The best 21 kW instantaneous water heaters, compared.
Top pick: Stiebel Eltron DCE-X 10/12 – Strongest stated flow, controls and relevant review evidence among these six lower-output listings, and still only 10/12 kW. Best value: Triton Instaflow 10.1 kW – Documented 2.7 L/min flow, IP25 and a small multi-point sink job, at a much lower shelf price than the Stiebel models. Updated 17.10.2026.
A 21 kW instantaneous water heater heats mains water as it flows, with no hot-water cylinder. In the UK that rating is a three-phase 400 V appliance. Flexiheat lists 21 kW as 3 × 30.3 A and about 10.1 litres per minute at a 30°C rise. Most homes have a single-phase supply, and a BS 1363 13 A plug is only about 3 kW, so this is not a socket product. How to size the supply before the brands is in the [21 kW instantaneous water heater buying guide](/hub/hot-water/21kw-instantaneous-water-heater/sub/21kw-instantaneous-water-heater-buying-guide). Quality table puts 21 kW near 10.0 L/min at 30°C or 7.9 L/min at 38°C. The same guide uses a rule of thumb of about 8–10°C incoming water in winter and 15–17°C in summer.
A 21 kW instantaneous water heater heats mains water as it flows, with no hot-water cylinder. In the UK that rating is a three-phase 400 V appliance.
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Personal buying guide
Question 1 of 2
What are you trying to heat?
The Stiebel Eltron DCE-X 10/12 provides the clearest documented performance in this set, with selectable 10–12 kW output, a stated 5 litres per minute and a 55°C maximum. Its backlit display, two memory keys and electronic control make daily adjustment more useful than a basic power switch. The 4.4-star average from 116 ratings is supported by more UK experience than the other higher-output entries, including successful use in flats and at basins. Shower feedback is mixed, though: some buyers use it successfully while others find the flow too restricted, particularly for a bath or a stronger shower. It cannot plug into a 13 A socket, and reviewers describe a dedicated high-current circuit, so the £412.86 shelf price is only part of the project cost. This is our top pick among the six smaller heaters because its specification and evidence are strongest, not because it meets the category's 21 kW requirement.
Pros
Cons
Best for: A flat, utility sink or carefully specified low-flow shower where an electrician confirms the dedicated supply.
Less suitable: A normal 13 A socket, fast bath filling, simultaneous high-demand outlets or a true 21 kW brief.
Specifications
Common questions
Is the DCE-X 10/12 a 21 kW heater?
No. The listing is 10/12 kW at 220/240 V, with a stated flow of 5 litres per minute and a 55°C maximum.
Can it plug into a 13 A socket?
UK reviewers say no. They describe a separate cable and a high-current breaker.
Do reviewers use it as a shower?
Some do, including after removing a cylinder. Others say the flow is too low for a shower or a bath.
The WINTEMP supplies a stated 11 kW and claims enough flow for one shower and a sink, placing it near the top of the single-phase listings in this comparison. Its LED control and temperature lock are practical features, and £176.47 is substantially less than either Stiebel Eltron model. The electrical instructions are a concern for a UK buyer, however, because the listing uses US-oriented language about a 50 A double-pole breaker and gives output in gallons per minute. Its 4.4-star average covers only 13 ratings, and every written review in the collected file is marked United States. That means the reviews do not establish British electrical compatibility or winter shower performance. Consider it only after a UK electrician has checked the model documentation and supply; it is not reliable evidence of a 21 kW purchase.
Pros
Cons
Best for: A carefully checked high-demand point-of-use installation where a UK electrician accepts the documentation and designs the circuit.
Less suitable: Buyers relying on UK installation evidence, a normal socket, or a genuine 400 V 21 kW appliance.
Specifications
Common questions
Is this 21 kW?
No. The listing is 11 kW at 240 V, with a stated maximum of 46 A.
Does it plug into a 13 A socket?
The bullets say to use a 50 A double-pole breaker. They do not list a 13 A plug.
Are the reviews in the saved listing from the UK?
No. The review texts are marked United States. The 4.4 from 13 ratings should not be read as British install experience.
The Stiebel Eltron DCE-X 6/8 offers selectable 6 or 8 kW output in a compact electronic unit with an LC display. Its energy class A listing, two-year warranty and supplied connection parts provide more documentation than the cheapest entries. UK feedback makes the installation limit clear: this is not a socket appliance, and one reviewer describes a substantial cable back to the consumer unit. The same evidence is less reassuring for bathing, with one buyer reporting only a brief useful flow before it falls to a trickle. At £344.99 it costs considerably more than several 10–11 kW listings, so the premium only makes sense when compact size, controls and brand support matter more than maximum output. It is a credible basin or carefully specified low-flow option, but the 6/8 kW model is not the 21 kW version.
