-of-use unit, or design a recirculation strategy.
This distinction matters. Many homeowners buy a tankless unit expecting zero wait time and are frustrated when the system still has pipe-lag. Tankless solves the “running out of hot water” problem. Recirculation or point-of-use placement solves the “waiting for hot water” problem.
The Sizing Math That Decides Everything
Tankless sizing starts with two numbers: flow rate and temperature rise. Flow rate is how many gallons per minute the shower and other fixtures need. Temperature rise is the difference between the incoming cold-water temperature and the hot-water temperature you want.
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A standard showerhead commonly uses 2.0 to 2.5 gallons per minute. Many low-flow showerheads use 1.5 to 1.8 GPM. A quality 1.5 or 1.8 GPM showerhead can make a borderline tankless system feel much better because the heater has less water to warm every minute.
Temperature rise is just as important. If your groundwater enters the house at 70 degrees F and you want 105 degree F shower water, the heater needs a 35 degree rise. If your winter inlet water is 40 degrees F and you want 115 degree F water available at the heater, that is a 75 degree rise. The same appliance produces much less usable flow at a 75 degree rise than at a 35 degree rise.
A Simple Shower Sizing Example
For one 1.8 GPM shower with 70 degree F inlet water and a 105 degree F shower temperature, the required rise is about 35 degrees. A smaller electric tankless unit may handle that in a warm-climate point-of-use installation.
Now change the conditions. Two 2.0 GPM showers plus a 0.5 GPM lavatory is 4.5 GPM. If winter inlet water is 40 degrees F and the heater is set around 120 degrees F, the required rise is roughly 80 degrees. That is a completely different load. A small electric unit will not keep up. In many homes, that scenario points toward a high-capacity gas tankless unit or a different water-heating strategy.
| Application | Likely flow | Warm-climate rise | Cold-climate rise | Practical recommendation |
|---|---|---|---|---|
| One 1.5 GPM shower | 1.5 GPM | 30-45 degrees F | 65-80 degrees F | Small to mid-size electric may work in warm areas; cold areas need careful sizing |
| One 1.8 GPM shower | 1.8 GPM | 35-50 degrees F | 65-80 degrees F | Mid-size electric is safer than a tiny unit; point-of-use helps |
| One 2.5 GPM shower | 2.5 GPM | 35-50 degrees F | 65-80 degrees F | Large electric or gas tankless depending on home capacity |
| Two showers at once | 3.6-5.0 GPM | 35-50 degrees F | 65-80 degrees F | Condensing gas tankless is usually the cleaner fit if gas is available |
| Two showers plus lavatory | 4.1-5.5 GPM | 35-50 degrees F | 65-80 degrees F | Whole-home high-output system; do not buy before load calculation |
Undersized tankless units often try to preserve outlet temperature by reducing flow. That means the shower may stay warm, but the water pressure can feel like a trickle. This is why sizing by advertised maximum GPM alone is risky. The published maximum is usually under favorable temperature-rise conditions.
Best Electric Instant Water Heaters for Shower Use
Electric tankless heaters are attractive because they do not need combustion air, gas piping, flue venting, or condensate handling. They are compact, quiet, and clean at the point of use. The tradeoff is electrical demand. A serious electric tankless water heater can draw more power than an oven, dryer, and air conditioner combined.
Best Small Point-of-Use Pick: Rheem RETEX-13 Class
A 13 kW electric tankless unit such as the Rheem RETEX-13 is a good fit for a single efficient shower in a warm climate, a pool bath, garage apartment, or addition where a long hot-water run would waste water and time. Retail specs list the RETEX-13 around 13 kW and 2.54 GPM, with a common retail price around $299.
This class of unit is not a universal shower solution. It typically requires 240V power and a 60A double-pole breaker. In warm incoming-water conditions and with a 1.5 to 1.8 GPM showerhead, it can work well. In a cold-climate winter, it may not deliver the flow and temperature most homeowners expect.
Best Mid-Size Electric Pick: Rheem RETEX-18/24 or Stiebel Eltron Tempra 20/24 Plus
For a stronger single shower or light whole-home use, we look at the 18 kW to 24 kW range. This is where units such as Rheem RETEX-18/24 and Stiebel Eltron Tempra 20/24 Plus become relevant. A Stiebel Eltron Tempra 20 Plus has been commonly listed around $579, and larger models step up from there.
