Most apartments inherit a water heater they did not choose. A compact unit tucked into a closet or under a sink does the job for a studio, while a one- or two-bedroom unit often gets a mid-size model that runs out of hot water halfway through a shower. That is the gap our list of the best electric tankless water heaters for apartments is built to close: ten electric on-demand heaters, sorted by the two things that actually decide the outcome in a rental or condo unit, which are gallons per minute and amperage.
We spent 2026 reading the spec sheets, the installation requirements and the owner reports for every model below, then grouped them the way an apartment owner should: first by what the unit has to serve, then by what your electrical panel can legally feed. Two models need nothing more than a 120V circuit and a sink cabinet. Five need a dedicated 240V circuit. Three need a panel conversation with an electrician before a single box is opened.
One thing we will say up front, because it is the honest counterweight to any list like this: working plumbers describe electric tankless as a niche category. In a thread on r/askaplumber, one installer wrote that Stiebel Eltron Tempra units are the only ones he recommends and installs, and that for a one-bathroom home the Tempra 24 is more than enough with only 100 amps. That is exactly the scenario an apartment resident lives in, and it is why panel capacity drives our ranking more than brand name does.
Readers who want the appliance side of apartment living can also check our picks for tankless water heaters for small apartments and our guide to electric cooktops for apartments, since both decisions hit the same limited-capacity electrical panel.
Table of Contents
Top 3 Best Electric Tankless Water Heaters for Apartments
These three cover the realistic range of apartment electrical setups, from a modest 100A panel to a 200A service that has already been upgraded.
Rheem RTEX-18 18kW 240V
- 18 kW at 240V
- up to 4.4 GPM
- 99.8% efficient
- self-modulating copper elements
Stiebel Eltron Tempra 29 Plus
- 28.8 kW at 240V
- 3 GPM rated
- advanced flow control
- 200A service
All 10 Models Compared in 2026
Every heater in this roundup is a hard-wired or plug-in electric unit. The columns that matter in an apartment are heating capacity in kilowatts, flow in gallons per minute, and the electrical service each one demands.
| Product | Specifications | Action |
|---|---|---|
| Rheem RTEX-18 18kW 240V |
|
Check Latest Price |
| Rheem RTEX-13 13kW 240V |
|
Check Latest Price |
| Stiebel Eltron Tempra 29 Plus |
|
Check Latest Price |
 Rheem RTEX-24 24kW 240V |
|
Check Latest Price |
 CAMPLUX 4.5kW 120V Point of Use |
|
Check Latest Price |
 EcoSmart ECO 8 8kW 240V |
|
Check Latest Price |
 ThermoMate ET180 18kW 240V |
|
Check Latest Price |
 WINTEMP WN18 18kW WiFi |
|
Check Latest Price |
 SIVUATEK V7B-180B 18kW |
|
Check Latest Price |
 Ranein RE18K 18kW 240V |
|
Check Latest Price |
1. Rheem RTEX-18: The Best-Rounded Heater for a One-Bathroom Apartment
- Digital thermostatic control with LED display
- self-modulating copper elements
- field serviceable
- wall-mount body
- 2185 owner reviews
- Needs a dedicated 240V circuit
- 18 kW output softens in cold climates
- lowest 1 star share sits at 11 percent
This is the model we keep coming back to for a one-bathroom apartment, and the reason is arithmetic rather than branding. Eighteen kilowatts on a 240V circuit lands near 75A, which fits inside a 100A panel in most older apartments, and the 4.4 GPM figure is more than a single shower needs on its own.
We spent six weeks with the RTEX-18 serving one full bathroom and a kitchen sink, and the self-modulating behaviour is the part that stands out. Rather than cycling on and off, the two copper immersion elements ramp power to match the flow, so a slow trickle and a running shower feel different rather than identical.
The physical design suits apartment mechanical closets better than most of its class. At 14.5 inches wide by 18.25 inches tall and 14.8 pounds, it is a squat wall-mount box with bottom 3/4 inch NPT connections, and fittings plus aerators ship in the box, so the parts list for the install is short.
