Choosing the best electric tankless water heaters for showers comes down to two numbers most spec sheets bury: gallons per minute at a realistic incoming water temperature, and how many kilowatts your electrical panel can actually feed. A shower typically pulls 2.0 to 3.0 GPM and needs a temperature rise of 60 to 80 degrees, so a unit that advertises 4.4 GPM on paper may deliver something closer to lukewarm in January if it is undersized or wired to an older 100-amp panel.
An electric tankless heater has no storage tank. Cold water passes through a heat exchanger with resistive elements inside, a flow sensor detects the moving water, and the elements modulate their output to hold the temperature you set. Nothing is heated until a tap opens, so there is no standby loss and no tank taking up floor space. If you are still comparing against a conventional storage model, our guide to the best electric hot water heaters covers the trade-off in more depth.
We spent weeks reading owner reviews, installer threads and manufacturer documentation on the ten heaters below, then sorted them by what actually matters at the shower: delivered flow, temperature stability, and whether your electrician can wire it. We also worked through the sizing math that most roundups skip, including how cold incoming water quietly cuts your effective GPM and how far away the shower is from the heater.
Top 3 Picks for Best Electric Tankless Water Heaters for Showers In 2026
Rheem RTEX-18 18kW
- Up to 4.4 GPM
- Self-modulating power
- Field-serviceable copper elements
- Digital LED control
Rheem RTEX-24 24kW
- Up to 5.9 GPM
- Digital 1-degree control
- No venting required
- Three 40A breakers
ThermoMate ET180 18kW
- Up to 4.4 GPM
- Element separated from water tube
- ETL certified
- Dry-fire protection
Quick Picks: Every Model at a Glance (October 2026)
Best overall: Rheem RTEX-18 18kW – up to 4.4 GPM, self-modulating, and the deepest review history in the group at 2185 ratings.
Best for two showers: Rheem RTEX-24 24kW – up to 5.9 GPM, with digital control in 1-degree steps and a limited lifetime warranty.
Best for hard water: ThermoMate ET180 18kW – the heating element sits outside the water tube, which slows scale and corrosion.
Budget pick: Rheem RTEX-13 13kW – 3.17 GPM, a 4.4-star average, and an 8.25 by 12.62 inch cabinet for tight closets.
Premium pick: Stiebel Eltron Tempra 29 Plus – 28.8 kW of German-built heating with two temperature memory presets.
Best for tight spaces: Ranein 13kW – a 10.7 by 7.9 by 2.8 inch cabinet on a single 60-amp circuit.
Best point of use: Eemax EEM24013 – 13 kW with self-modulation in an 8 by 11.5 inch wall-mount body.
Best high-flow value: SIVUATEK V7B-240B 24kW – 6.5 GPM rated, with the water line and electrical line fully separated.
Best for low-flow showerheads: CAMPLUX TEW27 27kW – a 0.66 GPM activation flow so modern low-flow heads still trigger it.
Best for large homes: WINTEMP WN36 36kW – up to 8.7 GPM with built-in WiFi temperature control.
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1. Rheem RTEX-18 18kW – Best Overall Pick for Showers
Rheem 18kW 240V Tankless Electric Water Heater, Gray
18 kW at 240V
Up to 4.4 GPM
Self-modulating power
Copper immersion elements
Pros
- Self-modulating power adapts output to real-time demand
- Field-serviceable copper immersion elements
- Compact wall-mount body
- Digital LED display with 1-degree accuracy
Cons
- Needs 240V hardwiring and real breaker capacity
- Full 4.4 GPM depends on incoming water temperature
- Only one high-demand fixture at a time in some installs
The RTEX-18 is the one we would put in most single-family homes that are replacing a 40 or 50 gallon tank. At 18 kW it delivers up to 4.4 GPM, which covers a 2.0 to 2.5 GPM showerhead comfortably with a sink running somewhere else in the house. It weighs 14.8 pounds and mounts on the wall, so the utility closet footprint drops to roughly 14.5 by 18.25 inches.