This range can be a good answer for a well-planned bathroom remodel, especially where the home already has suitable electrical capacity. It is not a purchase to make from a product page alone. The electrician needs to verify panel capacity, conductor size, breaker requirements, voltage drop, and whether the home’s service can support the added load.
Best Large Electric Pick: Rheem RETEX-36 or Tempra 29/36 Plus
Large electric tankless heaters can serve bigger loads, but the power requirement is substantial. The Rheem RETEX-36 is listed at 36 kW and 7.03 GPM in retail specs, with a retail price around $749.99. Stiebel Eltron’s larger Tempra models also require careful electrical planning, and technical literature points to copper conductors rated 75 degrees C or greater, voltage-drop considerations, and service requirements that can reach 200A to 300A depending on the model and configuration.
For a homeowner, the appliance price is only part of the story. If the panel is full, the service is undersized, or the route from panel to heater is difficult, the electrical upgrade can cost more than the water heater. We do not recommend buying a large electric tankless unit until an electrician performs a real load calculation.
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When a house needs two showers at once, long back-to-back showers, or a primary bath with body sprays or a soaking tub nearby, condensing gas tankless is often the strongest tankless option. Gas brings high BTU input without the same electrical-service burden as large electric tankless systems.
Rank #2
- 【Electric Tankless Water Heater】The V7B-240B is compact design for point of use anywhere, in the meanwhile it is able to provide luxury hot water with its powerful 24kW heating system. It is great idea to install this tankless water heater electric a few ft away from shower tub or washing machine, to get sufficient hot water of higher than 120℉ in seconds.
- 【Smart Tankless Water Heater】Tankless water heater electric V7B-240B wastes no energy for preheating like tank water heater, instead, this instant water heater adjusts power input based on real time flow rate and temp setting, thus, no fluctuation out of it even when flowrate changes, which commits comfortable experience, safety and optimal energy efficiency of 99.8%, saving up to 50% on water heating cost.
- 【Safe& Durable Instant Water Heater】This electric water heater is newly designed with multiple protection devices, such as leakage protection, dry heating protection, high temp protection & auto stop. Every piece of V7B-240B goes through strict tests to ensure 100% safety and it is listed by ETL, suitable for residential, restaurant, school, hospital, office and other public facility.
- 【Unique Electric Water Heater】The electric tankless water heater V7B-240B comes with a specially designed heating chamber, which runs water line and electricity line separated for the benefits of no leakage, no inner corrosion and minimized scale deposit, so you can expect outstanding performance in years to come with nearly zero maintenance.
- 【Point of Use Water Heater】This electric tankless water heater is compact design for space saving & point of use, avoids energy & water waste. Comes with standard 3/4" NPT pipe fittings and it requires 240V supply via 3 pieces of “40A Double Pole Circuit Breaker” and 3 sets of “8AWG/2 wires with Grounding”, please check and confirm that your circuit breaker panel meet these needs.
A high-capacity model such as the Rinnai RU199iN is a good example of the category. It is an indoor condensing natural gas unit with 0.95 UEF, 15,000 to 199,000 BTU input, and published flow information up to roughly 9.8 to 11 GPM depending on conditions. Rheem IKONIC condensing models occupy a similar whole-home category.
For 2026, ENERGY STAR criteria for whole-home gas instantaneous water heaters include UEF of at least 0.95, maximum GPM of at least 2.8 over a 67 degree F rise, heat-exchanger warranty of at least 6 years, parts warranty of at least 5 years, and ANSI Z21.10.3/CSA 4.3 safety compliance. Eligible tankless gas water heaters with UEF of 0.95 or higher may qualify for a federal tax credit of 30% of project cost up to $600, subject to current tax rules and product eligibility.
The added work is real. A gas tankless installation may require a larger gas line, gas meter review, Category IV venting, intake air routing, condensate drain, condensate neutralizer, exterior termination clearances, and manufacturer-specific service clearances. In finished homes, vent routing and condensate disposal can decide whether the job is simple or expensive.
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Nationally, a whole-house tankless water heater installation commonly lands around $1,800 to $5,500 in 2026. Electric installations often fall around $1,800 to $4,200, while gas installations commonly run about $2,700 to $5,500 depending on venting, fuel-line work, access, and local labor rates.
Service pricing in 2026 commonly lists tankless installation around $2,400 to $5,400 nationwide, compared with about $1,600 to $2,400 for a standard tank replacement. Electric tankless equipment may cost $500 to $1,500 for many whole-home units, with installation from $150 to $1,000 in simple cases, for a total around $650 to $2,500. Natural gas tankless systems are often $750 to $2,000 for the unit and $900 to $1,500 for installation, for a total around $1,650 to $3,500. Propane systems often fall around $1,400 to $3,500 total.