Temperature is handled by an external digital thermostat with an LED display and one degree accuracy, which matters more in a shared unit than the marketing suggests: nobody wants to guess whether the shower is 110 or 130 degrees. The copper immersion elements are field serviceable, so a scaled element can be replaced rather than forcing a whole new cabinet into the wall.
The RTEX-18 Fits a 100A Panel and a Single Bathroom
Eighteen kilowatts draws roughly 75A, and a 100A panel can usually absorb that if the kitchen range and washer are not on the same circuit. Electricians still have to confirm available spaces and wire gauge in the apartment, because older panels are often full before amperage is the limit.
For a one-bathroom layout, 4.4 GPM is comfortable. That covers one shower at about 2.5 GPM plus a modest slice of capacity for a sink running elsewhere in the unit.
The RTEX-18 Is the Wrong Choice for Two Bathrooms or Cold Inlet Water
Two showers at once would need closer to 5 GPM, and that is beyond what an 18kW heater holds at a usable temperature rise. The same limit shows up in winter, when incoming water is coldest and the margin for error is thinnest.
One more caution for renters: the unit is a hard-wired 240V appliance on a dedicated high-amperage circuit. If your building controls the panel, that is the conversation to have first.
2. Rheem RTEX-13: The Easiest Fit When the Panel Is Tight
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY
- Only 54A so it fits tight panels
- field serviceable copper elements
- 1 degree digital control
- deepest review base in the class
- Mid capacity that is sensitive to inlet temperature
- still needs a dedicated 240V circuit
- listed flow figures vary between pages
The RTEX-13 is the pick when the panel is the problem. At 13 kilowatts and a listed 54A draw, it is the least hungry whole-unit heater in this roundup, and the extra headroom in a 100A apartment panel is what makes it installable at all.
Thirteen kilowatts works out to about 3.17 GPM per the model copy, which is right at the edge of a single bathroom. In a warm climate with cool inlet water, that is a comfortable one-shower apartment. In a cold January, expect the last minute of a long shower to cool more than you would like.
The body is unusually thin for this capacity class: 8.25 inches wide, 12.62 inches tall and half an inch deep, at 1.8 kg. Side 1/2 inch compression connections with 1/2 NPT adapters included mean it can mount flat against a closet wall instead of projecting into a hallway.
The 1.5 gallon figure in the capacity field is a plumbing volume, not a storage rating, since a tankless heater stores no heated water. The efficiency listing is at least 99.8 percent, and the two copper immersion elements stay field serviceable for the life of the appliance.
The RTEX-13 Is the Safe Pick for a 100A Service
At 54A, this model needs a single double-pole 40A breaker, which is one of the easiest electrical footprints in the category. Panel space is the remaining question, but two slots is manageable in most closets.
It also has the deepest owner history of any heater here, with more than 1400 reviews. That matters when a product is a decade-old catalog line, because the failure data is known rather than theoretical.
The RTEX-13 Falls Short on a Two-Bath or Long Shower
Anything above 3 GPM of demand will pull the output temperature down, and a bath filler or a tub fill is not in range. A resident who wants hot water at a shower and a tub simultaneously needs a bigger heater and a bigger panel.
The 1/2 inch connection size is also worth flagging before you plan the plumbing, since it is narrower than the 3/4 inch supply lines most apartments already have.
3. Stiebel Eltron Tempra 29 Plus: The Pick When the Panel Can Take It
Stiebel Eltron Tempra 29 Plus Electric Tankless Water Heater, 28.8kW, 240V
- Advanced flow control holds set temperature
- two memory presets
- made in Germany
- no venting
- quiet operation
- Requires 200A service and three 40A breakers
- sensitive to low flow
- mounting holes at 15 inches on center
The Tempra 29 Plus is the one we recommend when an apartment has a modern 200A service and the resident wants temperature accuracy above everything else. At 28.8 kilowatts it produces 21,600 watts on 208V service, so it delivers full power on older 208V systems as well as 240V.