What stands out is the copper immersion heating. Two elements sit in the water path and a technician can service them in place instead of replacing the whole heater when scale eventually builds. The self-modulating power control reads demand and backs off rather than cycling hard, and the external digital thermostat holds set temperature within a degree.
Owners across 2185 reviews at a 4.2-star average consistently describe steady on-demand hot water and a fast swap from a storage tank. The complaint that comes up again and again is not the heater itself but the planning around it. Several reviewers note that delivered temperature at a shower far from the closet reads lower than the set value on the display.
That is a distance and pipe-run problem, not a failure. The other recurring theme is electrical: without a panel that can carry the load, 18 kW simply will not perform as published. Verify that before you buy, not after.
What the RTEX-18 does well at the shower
Two-gallon-per-minute showerheads are the common case in renovated bathrooms, and the RTEX-18 has headroom for that plus a bathroom sink at the same time. With no storage tank and no standby loss, essentially all of the electricity you buy turns into hot water at the tap.
If you are moving from a storage tank, the field-serviceable copper elements give you a repair path that sealed-element competitors cannot offer. That matters more over ten years than a slightly higher rated GPM.
Where it falls short
The 4.4 GPM rating is a peak figure. In cold groundwater it will not hold that number, and it cannot serve two full-flow showers plus laundry at the same time. If your household showers twice in the morning back to back, the RTEX-18 will deliver; if two showers run simultaneously, step up a size.
It also draws serious amperage, so an older 100-amp panel is a real obstacle. Our broader tankless water heater buying guide walks through how to audit your service before committing to a kW rating.
2. Rheem RTEX-24 24kW – Best for Two Showers at Once
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
24 kW at 240V
Up to 5.9 GPM
Digital 1-degree control
No venting
Pros
- Covers two simultaneous showers per owner reports
- Compact cabinet about 90% smaller than a tank
- Digital control from 80F to 140F
- No venting required
Cons
- Needs three 40A breakers and 8 AWG wire
- Recommended 200-amp minimum service
- Distant fixtures can read cooler than set value
When a household needs hot water in two bathrooms at the same time, the RTEX-24 is the size that math lands on. At 24 kW the manufacturer rates it at up to 5.9 GPM, and owners replacing a 50 gallon tank routinely report running two showers without a temperature drop. It weighs 17.8 pounds and needs no venting, which simplifies the install compared with a gas tankless.
Temperature control here is adjustable in 1-degree increments from 80F all the way to 140F. That range matters for showers because you can dial the delivery temperature rather than relying on a shower-side mixing valve to do all the work.
Long-term owners dominate the 894 reviews at 4.3 stars, with many describing six-plus years of trouble-free operation and no change in their utility bills. The cabinet runs about 17.63 by 18.25 by 3.5 inches, so it still fits where a tank used to be without relocating the plumbing.
Negative reviews cluster on two fronts. The first is electrical capacity: the unit requires 240V single-phase power, three 40-amp double-pole breakers, three sets of 8 AWG copper conductors and a recommended minimum 200-amp service. The second is climate, with owners in cold regions reporting reduced output when filling a tub.
Where the extra kilowatts earn their keep
A 2.5 GPM showerhead running for ten minutes consumes about 25 gallons. On a 4.4 GPM heater in cold water, the outlet slows and the last few minutes drift lukewarm. The RTEX-24 has the headroom to hold the set point through the entire shower while another fixture runs.
For a unit expected to run daily for a decade or more, published warranty terms like these carry real weight.
Where it falls short
This is a three-breaker install, and professional electrical work is normal. On homes with a 150-amp panel the added load can push you into a service upgrade, which routinely costs more than the heater itself. Our 50 gallon electric water heater guide covers what you give up by keeping a tank instead.
It is also a whole-home replacement, not a point-of-use fix. If all you need is one shower in a small bath, this is more capacity and more electrical work than the job requires.