Those numbers move quickly once the job leaves the easy path. Labor-only ranges around $800 to $2,000 are common for tankless installs, but relocation, fuel conversion, expansion tanks, earthquake strapping where required, vent upgrades, drywall repair, attic or crawlspace access, electrical upgrades, gas-line work, and water treatment can add meaningful cost.
| Cost item | Typical 2026 range | What changes the price |
|---|---|---|
| Small electric point-of-use unit | $299-$700 unit cost | kW rating, controls, brand, warranty |
| Large electric tankless unit | $700-$1,500 unit cost | Capacity, number of circuits, service requirements |
| Gas condensing tankless unit | $750-$2,000+ unit cost | BTU input, UEF, recirculation features, warranty |
| Tankless installation | $1,800-$5,500 installed | Electric service, gas line, venting, condensate, access |
| Permit fees | $50-$250 typical | City, scope, electrical/gas inspections, high-cost markets |
| Maintenance setup | $100-$400+ added scope | Isolation valves, flush kit, scale-control device, drain access |
We prefer to include isolation valves and flushing ports from day one. They make annual descaling simpler and cheaper. In hard-water areas, we also recommend water testing and a scale-control plan before installation. Hard water is one of the practical reliability issues that many product comparisons miss.
Electric vs Gas: Which Is Better for a Shower?
Electric tankless is usually better for one nearby shower, a bathroom addition, or an all-electric home with spare panel capacity. It is compact, avoids combustion venting, and can be placed close to the fixture. The biggest risk is underestimating electrical demand.
Gas tankless is usually better for whole-home use, especially in colder regions or multi-bath homes. A condensing gas unit can deliver high output through long showers and simultaneous use. The risks are installation complexity, vent routing, condensate handling, gas capacity, and maintenance.
In cold climates, we often lean toward condensing gas for whole-home shower performance when gas is available and allowed by local rules. In warm climates, a well-sized electric point-of-use unit can be a clean, cost-effective answer for a single shower. In many all-electric homes, a heat-pump water heater may beat tankless for operating cost and incentives, especially when space is available and endless shower duration is not the main goal.
Best Choice for Remodels and Additions
Bathroom remodels and additions are where the decision gets more interesting. If the new shower is far from the existing water heater, simply tying into the old hot-water line may create a long wait and waste water every morning. We usually compare three options.
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For a primary-suite addition, we also look at the shower valve, showerhead flow, tub filler, and fixture schedule. A single 1.8 GPM shower is one thing. A 2.5 GPM rain head, hand shower, and 6 GPM tub filler is another. The water heater, supply piping, drain planning, and ventilation all need to match the real design.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Installation Details That Separate Good From Bad
A good tankless installation is clean, serviceable, and permitted. For electric units, that means correct breaker sizing, conductor sizing, copper conductors where required by the manufacturer, proper disconnecting means, water-tight connections, and a load calculation before the unit is purchased.
For gas units, it means correct gas-line sizing, approved vent materials, proper intake and exhaust clearances, condensate drainage, neutralization where required, pressure relief, service valves, and manufacturer-required clearances. Condensing gas tankless units produce acidic condensate. That water needs a safe path to a drain, and many jurisdictions or manufacturers require neutralization before discharge.
Plumbing codes also matter. Water heaters generally need approved temperature and pressure relief provisions, proper installation clearances, safe discharge piping, listed components, and inspection where local law requires it. Some areas require expansion tanks, seismic strapping, drain pans, or specific vent termination distances. Permit fees are often modest, commonly $50 to $250, but the inspection is not optional work in many municipalities.
Rank #3
- [Compact & Space-Saving Design]The compact, space-saving design of this tankless water heater is ideal for small homes, apartments, and commercial spaces. It mounts easily on the wall, freeing up space in your bathroom or kitchen.
- [Safe & Reliable]Equipped with multiple safety features, including overheat protection and automatic shut-off, this tankless water heater ensures your safety while delivering hot water reliably and efficiently. The durable build guarantees long-lasting use.
- [Instant Hot Water On Demand]Enjoy consistent and endless hot water with our 24000W Electric Tankless Water Heater. Perfect for showers, bathrooms, sinks, and faucets, it provides instant, on-demand hot water whenever you need it, with no waiting time.