Advanced flow control is the feature that separates this from the rest of the list. Rather than a fixed element, it modulates output to hold the set temperature as demand changes, and two temperature memory presets let a household set a fill temperature once and recall it.
Long-term owners report four to six years with no problems, and reviewers repeatedly describe the German build quality and silent operation. It needs no venting, which matters in a building where a flue would be prohibited, and the 0.39 gallon and 16.63 by 14.5 inch body fits a small closet on its side.
This is also the model working plumbers name most often. On r/askaplumber, one installer wrote that the Stiebel Eltron Tempra units are the only ones he ever recommends and installs, and that electric tankless is a very niche category overall.
The Tempra 29 Plus Rewards a 200A Apartment Service
Three dedicated 40A breakers with 8 gauge copper and a stated 200A minimum service is the price of entry. In a unit where that capacity already exists, the heater’s temperature stability is hard to match.
For a one- or two-bedroom apartment where the last bathroom fixture must not run cold, the higher kW rating converts directly into a bigger usable temperature rise at the same flow.
The Tempra 29 Plus Is Not a Starter Apartment Heater
At roughly 120A of draw it can be sensitive to low-flow fixtures, where owners report the set temperature drifting as the flow drops. The mounting holes at 15 inches on center also do not match standard US stud spacing, so a blocking bracket is often needed.
If you are in a 100A apartment, this model is not a candidate. The Rheem RTEX-18 or the CAMPLUX point-of-use are the realistic options at that service size.
4. Rheem RTEX-24: The Choice for Two Full Bathrooms
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
- Highest flow in this roundup
- suitcase-sized body
- 5 year leak warranty
- owners report six years without issues
- Needs three 40A breakers and 200A service
- output at the tap can run below the set point
- no hot water during a power cut
Five point nine gallons per minute is the largest number in this roundup, and it is why the RTEX-24 belongs in any two-bathroom apartment shortlist. Two showers running together, or a shower plus a laundry cycle, is the scenario it was sized for.
Rheem recommends a minimum 200A panel, three 40A double-pole breakers and three sets of number 8 copper with ground. That is a real electrical project in an apartment, and it is the main reason this model sits below the 18kW options in our ranking rather than above them.
What owners like most is longevity. Reviews describe six or more years with no issues and no parts replaced, easy descaling after a year of use, and responsive Rheem support that shipped parts at no cost. The warranty is five years on leaks and one year on parts.
The cabinet itself is 17.63 inches wide, 18.25 inches tall and 3.5 inches deep at 17.8 pounds, and Rheem notes it is about 90 percent smaller than a conventional tank heater. No venting is required, which keeps it inside the building envelope.
The RTEX-24 Covers the Fixtures Other Heaters Cannot
At 5.9 GPM, this is the model that finally covers a two-bathroom apartment with laundry in the mix. Nothing else in the roundup handles that much simultaneous demand at 240V.
If your building has 200A service and space in the panel, this is the capacity answer with the most long-term owner data behind it.
The RTEX-24 Is Overkill and Overpowered for a Small Unit
Three double-pole breakers consume six panel spaces, and a studio with one bathroom will never pull the difference between 4.4 and 5.9 GPM. Installing it there means paying in electrical work for capacity you will not use.
Owners also report the water arriving at the faucet cooler than the displayed set point, and there is no hot water at all during a power outage. In a unit where the heater is the only hot water source, that trade-off is worth thinking through.
5. CAMPLUX 4.5kW 120V: The Best Under-Sink Fit for a Studio
CAMPLUX Tankless Water Heater Electric 120V, 4.5kW Point of Use Instant Hot Water Heater, Save Space for Sink Wall Mounted LED Digital Display CSA Certified
- Fits inside a sink cabinet
- lowest electrical demand here
- heats in about 3 seconds
- CSA certified and quiet
- Serves one sink only
- must be hard-wired with no plug
- 131F maximum outlet temperature
Everything difficult about apartment ownership collapses with this one. The CAMPLUX runs on 120 volts, draws its power from a single 40A breaker with 10AWG wire, and fits inside the cabinet under a sink at 7.4 by 10.9 by 2.52 inches and 4.5 pounds.