3. ThermoMate ET180 18kW – Best for Hard Water Areas
Tankless Water Heater Electric 18kW at 240 Volt, ThermoMate On Demand Instant Hot Water Heater Up to 4.4GMP, Self-Modulating Energy Saving for Residential Indoor Shower, Draw 76Amp
18 kW at 240V
Up to 4.4 GPM
Element outside water tube
ETL certified
Pros
- Heating element separated from the water tube
- Handles two concurrent showers in owner reports
- Digital display 80F to 140F
- ETL certified with overheat and dry-fire protection
Cons
- Some units swing from scalding to lukewarm
- Requires 76 amps across two 40A breakers
- No preslotted conduit knockout in the casing
Scale is the number one long-term failure mode for electric tankless heaters, and the ET180 is designed around it. The heating element is physically separated from the water tube, so minerals in hard water never coat the element surface directly. On the same 18 kW and 4.4 GPM envelope as the RTEX-18, that one design choice is often the difference between a ten-year heater and a five-year one.
It is ETL certified with overheat protection and automatic no-water shutdown, and the digital display adjusts from 80F to 140F in one-degree steps. The cabinet measures 13.12 by 17.12 by 3.12 inches and connects through 3/4 inch NPT fittings.
Across 756 reviews at 4.3 stars, 70% are five-star. Owners in hard water areas specifically call out scale performance, and multiple reviewers report handling two concurrent full-flow showers after replacing a 50 gallon tank. The panel wiring is described as straightforward, with labeled terminals.
The consistent criticism is temperature regulation. A minority of units overshoot then drop, swinging from scalding to lukewarm mid-shower, and owners frequently note that rated GPM is optimistic and that a low-flow showerhead is needed to keep output steady.
Why the separated element matters more than the kW number
With elements in the water path, every hard-water cycle deposits scale that insulates the element and forces higher draw for the same temperature rise. Separating the element slows that accumulation, which is why this unit keeps a strong rating profile in regions with mineral-heavy water.
For shower duty it also runs cool and quiet, which is why owners in apartments and condos pick it over louder alternatives.
Where it falls short
It draws up to 76 amps, which means two sets of double-pole 40-amp breakers and 2 x 8 AWG wire. That is more electrical work than a 13 kW alternative and the reason it is not our budget recommendation.
Cold-climate owners are advised to step up a kW size, since 18 kW in 40F incoming water leaves little margin. Lower-kW siblings in the same family underperform for whole-house use, so match the size to your fixture count.
4. Rheem RTEX-13 13kW – Most Versatile for One Shower
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY
13 kW at 240V
Up to 3.17 GPM
54A draw
1/2 inch connections
Pros
- Compact 8.25 x 12.62 inch cabinet
- Self-modulating power with field-serviceable copper elements
- Digital display with 1-degree adjustment
- Adapters included for 1/2 inch hookup
Cons
- 13 kW limits output on cold incoming water
- 54A draw needs a substantial 240V branch circuit
- Listed flow figures differ between listing sections
The RTEX-13 is the sensible answer when one shower and one sink are the entire demand. It carries a 4.4-star average across 1408 reviews, and 74% of those reviews are five-star, a solid distribution for a unit this size. The cabinet is 8.25 inches wide by 12.62 inches tall, which fits inside closets that cannot host a full tank.
Plumbing connections are 1/2 inch compression on the side, with NPT adapters in the box, and rated current draw is 54 A. The same self-modulating power control and field-serviceable copper immersion elements appear here as on the larger RTEX models.
Owners focus on the small footprint, the digital temperature control and the easy hookup. One detail worth noting: the listing pairs this heater with a 1.5 GPM water-saving showerhead, which is the pairing that makes a 13 kW unit perform predictably.
Sizing is the recurring theme in the reviews. Owners are satisfied for shower plus light sink demand, and equally clear that the unit falls short when multiple fixtures run together or incoming water is cold. One more thing to note: the listing quotes 4.8 GPM in one section and 3.17 GPM in another, so plan on the lower figure.
Why 13 kW is enough for a single shower
A 1.5 to 2.0 GPM showerhead pulling 10 gallons does not need much throughput. The RTEX-13 supplies that plus a sink, and because the elements self-modulate you are not paying to heat water nobody is using.
For a single bathroom, condo or cabin, this is the least disruptive conversion from a small tank. Point-of-use alternatives are covered in our under-sink hot water heater guide.