- [Easy Installation & Maintenance]With user-friendly installation instructions and minimal maintenance requirements, this water heater can be set up quickly and easily, making it an ideal choice for homeowners and professionals alike.
- [Energy Efficient & Cost-Effective]This 220V water heater is designed to save energy while delivering powerful performance. With no need for a storage tank, it heats water only when needed, reducing energy wastage and lowering your electricity bill.
When to DIY vs When to Hire a Pro
A homeowner with plumbing and electrical experience may be able to replace a small point-of-use electric unit in a like-for-like setup, provided the circuit, breaker, wire, disconnect, and plumbing connections already match the manufacturer’s requirements and local code. Even then, the permit question should be checked before work starts.
Hire a licensed pro when the project involves a new 240V circuit, a panel calculation, service upgrade, gas piping, venting, condensate routing, a whole-home heater, relocation, fuel conversion, or any work inside a finished wall that affects multiple trades. Gas tankless installation is not a casual DIY project. Mistakes can create combustion, carbon monoxide, gas leak, water damage, and warranty problems.
For most whole-home tankless installations, the right team is a plumber plus an electrician for electric units, or a plumber/HVAC-qualified installer for gas units, depending on local licensing. On construction and remodel jobs, we coordinate the water heater choice before rough-in so the electrical, gas, vent, drain, and framing details are not being forced in at the end.
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Maintenance and Lifespan Expectations
Tankless water heaters need maintenance. In clean, soft-water conditions, annual inspection and periodic flushing may be enough. In hard-water markets, descaling may be needed every year, sometimes more often depending on hardness and usage. Isolation valves and flush ports make this practical.
We also check inlet screens, combustion air paths, vent terminations, error codes, condensate neutralizers, and drain lines. For gas units, annual service helps confirm safe operation and efficient combustion. For electric units, maintenance is simpler, but scale buildup can still reduce performance and shorten element life.
A quality tankless unit can last many years when installed correctly and maintained. But a neglected tankless heater in hard water can lose performance much earlier than expected. The maintenance plan should be part of the buying decision, not an afterthought.
Our Bottom-Line Recommendation
For one shower in a warm climate, especially near the point of use, a 13 kW to 24 kW electric tankless unit can be the best instant water heater choice. Pair it with a quality 1.5 to 1.8 GPM showerhead, keep the pipe run short, and verify the electrical circuit before buying.
For a whole-home system with two or more showers, a condensing gas tankless unit in the 180,000 to 199,000 BTU class is usually the stronger performer where gas is available. Look for 0.95 UEF or better, confirm ENERGY STAR eligibility if you plan to claim incentives, and include venting, condensate neutralization, and maintenance valves in the estimate.
For large all-electric homes, do not assume a 36 kW tankless heater is the answer just because the product page shows high GPM. Start with a load calculation and compare it against a heat-pump water heater, central tank replacement, or hybrid approach. The best water heater is the one that fits the house, the climate, the panel, the plumbing layout, and the way your family actually showers.
Frequently Asked Questions
What size instant water heater do I need for one shower?
For one shower, start with the showerhead flow rate and temperature rise. A 1.5-1.8 GPM shower in a warm climate may work with a 13-24 kW electric tankless unit. A 2.0-2.5 GPM shower or cold incoming water usually requires a larger unit.
Will a tankless water heater give instant hot water at the shower?
Not always. Tankless means the heater makes hot water on demand, but cold water still sitting in the pipe must clear before hot water reaches the shower. For faster delivery, use point-of-use placement or a recirculation system.
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Is electric or gas tankless better for showers?
Electric tankless is often best for one nearby shower or a small addition where panel capacity is available. Gas tankless is usually better for whole-home use, multiple showers, and colder climates because it can deliver higher output.
How much does it cost to install a tankless water heater in 2026?
Whole-house tankless installation commonly runs about $1,800-$5,500 in 2026. Electric installs often fall around $1,800-$4,200, while gas installs are commonly $2,700-$5,500 depending on venting, gas-line work, electrical needs, permits, and access.
Can I install an instant water heater for a shower myself?
A simple like-for-like point-of-use electric replacement may be possible for a skilled homeowner, but new circuits, panel work, gas piping, venting, condensate routing, and whole-home installations should be handled by licensed professionals.
Do tankless water heaters need maintenance?
Yes. Most systems need periodic flushing, inlet screen checks, and inspection. In hard-water areas, annual descaling and a scale-control plan are especially important. Isolation valves should be installed from day one.
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