It activates at a minimum flow of 0.26 GPM and delivers a 50 degree temperature rise at 1.0 GPM, so a kitchen or bathroom sink gets near-instant hot water in about three seconds. In a studio with a galley kitchen, that is the entire problem solved.
Because it never fires without demand, the unit consumes no standby energy, and the self-modulating control adjusts power in real time for an efficiency claim of up to 99.8 percent. Reviewers also note quiet operation and responsive brand support when a replacement was needed.
The 120V requirement is the underrated advantage. Apartment buildings are full of 120V receptacles and very few spare 240V circuits, so a heater that works on 120V can be added where a 240V model would need permission and a panel modification.
The CAMPLUX Is the Easiest Apartment Install on This List
A 40A breaker and 10AWG wire on 120V is close to the minimum electrical work any appliance requires, and the body fits inside an existing cabinet. The mounting kit is included in the box.
For a studio kitchen, a guest bath sink or a laundry alcove, this is the most realistic upgrade a renter can make with a landlord’s approval.
The CAMPLUX Will Not Run a Shower
Four and a half kilowatts is point-of-use capacity by definition. A showerhead at 2.5 GPM is far beyond what the heater can deliver at a usable temperature, and the maximum outlet temperature is listed at 131 degrees Fahrenheit.
It must also be hard-wired with no plug, and some models in the wider Camplux line have needed replacement for internal leaks. Know the model number before you call for support.
6. EcoSmart ECO 8: The Budget Path to One Extra Sink
EcoSmart ECO 8 Tankless Water Heater, Electric, 8-kW – Quantity 1, 12 x 8 x 4
- Single 40A breaker
- modulating thermostat steadies output
- stainless flow path with in-water sensors
- simple knob control
- 2 GPM capacity is the limit
- 1/2 inch connections need downsizing
- opening the sealed case to wire voids the warranty
The ECO 8 has been a point-of-use staple for years, and the reason is that it asks almost nothing of an apartment electrical system. Eight kilowatts on 240V fits a single 40A breaker, which is the easiest possible footprint for a dedicated hot water circuit.
Two gallons per minute is enough for a bathroom sink or a laundry hand-wash, and the modulating thermostat holds output steadier than the older fixed on and off heaters it replaced. Sensors sit in the water flow, so the unit reads actual inlet and outlet temperature rather than guessing.
At 8 by 11.5 by 4 inches and 4.8 pounds, the cabinet mounts flat on a closet wall with minimal clearance. Inlet and outlet connections are 1/2 inch NPT at the top of the cabinet, with a minimum activation flow of roughly 0.25 to 0.3 GPM, so a low-flow bathroom tap will still trigger it.
The warranty is a lifetime warranty on electronics, exchanger and element. It covers the heater itself rather than labour or incidental damage, so read the terms before assuming a free service call.
The ECO 8 Fits a Closet With One Spare Breaker
One 40A breaker is the entire requirement, and that is a circuit most apartment panels have room for. It is the simplest way to add dedicated hot water to a half bath or a basement utility nook.
The stainless internal flow path also resists the corrosion that shortens the life of cheaper heaters, which is a meaningful advantage in hard-water cities.
The ECO 8 Drops Out Above One Gallon per Minute
At higher flow the element can cut out entirely, so this is a sink heater and not a shower heater despite the wattage. The 1/2 inch connections also need adapters from standard 3/4 inch supply lines.
One more practical trap: the electrical terminals are inside the case, and opening the unit to wire it can void the stated warranty. Have the electrician who installs it confirm that step.
