Independent comparisons. We may earn from shop links. More info

240V tankless water heater UK

Updated: 21 October 2026

You have a WINTEMP or GRS page open on amazon.co.uk next to a Stiebel Eltron unit, and every title says about 27 kW. A 240V tankless water heater UK shoppers meet in this set is a different electrical product from the 400 V heater, even when the kilowatt figure matches. Checked on 19 October 2026, WINTEMP and GRS specify 240 V and US units, while the Stiebel listing specifies 400 V. Those prices and stars are on the [27kW instantaneous water heater comparison](/hub/hot-water/27kw-instantaneous-water-heater). Peter F. read those listings and public UK sources, and did not run a lab test. A qualifying purchase can earn a commission at no extra cost to you.

You have a WINTEMP or GRS page open on amazon.co.uk next to a Stiebel Eltron unit, and every title says about 27 kW.

Our top picks

WINTEMP WINTEMP YQ-WN27K

Best overall

WINTEMP WINTEMP YQ-WN27K
1.8/10about £408.53
View offer
GRS GRS 27 kW

Rank 2

GRS GRS 27 kW
1.6/10about £388.71
View offer
Stiebel Eltron Stiebel Eltron DHE 27 Set

Rank 3

Stiebel Eltron Stiebel Eltron DHE 27 Set
6.9/10about £821.57
View offer
On this page
  1. 1.What each listing actually specifies
  2. 2.Why 240 V is the wrong shortcut to 230 V
  3. 3.How to read gallons, Fahrenheit, ETL and the reviews
  4. 4.Which to choose, and when neither fits

What each listing actually specifies

WINTEMP, ASIN B0FPX5C9Y1, is specified at 240 V and 27 kW, with 6.5 gallons per minute, 140 °F and an ETL mark, at £408.53 and 5.0 from one rating with no written review. GRS, ASIN B0C35HR76V, is also 240 V and 27 kW, with a 2 litre capacity field, at £388.71 and 4.0 from 69 ratings whose text is marked United States. Stiebel Eltron DHE 27 Set, ASIN B08L5B34YH, is specified at 400 V, at £821.57, with 2.9 from two ratings and no written text. Stiebel's UK page states 400 V and 3/PE. The amazon wattage field showed 24 kW while the title said 27 kW. The datasheet rates 24.4 kW at 380 V and 27 kW at 400 V.

Why 240 V is the wrong shortcut to 230 V

A UK dwelling is usually a single 230 V phase. A North American 240 V water heater is built for split-phase practice, two legs with 240 V between them. A 27 kW load on 230 V single-phase is about 117 A. Quality Heating's single-phase sheet already puts a 13.5 kW unit at 62 A. Flexiheat states that its 10 to 36 kW tankless range, including 27 kW, needs three-phase and will not run on ordinary 230 V. Connecting the WINTEMP to a UK three-phase board does not repair that. Three-phase here is 400 V between phases. Whether the building has it is on the three-phase page.
WINTEMP WINTEMP YQ-WN27K

Testsieger

WINTEMP WINTEMP YQ-WN27K
1.8/10about £408.53
View offer

How to read gallons, Fahrenheit, ETL and the reviews

The WINTEMP bullets quote 6.5 gallons per minute, about 24.6 litres per minute. At 27 kW that is only a small temperature rise. Flexiheat's 27 kW class is about 13 litres per minute at a 30 °C rise. The sum for your mains and the outlets you open together is on the buying guide. ETL on those bullets is a North American electrical listing mark. Lochinvar's note on Regulation 4 is the UK point: ETL or UL is not WRAS or KUKreg4. The GRS score of 4.0 from 69 sits on review text marked United States, not British owners on a UK board.

Which to choose, and when neither fits

Choose the 400 V class only when three-phase is actually there. In this set that is the Stiebel Eltron DHE 27 Set. Check the nameplate against the 24 kW field and the 27 kW title. It is not a lab-tested winner, and it is not a unit to cable yourself. If you do not have 400 V, leave both sides of this comparison. Genuine single-phase instantaneous heaters in these sources stop around 7 to 13.5 kW, and 13.5 kW is already about 62 A. Any new circuit, or any connection to the public water supply, needs a qualified person.

Common questions

Why is the 240 V unit about half the price?
On 19 October 2026 the GRS listing was £388.71 and the WINTEMP listing was £408.53, against £821.57 for the Stiebel Eltron DHE 27 Set. The gap looks like a discount on one heater. It separates a 240 V listing, written in gallons and Fahrenheit, from a 400 V three-phase listing. You are comparing two electrical identities that share a kilowatt headline. Prices on amazon.co.uk move, so read the voltage on the page in front of you.
Is 240 V close enough to UK 230 V?
The sticker numbers sit near each other. The current decides the question. A 27 kW load on a single 230 V phase is about 117 A, which does not fit a normal domestic cut-out. Flexiheat states that a 27 kW tankless unit in its range needs three-phase and will not run on ordinary 230 V. Electronic controls, the neutral and 50 Hz against 60 Hz stay case by case even for a simple US storage tank. For these listings, 240 V on the label is not a UK supply.
What does ETL mean on amazon.co.uk?
ETL on the WINTEMP bullets is a North American electrical listing mark. It is not WRAS and it is not KUKreg4. Lochinvar's Regulation 4 note draws that line for water-heating products: a foreign electrical listing does not show that the fitting may connect to the UK public mains. The Water Supply (Water Fittings) Regulations 1999 still apply to that connection. An ETL line is a prompt to ask which UK approval, if any, the fitting actually carries.
Can an electrician fit the WINTEMP on a UK three-phase board?
UK three-phase for this category is 400 V between phases. The WINTEMP listing is specified at 240 V, so connecting it to that board tries to make the wrong appliance fit. It does not turn a 240 V nameplate into the 400 V heater. The listing in this set that matches 400 V is the Stiebel Eltron DHE 27 Set, and even that unit needs a qualified person for the circuit and the water connection. If the building is single-phase only, neither listing is the one to install.
What should you buy if you only have single-phase?
Leave the 27 kW category. A normal UK single-phase supply cannot run a 27 kW instantaneous heater, whether the listing says 240 V or the UK unit is specified at 400 V. Genuine single-phase instantaneous heaters in these sources stop around 7 to 13.5 kW, and Quality Heating's sheet puts 13.5 kW at 62 A. A thread adaptor does not make the 240 V unit a fitting accepted for the UK public mains. Buy a heater your supply can feed, and ask a qualified person before any new circuit.

Related pages

Best 27kW instantaneous water heater

27kW instantaneous water heater

27kW instantaneous water heater UK: the buying checks

How to use an instantaneous water heater at 27 kW output

A 27kW water heater three phase supply for UK homes

Sources

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.

  1. Approved Document G, with 2024 amendments (retrieved 19 October 2026)
  2. Approved Document P, 2013 (retrieved 19 October 2026)
  3. Water Supply (Water Fittings) Regulations 1999 (retrieved 19 October 2026)
  4. HSE, landlords and legionella (retrieved 19 October 2026)
  5. Stiebel Eltron UK, DHE 27 Set (retrieved 19 October 2026)
  6. Stiebel Eltron UK comparison sheet (retrieved 19 October 2026)
  7. Stiebel Eltron UK, three-phase water heating (retrieved 19 October 2026)
  8. Flexiheat UK, electric instantaneous water heaters (retrieved 19 October 2026)
  9. amazon.co.uk, Stiebel Eltron DHE 27 Set B08L5B34YH (retrieved 19 October 2026)