Pros
Cons
Best for: A compact basin or low-flow point-of-use installation where precise control and documented fittings justify the premium.
Less suitable: Filling a bath, serving several outlets together or replacing a 21 kW three-phase heater.
Specifications
Common questions
Can I plug the 6/8 kW model into a wall socket?
A UK reviewer says no. They describe a 6 to 10 mm² cable back to the consumer unit, as with a shower.
Will it fill a bath?
One UK buyer assumed it would because the title says showers. They report a few seconds of flow, then a trickle.
Is this the 21 kW version?
No. The model is the DCE-X 6/8, part 238158.
The ThermoMate ET055 is a 5.5 kW point-of-use heater intended to serve a cloakroom basin or one small sink at a stated flow of 3 litres per minute. Its 4.0-star average from 248 ratings gives it the broadest review base in this comparison, and UK buyers do report useful cloakroom installations. The limitation is heat at normal flow: reviewers describe lukewarm water unless they reduce the tap considerably, while the listing itself states a maximum of only 25°C. The title says there is no plug, and one buyer had to call an electrician after overlooking that detail. It also arrives without hoses according to the listing, so the £109.74 price is not the complete installed cost. This can suit hand washing on a professionally wired circuit, but it is neither a shower solution nor a 21 kW heater.
Pros
Cons
Best for: A cloakroom basin or low-demand hand-washing sink with a dedicated circuit fitted by an electrician.
Less suitable: Dishwashing at a strong flow, showers, baths or any installation requiring true 21 kW output.
Specifications
Common questions
Can I plug this into a socket?
The title says no plug. A UK reviewer had not noticed that and called an electrician to wire it in.
Is 5.5 kW a 21 kW heater?
No. The listing wattage is 5,500 W.
What temperature does the listing state?
The specification says a maximum of 25°C. UK reviews describe lukewarm water unless the flow is cut right back.
The Triton Instaflow provides 10.1 kW and a stated 2.7 litres per minute for an under-sink multi-point installation. That output is more useful than the 5.5–6 kW entries for a kitchen sink or two modest hand basins, and the listing adds energy class A and IP25. It must not be mistaken for a dedicated electric shower: Triton describes this multi-point range as unsuitable for use as an electric shower. The 3.5-star average from 23 ratings is only moderate, so the specification carries more weight than customer enthusiasm. Its £154.99 price is below the higher-output Stiebel models, while still requiring a correctly designed fixed electrical installation. It is the best-value pick here for a clearly bounded small multi-point job, but it remains a 10.1 kW product rather than the 21 kW heater named by the category.
Pros
Cons
Best for: A kitchen sink or two modest hand basins where a qualified installer confirms the circuit and plumbing.
Less suitable: A dedicated shower, high simultaneous demand or a household that has specified a 21 kW three-phase heater.
Specifications
Common questions
Will this fit under a sink?
The title says undersink multi-point instantaneous heater.
What flow does the listing state?
2.7 litres per minute at 10.1 kW. That is not a 21 kW flow.
Is there an ingress rating?
The listing states IP25. It does not state the supply voltage.
The Normodic supplies 6 kW of instantaneous heating in a compact under-sink format, so its realistic role is one basin or sink rather than a 21 kW whole-home job. Its listing offers a 30–55°C setting range and IPX4 wording, but it does not state a flow rate that would show what temperature it can maintain in winter. The title also says 220 V, which needs checking against the actual rating plate and the proposed UK supply before installation. At £49.99 it is the least expensive listing here, although electrical work may cost more than the appliance. There is no star-rating line or review text in the collected UK data, leaving reliability and real-world flow unverified. Treat it as a specification-light basin heater to investigate, not as a budget substitute for a three-phase 21 kW unit.
Pros
Cons
Best for: A single basin or modest sink where an electrician verifies the rating plate, circuit and installation position.
Less suitable: A shower, simultaneous outlets or anyone who specifically needs a documented 21 kW three-phase heater.
Specifications
Common questions
Is this a 21 kW heater?
No. The listing is 6,000 W, and the title voltage is 220 V.
Can it go under a sink?
The item highlight describes an under-sink water heater. There is no review in the saved listing.
Does the listing include a UK star rating?
The saved listing has no customer-review line and no review texts.
Scores compare the six amazon.co.uk listings captured for this category. None is a 21 kW heater: all are rated between 5.5 and 12 kW. The score measures how credible and useful each listing is for its documented smaller job. It does not treat a lower-output product as equivalent to a 400 V three-phase 21 kW unit. Scores are reduced for a missing flow figure, contradictory or US-oriented electrical information, no UK review evidence, repeated weak-flow reports and unclear support.
Documented performance and job fit
30%Stated output, flow, temperature range and whether the intended outlet is clear. A lower kilowatt figure is scored for that smaller job, not as a 21 kW substitute.