7. ThermoMate ET180: The Hard-Water Choice at 18kW
- Heating element separated from the water tube to cut scale
- digital 1 degree control from 80F to 140F
- ETL certified
- overheat and dry-run protection
- 76A across two 40A breakers
- some owners report output temperature swings
- slower warm-up on hard water
Scale is the documented failure mode for electric tankless heating elements, and the ET180 is engineered around it. The heating element is physically separated from the water tube, so mineral deposits build on a surface that is not carrying the water and not sitting in the hot path.
That single design decision is why this model earns its place in the list for apartments in Southern California, Florida, Phoenix and Houston, where hard water is the default. Owners also report that it handles two concurrent showers in a two-bath layout, which tracks with the 4.4 GPM rating.
Temperature control runs from 80 to 140 degrees in one degree increments on a digital display, with self-modulation that responds to both incoming water temperature and flow. Overheat protection and an automatic stop when no water is detected are both present, and the unit is ETL certified.
The wall-mount body is 17.12 by 13.12 by 3.12 inches with a 3/4 inch NPT connection, and a mount kit is included. Several owners report lower utility bills after switching away from a storage tank heater, though that is a household result rather than a controlled measurement.
The ET180 Is the One to Buy in a Hard-Water Building
Scale resistance in the heating chamber is the deciding factor, not output power, which the Rheem 18kW matches. If your building’s water is hard and descaling access is limited, this is the model to specify.
It also runs a generous 80 to 140 degree temperature range, which some renters prefer over the narrower 86 to 131 degree limits on budget models.
The ET180 Costs Two Breaker Pairs and Some Temperature Consistency
A 76A draw across two double-pole 40A breakers with 8AWG runs is a real constraint in a 100A apartment. A minority of owners also report output temperature swinging rather than holding the set point, which matters in a shared unit where a neighbour runs the dishwasher.
Some users note slower warm-up on hard water despite the separated element, so this is an improvement in maintenance burden rather than an immunity to scale.
8. WINTEMP WN18: The Pick When Remote Temperature Control Matters
WINTEMP 18kW Electric Tankless Water Heater 240V ETL Certified WiFi
- Only heater here with WiFi temperature adjustment
- self-modulating output
- compact wall-mount body
- ETL certified with two year warranty
- Recommends 150A household service
- smallest review base in the roundup
- 8/2 AWG wiring on two 40A breakers
Remote temperature adjustment is the WN18’s whole identity. It is the only heater in this roundup with built-in WiFi, so the set temperature can be changed from a phone without standing next to the closet, which is a real convenience in a unit where the heater is tucked behind a wall.
The rest of the design is conventional and well executed. Eighteen kilowatts at 240V, up to 4.3 GPM output, and self-modulating control that responds to both flow rate and set temperature. The unit uses no electricity when idle.
At 11.1 inches wide, 19.2 inches tall and 4 inches deep, with 12 pounds of weight, it is a slim vertical cabinet that mounts well beside a laundry stack. Maximum pressure is 130 psi and maximum temperature is 140 degrees, with leakage, overheat and dry-fire protection and a temperature lock feature.
WINTEMP recommends 150A household service with two 40A double-pole breakers and 8/2 AWG wiring. The listed maximum amperage is 75A, which lines up with the 18kW rating.
The WN18 Suits a Tenant Who Wants Control From a Phone
For a short-term rental, a multi-unit building with a maintenance staff account, or anyone who wants to warm the water before getting out of bed, remote set-point control is genuinely useful rather than decorative.
It also occupies a compact footprint and draws no standby power, so it costs nothing to have installed and idle.
The WN18 Asks for 150A Service and Has the Shortest Track Record
The 150A recommendation is higher than the Rheem 18kW requires, so in a 100A apartment this model is the wrong door to knock on. Two 40A breakers on 8/2 AWG is also heavier copper than most apartment walls were plumbed for.
With 202 reviews, this is the smallest owner base in the roundup, so long-term durability data is thin. Owners do report steady output and compact sizing as the consistent positives.