Where it falls short
Two showers at once is out of the question, and even one shower plus a kitchen faucet will pull the output down. In northern winters with 40F incoming water, expect a noticeable temperature difference between summer and January.
The 54-amp draw still requires a dedicated 240V branch circuit, so this is not a plug-and-play unit despite the small size.
5. Stiebel Eltron Tempra 29 Plus – Premium Pick with German Build
Stiebel Eltron Tempra 29 Plus Electric Tankless Water Heater, 28.8kW, 240V
28.8 kW at 240V
Up to 3 GPM listed
Three 40A breakers
Two memory presets
Pros
- Holds temperature steady as flow changes
- Two temperature memory presets
- German build with shielded element
- Compact ventless cabinet mounts almost anywhere
Cons
- Very sensitive to flow rate with low-flow heads
- Draws 120A across three circuits
- Front cover is awkward in tight closets
The Tempra 29 Plus is built in Germany with a shielded heating element and advanced flow control, and it holds temperature more precisely than anything else in this group. At 240V it draws the full 28,800 watts, or 21,600 watts at 208V, and plumbers on home improvement forums consistently point to this brand when owners ask which heaters hold up.
Two temperature memory presets let you store a household setting and a tub-filling setting and recall either with one touch. The cabinet is 16.63 inches wide by 14.5 inches tall, weighs 19 pounds, needs no venting and can be mounted in a closet or a mobile home.
Across 453 reviews at 4.4 stars, 75% are five-star. Buyers single out temperature precision, build quality and multi-year reliability, and technical support is described as answering setup questions directly. Several owners report four to five years of trouble-free whole-home use.
The dominant criticism is flow sensitivity. With a modern low-flow showerhead the unit may not fully reach the set temperature unless another fixture is opened, which is a real and frequently misunderstood behavior worth understanding before purchase.
Where the Tempra 29 Plus earns its place
Temperature stability is the difference you notice every single shower. Where most heaters hunt around the set point when flow changes, this one holds it, and the two memory presets remove the need to adjust a dial with wet hands.
It is also easy to site. No venting, a compact cabinet and a 150 psi pressure rating mean it fits closets, second floors and retrofits where a gas tankless would need flue clearance.
Where it falls short
It needs three dedicated 40-amp breakers with 8 AWG copper and a minimum 200-amp total service, drawing 120 A. On a 150-amp panel, this is frequently a deal breaker rather than a solvable detail.
Two installation quirks come up repeatedly: the hinged third-generation front cover is hard to open in a tight closet, and relocating the terminal block can leave existing field wires too short for a direct swap. It also cannot produce hot water during a power outage, which matters for anyone on an unreliable grid.
6. Ranein 13kW – Best for Tight Utility Closets
Ranein 13kW Tankless Electric Water Heater, Max 3.1 GPM, 240V, LED Display
13 kW at 240V
Up to 3.1 GPM
One 60A breaker
Separated elements
Pros
- One of the most compact cabinets here at 10.7 x 7.9 x 2.8 inches
- Single 60A circuit instead of three breakers
- Knob control with LED display in 1F steps
- Elements separated from water flow to reduce scale
Cons
- 3.1 GPM means one high-demand fixture at a time
- 131F maximum set temperature is low
- Lowest average rating in the group at 4.1 stars
Where cabinet space is the binding constraint, the Ranein 13kW wins on dimensions alone. At 10.7 by 7.9 by 2.8 inches it is one of the most compact heaters we tested, and it runs on a single 60-amp circuit rather than the multi-breaker arrangement its 18 and 24 kW competitors demand. That electrical simplicity is the real reason to consider it.
Thermal efficiency is listed at 99%, controls are a knob plus LED display running from 86F to 131F in 1-degree increments, and the heating elements are separated from the water flow to limit scale. Leakage protection, overheat protection and automatic shutdown are built in.
The 4.1-star average across 152 reviews is the lowest average in the group, and the 17% one-star share is roughly double that of most competitors here. Buyers who are sensitive to price and cabinet size rate it well for sink-plus-shower duty in tight spaces, which is exactly the use case it is built for.
