The best 27kW instantaneous water heaters, compared.
Top pick: WINTEMP YQ-WN27K Best value: Not awarded – Price is scored only when the supply is 400 V. The cheaper WINTEMP and GRS listings are 240 V, so neither is best value for this category. Updated 21.10.2026.
A 27kW instantaneous water heater heats mains water only while a tap is open. On a UK supply that output is a three-phase 400 V appliance. Stiebel UK sheet for the 27 gives about 39 A per phase and 40 A fuses. Most flats in Britain have a single-phase 230 V supply. A 27 kW load on that supply is about 117 A, outside a normal domestic cut-out, and the unit does not belong on a 13 A plug. Three-phase has to come from the distribution network operator before this class can run. A best-value pick is not awarded, because the price weight applies only on a 400 V supply. Winter flow on comparison sheet is 13.8 litres per minute at a 28 °C rise, one shower. What to confirm first is in the [27kW instantaneous water heater buying guide](/hub/hot-water/27kw-instantaneous-water-heater/sub/27kw-instantaneous-water-heater-buying-guide). It does not store a cylinder of hot water. Until a distribution network operator provides three-phase, this category is the wrong purchase for that flat.
A 27kW instantaneous water heater heats mains water only while a tap is open. On a UK supply that output is a three-phase 400 V appliance. Stiebel UK sheet for the 27 gives about 39 A per phase and 40 A fuses.
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Personal buying guide
Question 1 of 2
What supply do you have?
The WINTEMP YQ-WN27K is listed at £408.53 as a 240 V, 27 kW tankless heater with a stated flow of 6.5 gallons per minute and a maximum of 140 °F. That voltage is not the 400 V three-phase supply a UK 27 kW instantaneous heater uses, and 27 kW on one 230 V phase is about 117 A, which does not fit a normal domestic cut-out. Amazon.co.uk shows 5.0 from 1 rating and no review text, so there is no owner account to read. The bullets name an LED display, Wi-Fi, leak, overheat and dry-fire protection, ETL certification and a 2-year warranty. ETL on those bullets is not a Water Regulations approval for a fitting on UK public mains. The depth field prints 4 with no unit, and the page gives no IP rating and no pressure. A 6.5 gallon-per-minute claim is about 24.6 litres per minute, which at 27 kW is only a small temperature rise, not about 13 litres per minute at a 30 °C winter rise.
Pros
Cons
Best for: You are comparing a 240 V gallon-and-Fahrenheit listing with a 400 V heater and need the specification written out.
Less suitable: A UK home on single-phase 230 V, or any install that needs a documented 400 V instantaneous heater.
Specifications
Common questions
What supply does the listing specify?
240 V, with wattage and heat output both given as 27 kW. Flow is 6.5 gallons per minute and the maximum temperature is 140 °F.
What safety points are in the bullets?
The bullets name leak, overheat and dry-fire protection, and they say the unit is ETL certified. There is no IP rating on the listing.
Does the listing say a professional must fit it?
The listing does not say whether a professional must fit it.
The GRS model JN is the cheapest row in this set at £388.71, listed at 240 V and 27 kW with a digital display. The title calls it a whole-house water tank while the bullets also call it tankless, and the capacity field is 2 litres, so the page does not describe one clear appliance. The star line is 4.0 from 69 ratings, and the review text is marked United States, which is not British owner experience. Those US texts mention hot water arriving quickly and a touchscreen control, and also a manual line that a licensed plumber and electrician are required or the warranty is void. Other reviews report failures at about six months and at about a year, and one measurement of about 54 A with flow under 1.5 gallons per minute. That measurement is not a UK test. At 6.92 kg it is heavier than the WINTEMP listing, with aluminium named as the material, and it is still a 240 V page rather than the 400 V heater this category is about.
Pros
Cons
Best for: You want the £388.71 row written out so a cheap 27 kW title is not mistaken for a 400 V heater.
Less suitable: A UK single-phase home, or anyone treating the 4.0 from 69 as British reliability.
Specifications
Common questions
Is this listed as a tank or a tankless heater?
The title says whole house water tank. The bullets also call it tankless. The capacity field is 2 litres, the voltage is 240 V and the output is 27 kW.
What do the reviews say about installation?
The star line is 4.0 from 69. The review text is marked United States. One review quotes a manual that requires a licensed plumber and electrician or the warranty is void.
Does the listing give a flow rate?
The specification block does not. Review text mentions 6.5 gallons per minute for a CP-27US and, in another review, flow under 1.5 gallons per minute.
The Stiebel Eltron DHE 27 Set, ASIN B08L5B34YH, is the only heater in this amazon.co.uk set specified at 400 V, which is the supply a 27 kW instantaneous unit needs in Britain. In this capture it is £821.57, with 2.9 from 2 ratings and no review text, so the star line is not a reliability record. The title and the style field say 27 kW, while the wattage field says 24,000 W and the heat-output field says 24 kW. Stiebel's UK sheet rates the DHE 27 at 27 kW on 400 V and 24.4 kW on 380 V, with about 39 A per phase, 40 A fuses and IP25, none of which the amazon fields print as a current or a flow. The listing does describe full electronic control, ECO and a multifunction display, plus a fitting kit, an installation manual and a wall bracket, with a 2-year warranty line. Stiebel's comparison sheet gives 13.8 litres per minute at a 28 °C rise, which covers one winter shower and can fall short if a sink opens at the same time. The listing model is 202657 and the current UK product page uses 204285, so match the carton to the nameplate before anyone connects it.
Pros
Cons
Best for: You already have 400 V three-phase, or a competent person has confirmed it, and you will match the carton to the nameplate.