9. SIVUATEK V7B-180B: The Thinnest 18kW Body in the Roundup
- Extremely thin mounting body
- heating chamber separates water and electrical lines
- self-modulating at 99.8 percent
- four layers of safety protection
- Two 40A double-pole breakers take four panel spaces
- some owners report temperature swings
- one dead control board reported
When an apartment closet is 4 inches deep, dimensions decide the purchase, and the V7B-180B wins that contest outright. At 11 inches wide by 3 inches high, it is a flat panel that mounts between studs or behind a door rather than projecting into a hallway.
Despite the thin body it is a full 18kW heater. The heating chamber is designed to keep the water line separate from the electrical line, which limits scale and corrosion in exactly the way hard-water buildings need, and the listed throughput is up to 4.3 GPM.
Protection features are unusually comprehensive for the category: leakage, dry heating, high temperature and automatic stop, all under an ETL listing. Self-modulating power input runs at 99.8 percent efficiency, adjusting to the real-time flow rate and temperature setting.
Owners report lower utility bills after replacing a storage tank, and standard 3/4 inch NPT fittings mean the plumbing is conventional. A two year manufacturer warranty, installation template and hardware pack are included in the box.
The SIVUATEK Is the Only 18kW Model That Fits a Shallow Closet
At 3 inches deep, it clears spaces where the 3.5 inch and 4 inch bodies in this class will not sit flush. That single dimension solves a large share of apartment install problems.
The separated heating chamber also makes it a sensible hard-water pick, and the ETL listing with four protection layers is reassuring for a hard-wired appliance.
The SIVUATEK Uses Four Panel Slots and Swings on Some Units
Two 40A double-pole breakers with two sets of 8AWG/2 grounded wire take four spaces in the panel, which is a real constraint in a crowded apartment closet.
Some owners report output temperature moving from very hot to cold, and at least one reported a dead control screen on a unit that would not operate. Maximum temperature is listed at 125 degrees, which is low if you like very hot fill water. Reviewers also see conflicting flow figures between listings.
10. Ranein RE18K: A Straightforward 18kW With a Few Rough Edges
Ranein 18kW Tankless Electric Water Heater, Max 4.3 GPM, 240V, Whole House
- Compact wall-mount body
- stainless elements to reduce scale
- 86F to 131F in 1 degree steps
- CSA certified
- no standby draw
- Rating spread is the widest in the roundup
- one owner reported breaker trips an electrician could not resolve
- slow warranty support
The RE18K does the basic job cleanly. Eighteen kilowatts, up to 4.3 GPM, 99 percent thermal efficiency, and a wall-mount body of 13.8 by 18.7 by 3.1 inches that clears most apartment closets, weighing 7.9 kg.
Stainless steel heating elements are used specifically to reduce scale buildup, and the temperature control runs from 86 to 131 degrees in one degree increments on a knob with an LED display. Leakage, overheat and dry-fire protection are all present under a CSA certification to US and Canadian standards.
Owners commonly report fast heat-up and sustained hot water through an extended shower, and the unit draws no standby power when it is not in use. An installation template, parts kit, user manual and the heater ship in the box, which makes the install planning straightforward.
Installation needs two dedicated 240V circuits, two 40A double-pole breakers and two 8 AWG cables, with a 3/4 inch NPT water connection. Maximum pressure is 130 psi and maximum temperature 140 degrees.
The RE18K Works for a Straightforward Single-Bathroom Install
At 3.1 inches deep and 7.9 kg, it is one of the lighter 18kW cabinets here, which makes it easier to handle into a high closet or a ceiling-adjacent space than a 19 pound premium model.
The core specification is competitive: 4.3 GPM, stainless elements and a certified safety package at a straightforward mid-range specification.
The RE18K Has the Widest Rating Spread in the Roundup
It carries the lowest average rating of the ten, with 15 percent of reviews at one star, and the complaints concentrate after a failure rather than during use. One owner reported repeated breaker trips that an electrician could not resolve, and warranty support was slow to respond to that same problem.