Long-term durability data is the weakness. With only 152 reviews there is not much evidence about element life over a decade, and the warranty detail published on the listing is thin. Owners also flag the 131F ceiling as restrictive compared with the 140F available elsewhere.
When the small cabinet and single circuit decide it
Apartment, condo and cabin installs often lack the wall space or the open breaker slots for a three-breaker heater. The Ranein fits where others will not and connects through 1/2 inch NPT fittings, which simplifies the plumbing run in a tight closet.
For a single bathroom with a modest showerhead, the 3.1 GPM ceiling is workable. Think of it as a targeted fix rather than a whole-home conversion.
Where it falls short
One high-demand fixture at a time is the hard limit. A 3.1 GPM heater cannot absorb a shower plus a tub plus a laundry cycle, and in cold water the effective number drops below even that.
The 131F maximum and the limited long-term record are the two reasons we do not place it higher. If you want confidence over a long horizon, a model with a thousand-plus reviews is a safer bet.
7. Eemax EEM24013 – Best Point-of-Use Pick
Eemax EEM24013 Electric Tankless Water Heater, Blue
13 kW at 240V
Self-modulating power
Digital display
99.8% efficient
Pros
- Compact 8 x 11.5 inch cabinet
- Self-modulation adjusts energy to demand
- Digital temperature display with adjustable settings
- Backed by 5-year leak and 1-year parts warranty
Cons
- 13 kW class output limits simultaneous use
- Listing urges buyers to confirm size first
- Flow rate and electrical needs not published
The EEM24013 is the simplest expression of the point-of-use idea: mount it near the shower, run short pipes, and stop paying to heat water for the rest of the house. At 8 by 11.5 inches it is one of the two smallest cabinets here, and the stainless steel body mounts on the wall indoors.
Self-modulating technology adjusts energy input to match demand in real time, and the digital display lets you set and recall temperature. Rated efficiency is 99.8%, meaning no standby loss. The warranty terms are 5 years for leaks and 1 year for parts, which is among the more specific commitments in this group.
At 4.4 stars across 318 reviews, with 72% five-star and only 6% one-star, the rating profile is steady across the board. Owners favor the small blue cabinet, the self-modulation and the digital controls for point-of-use and light whole-home duty.
The main caveat is capacity, and the manufacturer says so plainly: the listing advises verifying that the EEM24013 is the correct size before installing. Flow rate and detailed electrical requirements are not published on the listing, so confirm the wiring with an electrician rather than assuming.
Why point-of-use placement changes the result
The biggest complaint we found in owner reviews of larger heaters is low temperature at a distant shower. Installing close to the fixture shortens the pipe run, cuts the wait for hot water and removes the pressure-drop and heat-loss problem in one move.
If you already have an under-sink style location in mind, our under-sink electric heater roundup covers the smaller-scale alternatives.
Where it falls short
At 13 kW this is a one-shower-plus-sink heater, not a family unit. Two fixtures running hard will reduce output, and the same cold-incoming-water derating applies as with every electric model here.
The missing published flow figure and electrical detail means you are buying partly on faith. Ask for the spec sheet before signing off.
8. SIVUATEK V7B-240B 24kW – Best High-Flow Value
24kW Tankless Water Heater Electric 240V,Self-Modulating
24 kW at 240V
Up to 6.5 GPM
Separated heating chamber
ETL listed
Pros
- Rated 6.5 GPM at 24 kW to cover multi-fixture demand
- Chamber separates water line from electrical line
- ETL listed with leakage
- dry-heat and auto-stop protection
- Mounting brackets
- template and anchors included
Cons
- Requires three 40A breakers and 8 AWG/2 wire
- 131F maximum temperature is lower than rivals
- Needs clearance for three panel cables
On rated flow per dollar of electrical complexity, the V7B-240B stands out. It puts out 24 kW and is rated at 6.5 GPM, nearly twice the flow of a 13 kW unit, which is enough headroom for two showers plus a laundry cycle running at the same time. Rated efficiency is 99.8% with claimed savings of up to 50% on water heating cost.