Less suitable: Your supply is single-phase 230 V, or you are choosing from two ratings with no written reviews.
Specifications
Common questions
What output does the listing give?
The style field says 27 kW and the title is the DHE 27 Set. The wattage field says 24000 W and the heat-output field says 24 kW. Voltage is 400 V.
Does the listing say it can go in a bathroom?
The listing does not say. No IP rating is printed on the page.
What is in the box?
The listing includes a fitting kit, an installation manual and a wall mounting bracket. The warranty line says 2 years.
Scores use the three amazon.co.uk listings kept in this comparison, checked 19 October 2026, plus the manufacturer figures named below. This is not a laboratory test. Weights sum to 100. A 240 V listing scores zero on the supply block, and its price is not scored. The highest score is the top pick. A best-value badge is not awarded: a lower shelf price on the wrong supply is not value for a UK 27 kW instantaneous heater.
Supply match
40%The supply has to be 400 V three-phase for a 27 kW instantaneous heater in Britain. A listing specified at 240 V scores zero on this block.
Listing agreement
15%The title, wattage and units need to agree, in litres and degrees Celsius a UK buyer can use. A 24 kW field against a 27 kW title, or gallons and Fahrenheit, scores lower.
Control and protection
15%Points come from control and protection lines actually printed on the listing, such as electronic control, ECO, a display, overheat protection or dry-fire protection.
Review evidence
20%Stars and counts are the amazon.co.uk figures. Very few ratings are deducted. Review text marked United States is not counted as British experience.
Price
10%Price is 10 points, and only if the supply matches 400 V. A cheaper 240 V listing does not earn a value score.
Confirm 400 V three-phase before you compare prices. A 27 kW instantaneous heater on a UK supply draws about 39 A per phase on the Stiebel DHE 27 sheet, with 40 A fuses. Winter flow is about 13 litres per minute at a 30 °C rise, which is one shower. A second tap at the same time can leave that shower short.
A 27 kW instantaneous water heater heats mains water only while a tap is open. It does not store a cylinder of hot water. On a UK supply that output is a three-phase 400 V appliance. Stiebel Eltron's UK sheet for the DHE 27 Set rates 27 kW at 400 V and 24.4 kW at 380 V, with about 39 A per phase and 40 A fuses. Most flats in Britain have a single-phase 230 V supply, which Stiebel's three-phase note states directly. A 27 kW load at 230 V is about 117 A.
That claim is about 24.6 litres per minute. At 27 kW it is only a small temperature rise. The winter figure to use is about 13 litres per minute at a 30 °C rise, or 13.8 litres per minute at 28 °C on Stiebel's sheet.
The GRS title says whole house water tank and the bullets say tankless, with a 2 litre capacity. The WINTEMP page is 240 V. Neither is the 400 V appliance this category is about, and neither belongs on a 13 A plug.
The GRS review text is marked United States. The Stiebel listing shows 2.9 from 2 ratings and no text, and its wattage field says 24 kW while the title says 27 kW. Match the carton to the nameplate.
Confirm 400 V, three-phase plus earth, and a dedicated circuit before you compare prices. Stiebel Eltron's UK sheet for the DHE 27 Set rates 27 kW at 400 V, about 39 A per phase and 40 A fuses, and 24.4 kW at 380 V. The amazon.co.uk row for ASIN B08L5B34YH is the only listing in this set that specifies 400 V. Its wattage field says 24,000 W and its heat-output field says 24 kW, while the title and the style field say 27 kW, and listing model 202657 does not match UK product number 204285.
Read litres per minute at a stated temperature rise, with a winter inlet near 10 °C. Stiebel's comparison sheet gives 13.8 litres per minute at a 28 °C rise. Flexiheat's 27 kW table gives about 13 litres per minute at a 30 °C rise, which is one shower up to about 40 °C. Stiebel's UK FAQ says a DHE 27 kW may struggle if a sink opens during that shower, so two outlets at once are a different job. The DHE 27 Set switch-on flow sits above about 2.5 litres per minute.
Three typical situations, each with a recommendation from this comparison.
You already have three-phase, or a competent person has confirmed it, and you will match the carton to the nameplate. The Stiebel Eltron DHE 27 Set is the top pick because it is the only listing here specified at 400 V. The star line is 2.9 from 2 ratings, with no review text.
You are comparing a 240 V gallon-and-Fahrenheit listing with a 400 V heater and you want the specification written out. The WINTEMP YQ-WN27K states 27 kW at 240 V, 6.5 gallons per minute and 140 °F. That supply is the wrong one for this category in a UK home.
You want the £388.71 row written out so a cheap 27 kW title is not mistaken for a 400 V heater. The GRS model JN is listed at 240 V. The title says water tank, the bullets say tankless, and the capacity field is 2 litres. The review text is marked United States.
The top pick is the Stiebel Eltron DHE 27 Set at about £821.57. It scores 69, the highest here, as the only listing specified at 400 V, with electronic control, ECO and a display on the page. The star line is 2.9 from 2 ratings and no review text, and the wattage field says 24 kW against a 27 kW title. A best-value pick is not awarded. The WINTEMP and the GRS are 240 V, and a lower price on the wrong supply is not value for this category. Skip the Stiebel if 400 V is not confirmed. Skip the other two if you need a documented 400 V instantaneous heater.
Not awarded
Price is scored only when the supply is 400 V. The cheaper WINTEMP and GRS listings are 240 V, so neither is best value for this category.
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 named below. amazon.co.uk prices, stars and review counts on these three listings were read on 19 October 2026. Page updated 21 October 2026.
This page contains affiliate links to amazon.co.uk (tag pcuk26-21). If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are taken from those listings. We did not run a lab test.