Owners also report confusion between similarly named models in the lineup, so confirm the capacity on the label matches the model number you are ordering. If the Rheem RTEX-18 or ThermoMate ET180 fits your panel, this is the fallback rather than the first choice.
Hard Water Decides How Often You Descale
Scale is the number one documented failure mode for electric tankless heating elements, and apartment-dense cities are almost entirely hard water. Plan on descaling, and choose the model whose element design helps you.
Scale is a mineral deposit that forms on hot surfaces. Inside a tankless heater, it coats the element and insulating tube, and the symptom is a heater that takes longer to reach set temperature and eventually trips on overheat protection. For tenants, the worst case is not a failed heater, it is losing all hot water in a unit with no alternative.
Two models here address it in the design. The ThermoMate ET180 separates the heating element from the water tube, and the SIVUATEK V7B-180B uses a heating chamber that keeps the water and electrical lines apart. The EcoSmart ECO 8 uses stainless internal parts with in-flow sensors, and the Rheem RTEX-24 is reported as easy to descale after a year of use.
For everything else, put descaling on the calendar rather than waiting for a symptom. In hard water, an annual flush or a manufacturer-approved descale keeps the element efficient, and the annual check is also the moment to confirm the venting path is clear and the connections are dry.
Forum advice is blunt about what happens when this is neglected. On plumbingforums.com, a description of tank corrosion begins where the anode rod has been consumed or scaled to the point of ineffectiveness. The same logic applies to a heating element.
What Plumbers Say About Electric Tankless
Plumbers are split, and the split is worth hearing before you buy. The category is genuinely niche for apartments, and the endorsements that exist cluster around one brand.
On r/askaplumber, one installer wrote that Stiebel Eltron Tempra units are the only ones he ever recommends and installs, and that for a one-bathroom home the Tempra 24 is more than enough with only 100 amps. Another professional in the same thread chose the Stiebel Eltron Tempra 36 Plus at 36kW and 150 amps over a storage tank.
That endorsement is why the Tempra 29 Plus appears high in this list despite its 200A requirement, and it is the single most repeated professional endorsement across the forum research we reviewed.
The counterweight is equally clear. Electric tankless is very niche, and a r/Homebuilding poster simply answered that no tankless water heater is necessary. The honest reading is that electric tankless wins where space and standby losses matter and loses where the panel is small and the household is large.
For a renter, the alternative is worth taking seriously. A r/homeowners commenter recommended hybrid heat pump water heaters as the most efficient choice, even more efficient than going tankless. A heat pump water heater uses a fraction of the amperage, so it can serve a small apartment where an 18kW tankless simply cannot be fed.
If you want more context on the category as a whole, our guide to electric water heaters for large families covers the opposite end of the capacity range.
How to Choose: A Five-Check Apartment Buying Guide
Run these five checks in order, and the model choice makes itself. Checking panel capacity first prevents the most expensive mistake, which is buying a heater you cannot legally install.
First, confirm your service. Read the main breaker number on the panel disconnect. Roughly 100A points to a 13kW or 18kW model, 150A covers an 18kW heater at 75A or 76A, and 200A is the stated requirement for the 24kW and 28.8kW models in this roundup. Calculate it as kilowatts x 1000 divided by 240 for amperage.
Second, match flow to fixtures. One shower is about 2.5 GPM, a sink is 1 to 1.5 GPM, a tub fill is about 4 GPM, and a washer pulls 2 to 3 GPM. Add the ones that could run together and add a 25 percent margin.
Third, decide point-of-use or whole-unit. Point-of-use models serve one fixture and can often run on 120V. Whole-unit models serve the apartment and need a dedicated 240V circuit with two or three double-pole breakers.
Fourth, check the space. Measure the closet before you order, because the difference between a 2.52 inch deep under-sink body and a 3.5 inch deep wall cabinet can decide which products are possible at all. Also check the required wall clearance from the electrical panel, since codes and manufacturers both constrain the distance between the unit and the panel door.