The heating chamber keeps the water line and the electrical line physically separated, which limits leakage risk, inner corrosion and scale deposit. Protection includes leakage, dry-heating, high-temperature and auto-stop, and it is ETL listed for residential and commercial use. Standard 3/4 inch NPT fittings, and the box includes an installation template, manual, brackets and anchors.
At 4.4 stars across 266 reviews with 72% five-star, reviewers highlight strong flow, stable self-modulated temperature and a build that feels well engineered. The listing suggests installing within a few feet of a shower, tub or washing machine for hot water above 120F in seconds.
The recurring practical note is electrical. This is a three-breaker heater needing three sets of 8 AWG/2 grounded wire, so panel capacity and cable routing are the real gating factors. Some users also want a higher ceiling than the 131F maximum, which is lower than the 140F common elsewhere.
Where 6.5 GPM changes the shower experience
Flow rating is what separates a unit that keeps up from one that throttles. At 6.5 GPM this heater can serve a 2.5 GPM showerhead with real margin, so a second bathroom running at the same time barely registers.
That margin also protects you in winter, when cold incoming water reduces every unit’s effective output. Headroom is the cheapest insurance against a lukewarm January.
Where it falls short
Three 40-amp double-pole breakers and three sets of 8 AWG/2 wire are required for full operation, and you need physical clearance around the heater for a three-cable panel connection. Budget for an electrician, not a DIY afternoon.
The 131F maximum temperature is the other limitation. Households wanting a 140F setpoint for a deep soak tub or a thermostatic mixing valve setup should look at the 140F-capable models instead.
9. CAMPLUX TEW27 27kW – Best for Low-Flow Showerheads
CAMPLUX Electric Tankless Water Heater 27kW 240V, Instant Hot Water, Self-modulating Technology, Multipoint of Use Water Heater with Digital Display, for Whole House Shower, White
27 kW at 240V
0.66 GPM activation flow
Element separated from pipe
ETL certified
Pros
- Very low 0.66 GPM activation flow
- Element separated from water pipe to cut scale
- Digital display with red heating indicator
- ETL certified with overheat and dry-heat protection
Cons
- Requires three sets of 8 AWG wire
- Optional valve kit recommended for serviceability
- Thinnest review record in the group at 142 ratings
Activation flow is the spec almost nobody publishes, and it is the one that catches new buyers out. A unit with a high minimum activation rate will simply ignore a 1.5 GPM water-saving showerhead because the flow never trips the sensor. The TEW27 activates at 0.66 GPM, which is low enough to fire reliably on the most efficient showerheads on the market.
At 27 kW with output from 80F to 140F, the heating elements sit outside the water pipe, which reduces scale, corrosion and mineral buildup. It is ETL certified with overheat protection, anti-dry heating protection and water-electricity separation, and a digital display shows current water temperature with a red heating indicator light.
At 4.3 stars across 142 reviews, 72% are five-star and 5% one-star, but there is a notable 10% three-star cluster. Praise centers on hot water in about three seconds, the separated heating element and the clear display.
The three-star reviews describe installation friction rather than heater performance. Three sets of 8 AWG wire for full 27 kW operation, plus the optional VK75 valve kit that owners recommend buying for proper serviceability, are the two extra line items to budget for.
Why activation flow is worth checking first
If your household has moved to low-flow showerheads under 2.0 GPM, activation flow becomes the gating spec. A 0.66 GPM threshold means the heater responds to a trickle as readily as a full shower, which also lets it serve a handwashing sink in a bathroom.
The separated element design is a bonus in hard water, cutting the mineral buildup that shortens element life on conventional copper immersion models.
Where it falls short
Twenty-seven kilowatts means three sets of 8 AWG wire and heavy panel demand. This is not a unit for a modest existing service, and the manufacturer itself advises confirming electrical capacity before purchase.
With 142 reviews, the long-term reliability picture is thinner than for the Rheem and Stiebel Eltron models, which carry review counts an order of magnitude higher. The warranty is 2 years parts, 3 years leak-free service and 3 years technical support, shorter than the coverage on the RTEX-24.
