27kW instantaneous water heater UK
Updated: 21 October 2026
You have a 27 kW title in the basket and you need to know if a normal UK supply can run it before the price matters. A 27kW instantaneous water heater UK homes can use is a 400 V three-phase appliance, about 39 A per phase, not a 230 V single-phase unit. The checks run from supply to winter flow, then to outlets and who signs the work off. Prices for the three amazon.co.uk listings are on the [27kW instantaneous water heater comparison](/hub/hot-water/27kw-instantaneous-water-heater). Peter F. checked those listings and public UK sources on 19 October 2026, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
You have a 27 kW title in the basket and you need to know if a normal UK supply can run it before the price matters.
You have a 27 kW title in the basket and you need to know if a normal UK supply can run it before the price matters. A 27kW instantaneous water heater UK homes can use is a 400 V three-phase appliance, about 39 A per phase, not a 230 V single-phase unit. The checks run from supply to winter flow, then to outlets and who signs the work off. Prices for the three amazon.co.uk listings are on the 27kW instantaneous water heater comparison. Peter F. checked those listings and public UK sources on 19 October 2026, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Supply | Stated output | Stated flow | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  WINTEMP WINTEMP YQ-WN27K | 240 V | 27 kW | 6.5 gallons per minute | 1.8 | about £408.53 | View offer |
 GRS GRS 27 kW | 240 V | 27 kW | n/a | 1.6 | about £388.71 | View offer | |
 Stiebel Eltron Stiebel Eltron DHE 27 Set | 400 V | Style 27 kW; wattage field 24 kW | n/a | 6.9 | about £821.57 | View offer |
On this page
Start with the supply
The first check is the supply, because 27 kW is an electrical size before it is a plumbing size. On the Stiebel Eltron DHE 27 Set the manufacturer states 400 V, three-phase plus earth, about 39 A per phase and 40 A fuses. In this amazon.co.uk set only B08L5B34YH is specified at 400 V, at £821.57. The WINTEMP at £408.53 and the GRS at £388.71 are both specified at 240 V. A lower price does not create three-phase. If you cannot point to 400 V on the cut-out, read the three-phase page before you compare kilowatts. Cable size is the electrician's design under BS 7671.
Turn kilowatts into winter flow
Convert kilowatts into litres per minute at a winter rise. Flexiheat's UK table gives about 13 litres per minute at a 30 °C rise. Stiebel's comparison sheet gives 13.8 litres per minute at a 28 °C rise, a main near 10 °C brought up to about 40 °C. The published rule of thumb is litres per minute equals (kilowatts times 60) divided by (4.18 times the rise). For 27 kW and 30 °C that is about 13. It is their rule of thumb, not a shower measurement. A 6.5 gallon claim is about 24.6 litres per minute, only a small rise at 27 kW.
Count the outlets that open together
Thirteen litres per minute is one heavy outlet. A high-flow shower can use most of that on its own, and whole house on a title does not add a second 27 kW. The switch-on flow on the DHE 27 Set sits above about 2.5 litres per minute, so a very fine spray can run cold because the heater has not started. Two showers at once are a different category.
If the supply and the flow both fit
Electrical safety and water-fittings law are separate checks. An ETL line on a 240 V listing is not a UK water approval. If the basket is a 240 V 27 kW unit, leave it. If 400 V and about 13 litres per minute at a 30 °C rise both fit the job, the named listings are on the best 27kW instantaneous water heater page.
Common questions
Can a normal UK house run a 27 kW instantaneous water heater?
Only if it has a 400 V three-phase supply that can take about 39 A on each phase. A typical flat or house is single-phase at 230 V. On that supply, 27 kW is about 117 A, which does not fit a normal domestic cut-out. Confirm the supply before you compare the £388 to £822 prices in this set. If the cut-out label is unclear, ask an electrician rather than guessing from the kilowatt title.
How many litres per minute does 27 kW give in winter?
About 13 litres per minute at a 30 °C rise, on Flexiheat's UK table, and 13.8 litres per minute at a 28 °C rise on Stiebel's comparison sheet. That is a main near 10 °C heated to about 40 °C. The figure is a published rule of thumb, (kW times 60) divided by (4.18 times the rise), not a lab test on this page. Always keep the temperature rise next to the flow. A number with no rise is not a winter promise.
Will a shower and a sink work together on 27 kW?
Often they will not, at full flow. Stiebel's own FAQ says a 27 kW DHE may struggle if a sink opens during a shower. One high-flow shower can already use most of the roughly 13 litres per minute available at a 30 °C rise. A second outlet shares the same kilowatts, so temperature or flow at the first outlet can fall. Buy this class for mostly one heavy draw at a time.
What is the difference between 24.4 kW at 380 V and 27 kW at 400 V?
They are the same heater class at two supply voltages, not two different shopping categories. Stiebel rates the DHE 27 at 24.4 kW when the supply is 380 V and at 27 kW when it is 400 V. The amazon.co.uk listing B08L5B34YH prints 24 kW in the wattage field and 27 kW in the title, which is why you match the nameplate on the box. Do not treat the lower figure as a quieter single-phase model.
Do I need a water-fittings approval as well as an electrical mark?
You need the fitting to satisfy Regulation 4 of the Water Supply (Water Fittings) Regulations 1999, which is a separate question from the electrical certificate. A WRAS or KUKreg4 approval is one way sellers show that. An ETL line in a product bullet is a US listing mark, not that UK water approval. The 15 litre exception in the regulations covers one unvented-storage clause only. It does not replace Regulation 4.
Does every new bath have to be limited to 48 °C?
Approved Document G sets 48 °C for water to a fixed bath when a dwelling is erected or formed by a change of use. That limit is not automatic for a new bathroom in a home that already exists. Ask the installer which case you are in before you add a blending valve on that ground alone. Scotland and Northern Ireland have their own water byelaws, so the same English paragraph is not the whole UK answer.
Who has to sign the electrical work off?
In England, a new circuit is notifiable under Approved Document P unless a registered competent person self-certifies it. The installation is still inspected to BS 7671. You do not sign that off yourself from a buying guide, and you do not put this heater on a 13 A plug. Scotland and Northern Ireland are not a copy of Part P, so the electrician states which rules apply at the address. Plumbing into the public supply is the same kind of professional job.
Related pages
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.
- Approved Document G, with 2024 amendments (retrieved 19 October 2026)
- Approved Document P, 2013 (retrieved 19 October 2026)
- Water Supply (Water Fittings) Regulations 1999 (retrieved 19 October 2026)
- HSE, landlords and legionella (retrieved 19 October 2026)
- Stiebel Eltron UK, DHE 27 Set (retrieved 19 October 2026)
- Stiebel Eltron UK comparison sheet (retrieved 19 October 2026)
- Stiebel Eltron UK, three-phase water heating (retrieved 19 October 2026)
- Flexiheat UK, electric instantaneous water heaters (retrieved 19 October 2026)
- amazon.co.uk, Stiebel Eltron DHE 27 Set B08L5B34YH (retrieved 19 October 2026)