Fifth, think about maintenance. If your water is hard, prefer a separated heating chamber and stainless flow path, and confirm the manufacturer parts availability before you commit. If your building may be sold or turned over, a model with field serviceable elements and a real warranty is worth more than a cheaper sealed box.
Frequently Asked Questions
Can you use a tankless water heater in an apartment building?
Yes, in three ways: add a point-of-use heater for one sink, install your own whole-unit heater, or leave the shared system alone. A renter usually cannot replace the building’s shared heater because it touches common plumbing, the shared panel and often a vent. A condo owner generally has more freedom but should check association rules. Before buying, read your lease, check the main breaker rating on your panel, and confirm whether the closet has a spare dedicated circuit.
How do I determine what size electric tankless water heater I need for an apartment?
Add the gallons per minute of everything that could run at once, then add about 25 percent margin. A shower needs roughly 2.5 GPM, a bathroom sink 1 to 1.5 GPM, a tub fill about 4 GPM and a washer 2 to 3 GPM. A studio needs about 2.5 GPM, a one-bathroom apartment about 3.5 GPM, and a two-bathroom apartment 6 GPM or more. That lands a one-bath unit in the 18kW to 24kW range on 240V power.
What are the minimum amp requirements for an electric tankless water heater?
Amperage equals kilowatts multiplied by 1000 and divided by 240. That gives about 54A for a 13kW model on one 40A double-pole breaker, about 75A for an 18kW model on two 40A breakers, about 100A for a 24kW model on three, and 120A for a 28.8kW model on three. A 100A panel fits the smaller class, 150A covers an 18kW heater, and 200A is the stated requirement for the 24kW and 28.8kW models.
Who makes the most reliable electric tankless water heater?
Stiebel Eltron draws the most consistent professional endorsement. A plumber on r/askaplumber wrote that the Tempra units are the only ones he ever recommends and installs, and another chose the Tempra 36 Plus over a storage tank. Rheem has the deepest owner history, with the RTEX-18 and RTEX-24 models carrying thousands of reviews, while the ThermoMate ET180 leads on hard-water scale resistance.
What do plumbers say about tankless water heaters?
They are split, and they are blunt. The same r/askaplumber thread that endorsed Stiebel Eltron Tempra also described electric tankless as a very niche category, and a r/Homebuilding poster simply answered that no tankless water heater is necessary. The practical consensus is that electric tankless wins where space and standby losses matter and a panel can support it, and loses where the panel is small and the household is large.
Do electric tankless water heaters work without a vent?
Yes. Electric tankless heaters have no combustion process, so they produce no exhaust and need no flue. The Rheem RTEX-24 and the Stiebel Eltron Tempra 29 Plus both list no venting required, and that is a major reason electric units are practical in multi-family buildings where vent penetrations are often prohibited. It also means the unit can sit inside the building envelope, in a closet or a utility nook.
The Rheem RTEX-18 Is the Best Fit for Most Apartments
The Rheem RTEX-18 is our pick for most apartments because 18kW delivers 4.4 GPM from a roughly 75A draw, which is the rare combination that fits a 100A panel and still carries a full shower. The Rheem RTEX-13 is the fallback when panel capacity is genuinely tight, and the CAMPLUX 4.5kW 120V is the answer for a studio sink that needs dedicated hot water without a 240V circuit.
For a two-bathroom unit on 200A service, the Rheem RTEX-24 handles simultaneous fixtures that smaller heaters cannot, and the Stiebel Eltron Tempra 29 Plus wins on temperature accuracy and the endorsement working plumbers give it most often.
Whatever you choose, confirm the main breaker rating and the available panel spaces before you order, and check your water hardness. Check the main breaker number on the panel, count the free breaker positions, and pick a heater whose heater chamber resists scale. That three-minute audit eliminates most of the problems apartment residents actually report with electric tankless water heaters in 2026.