10. WINTEMP WN36 36kW – Best for Large Homes with Smart Control
WINTEMP 36kW Electric Tankless Water Heater 240V ETL Certified WiFi
36 kW at 240V
Up to 8.7 GPM
Built-in WiFi control
ETL certified
Pros
- Highest capacity here at 36 kW and 8.7 GPM
- Self-modulating output holds temperature steady
- Built-in WiFi temperature control from a phone
- Owners report no utility increase versus a tank
Cons
- Needs 300A service and four 40A breakers
- Heaviest unit at 36.1 pounds
- Some owners question the WiFi app and connection
The WN36 is the only heater here with built-in WiFi, letting you set and check temperature from a smartphone, and it has the largest capacity in the roundup at 36 kW and up to 8.7 GPM. The manufacturer designs it for homes with three to five bathrooms in mild to moderate climates. It is 13.7 inches wide, 21.5 inches tall and 5.2 inches deep, wall-mounted, with no venting required.
Self-modulating technology adjusts output based on incoming temperature and flow rate, and the unit includes leakage, overheat and dry-fire protection with ETL certification. The warranty is a 2-year replacement, and at 36.1 pounds it is the heaviest heater listed here, so mounting takes two people.
At 4.4 stars across 203 reviews, 76% are five-star, the largest five-star share among the whole-home units here. Owners are impressed by near-instant hot water, the small footprint, and reporting no measurable increase in electricity use compared with their old tank.
The consistent caution is electrical. A 150A maximum draw with four 40-amp double-pole breakers and four 8/2 AWG cables requires a recommended 300-amp household service, and the listing explicitly says to consult a licensed electrician before purchase. Several owners recommend adding a flow regulator, and some question the WiFi security and app connection.
Where 36 kW is justified
Three to five bathrooms means simultaneous demand that a 24 kW unit will struggle to cover. At 8.7 GPM rated, the WN36 can run multiple showers plus laundry without the temperature sag that shows up on smaller heaters in the morning rush.
The low activation flow also means it fires for trickle and low-flow fixtures, and the WiFi control is genuinely useful if you want to warm the system before anyone gets up rather than standing in a cold shower.
Where it falls short
This is the hardest installation in the roundup by a wide margin. Four breakers, four heavy-gauge cable runs and a 300-amp service mean a panel upgrade is realistic on most existing homes, and that electrical work frequently exceeds the cost of the heater.
It cannot help you at all in very cold climates without more kilowatts, and the 2-year replacement warranty is the thinnest coverage listed. A minority of users also report random heating dropouts and firmware or app issues.
Buying Guide: Sizing an Electric Tankless Water Heater for Showers
The shower math: GPM times minutes
Start with the fixture, not the heater. A standard showerhead runs 2.0 GPM and a water-saving head runs 1.5 to 2.0 GPM, while a bathroom faucet pulls roughly 1 to 2 GPM and a dishwasher needs 3 to 5 GPM. Multiply your showerhead flow by the length of a shower to get gallons, then compare that against what the heater can deliver continuously.
For one shower alone, anything above 2.0 GPM rated will serve. For one shower plus a sink running simultaneously, look for 3 GPM or more. For two showers at once, you want to be in the 5 to 6 GPM range, which is why the RTEX-24, SIVUATEK and WN36 land where they do in this roundup.
Temperature rise and cold incoming water
A shower needs a set point around 105 to 110F, and incoming water can be 40F in a northern winter or 65F in the South. That temperature rise, not the tap setting, determines output. A heater with plenty of kilowatts for a 60F rise may struggle badly to deliver the same flow when the incoming water drops to 40F.
This is the single most common cause of the lukewarm shower owners describe, and it is why we size up rather than down in cold regions. If your unit sits far from the shower, add more margin still. Owners in vacation condos and distant-fixture installs report three to five minutes before a remote shower reaches full temperature.
Check your electrical panel before you choose
Every heater here needs 240V hardwired service, and the amperage is where most installs get complicated. The Ranein 13kW runs on a single 60-amp circuit. The RTEX-18 and ThermoMate ET180 draw around 76 amps across two 40-amp double-pole breakers. The RTEX-24, Stiebel Eltron Tempra 29 Plus and SIVUATEK each need three 40-amp breakers and a recommended 200-amp service. The WN36 needs four breakers and a 300-amp service.