UK customer evidence
25%The amazon.co.uk star line, the rating count and whether the written reviews describe British use. US review texts are not treated as UK install experience.
Installation clarity and suitability
20%Voltage, fixed-wiring requirements, protective information and whether the listing agrees with itself. A 13 A plug is about 3 kW, so these units are fixed-wiring jobs.
Controls and practical features
10%Useful temperature control, displays, memory settings and a clear mounting position, where the listing actually states them.
Value
15%The amazon.co.uk shelf price set beside the specification, the review evidence and the likely installation complexity. Electrician's labour is not in the shelf price.
Start with the supply you actually have, then the winter temperature rise, then the outlets you will run together. A title that says 21 kW on an amazon.co.uk search is not the same thing as a 400 V three-phase rating plate. The six listings below are 5.5–12 kW.
A 21 kW instantaneous water heater heats mains water as it flows and does not use a hot-water cylinder. In the UK that rating is a three-phase 400 V appliance. Flexiheat lists 21 kW as 3 × 30.3 A and about 10.1 litres per minute at a 30°C rise. Quality Heating’s table puts 21 kW near 10.0 L/min at 30°C or 7.9 L/min at 38°C. The same guide uses a rule of thumb of about 8–10°C incoming water in winter and 15–17°C in summer. Size for the winter rise.
The six listings in this capture are 5.5–12 kW. A star rating on one of them does not make it the 21 kW heater. Read the wattage on the listing before the title.
A BS 1363 13 A plug is only about 3 kW. The ThermoMate title says there is no plug. The Stiebel and WINTEMP files describe a dedicated high-current circuit.
Quality Heating uses about 8–10°C incoming water in winter and 15–17°C in summer as a rule of thumb. A title that says suitable for showers is not a bath filler. One Stiebel 6/8 buyer reported a brief flow, then a trickle.
A real 21 kW electric heater in this research is a 400 V three-phase appliance. Flexiheat lists that rating as 3 × 30.3 A and about 10.1 L/min at a 30°C rise. Most UK homes are single-phase, and this scrape tops out at 10–12 kW: the Stiebel Eltron DCE-X 10/12 at 220/240 V and the WINTEMP at 240 V. A BS 1363 13 A plug is only about 3 kW. Read the temperature rise, not a bare flow number.
Count the outlets you will run together. Quality Heating aims a 21 kW row at one bathroom plus a kitchen used close together. A 5.5–8 kW unit is a basin or one sink. The Triton Instaflow lists 2.7 L/min at 10.1 kW, and Triton describes that multi-point range as unsuitable for use as an electric shower. The ThermoMate ET055 states 3 L/min at 5.5 kW, with UK reviews of lukewarm water unless the flow is cut back. Every new circuit should be designed to BS 7671.
Three typical situations, each with a recommendation from this comparison.
You are removing a cylinder and want a compact electronic heater for a basin or a carefully specified low-flow shower. The Stiebel Eltron DCE-X 10/12 states 10–12 kW, 5 L/min and a 55°C maximum. It still needs an electrician, and it is not 21 kW.
A distant hand-washing sink is a 5.5–8 kW job. The ThermoMate ET055 states 5.5 kW and 3 L/min, with 4.0 from 248 ratings and UK cloakroom comments. The title says there is no plug, and the listing maximum is 25°C.
You want a kitchen sink or two modest basins, and you want the specification clear at a lower shelf price than the Stiebel models. The Triton Instaflow states 10.1 kW, 2.7 L/min and IP25 at about £154.99. Triton says it is not for use as an electric shower.
Among these six lower-output listings, the Stiebel Eltron DCE-X 10/12 is the top pick: the strongest combination of stated flow, controls and relevant UK review evidence, at about £412.86. The Triton Instaflow 10.1 kW is the best value, because its documented 2.7 L/min flow, IP25 listing and small multi-point use are relatively clear at about £154.99. Neither pick is a 21 kW heater. A real 21 kW electric unit in this research is 400 V three-phase, and no such unit was in the amazon.co.uk scrape. Call an electrician before you order a dedicated circuit.
Stiebel Eltron DCE-X 10/12
Strongest stated flow, controls and relevant review evidence among these six lower-output listings, and still only 10/12 kW.
Triton Instaflow 10.1 kW
Documented 2.7 L/min flow, IP25 and a small multi-point sink job, at a much lower shelf price than the Stiebel models.
Editorial
Peter F. is the editor for DIY & tools at productscompared.co.uk. Comparisons in this category are checked against amazon.co.uk listings, review evidence and public UK sources.
Guides on what to look for, how to use a product, and common mistakes.
Public UK sources from the research digest, plus the six amazon.co.uk listings in this comparison. The Veito amazon.co.uk offer was checked on 16 October 2026.
This page contains affiliate links to amazon.co.uk. If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are the UK listing figures in this comparison. We did not lab-test these heaters.