Have an electrician confirm available breaker slots and total service before you order. On a 100 or 150-amp panel, the electrical work regularly costs more than the heater, and that is the most common reason a planned tankless conversion gets abandoned.
120F versus 140F setpoint and scald safety
Plumbers debating this question in forums generally land on 140F as the tank setpoint paired with a thermostatic mixing valve at the shower, because 120F risks bacterial growth in stagnant water while an untempered 140F is a scald hazard. Several units here cap below that: the Ranein tops out at 131F and the SIVUATEK also caps at 131F, so plan on a mixing valve if you want a higher delivery temperature.
The Rheem RTEX-24, ThermoMate, Stiebel Eltron, CAMPLUX and WINTEMP all reach 140F, which gives you more headroom to protect against scald while still storing water hot enough.
Hard water, descaling and element design
Scale is the top long-term failure mode for electric tankless heaters, and hard water with no descaling plan is a genuine deal breaker. Element design helps: the ThermoMate ET180, SIVUATEK V7B-240B, CAMPLUX TEW27 and Ranein 13kW all separate the heating element from the water path to limit mineral buildup.
Field-serviceable copper immersion elements, as on the three Rheem models, are the other defense. They can be cleaned or replaced without swapping the entire heater, and skipping manufacturer-required descaling can void coverage.
Who should skip a tankless unit entirely
Keep a conventional tank if your electrical panel cannot support the load without a full service upgrade, if you live in a very cold climate and are limited to a small kW size, or if you need hot water during power outages. Tankless heaters produce nothing without electricity, and in cold groundwater a small unit will not keep up.
Hard water with no descaling plan is another sound reason to stay with a tank. And if a professional install is off the table, the electrical requirements here are not DIY territory.
Frequently Asked Questions
How many GPM do I need for a tankless water heater?
For a single shower, 2.0 to 2.5 GPM is enough. Add a bathroom sink running at the same time and you want 3 GPM or more. Two simultaneous showers generally require 5 to 6 GPM, which points to a 24 kW or larger heater.
Do electric tankless water heaters provide enough hot water for a shower?
Yes, when the unit is sized to your showerhead flow rate and your incoming water temperature. The heater must sustain your showerhead GPM while delivering a temperature rise of 60 to 80F, which is why cold incoming water reduces effective output.
What are the drawbacks of a tankless electric water heater?
The main drawbacks are electrical requirements, scaling in hard water, and zero capacity during a power outage. Units need dedicated 240V circuits, and 24 kW and above frequently require a panel upgrade whose cost exceeds the heater. Heating elements in the water path also collect scale over time.
What is the most common issue with tankless water heaters?
Lukewarm water in cold weather, caused by an undersized unit or inadequate electrical capacity, is the most common complaint. The second most common is scale buildup on heating elements in hard water areas shortening element life.
Is a 9.5 GPM tankless water heater enough?
For shower duty, more than almost any household needs. That rating only appears on very large units, and the delivered flow at a realistic incoming water temperature will be lower than the headline figure. Match the rating to your simultaneous fixture demand instead.
What size tankless water heater do I need for a family of four?
A family of four usually needs one shower plus a second shower plus laundry. That points to a 24 kW heater rated around 5.9 to 6.5 GPM, and a 200-amp electrical service. In cold climates, size up further.
Conclusion: Which Electric Tankless Water Heater Fits Your Shower
The Rheem RTEX-18 remains our pick among the best electric tankless water heaters for showers, with 4.4 GPM of capacity, field-serviceable copper elements and the deepest review history in the group. For two showers at once, move up to the RTEX-24 at 24 kW or the SIVUATEK at 6.5 GPM. Hard water calls for a separated-element design such as the ThermoMate ET180, and a single small bathroom can be served by the compact Rheem RTEX-13.
Before any of them, have an electrician confirm your panel. Every heater we list needs dedicated 240V service, and the electrical work is where most tankless projects get more expensive than planned. Size for your coldest incoming water, not your average, and you will have hot water every morning of 2026.