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What size instantaneous water heater do I need

Updated: 21 October 2026

You have an 18 kW figure in a title, and you still need to know whether that heater can run the shower without taking the consumer unit with it. What size instantaneous water heater do I need is a question of flow at a temperature rise, because the heater warms mains water as it passes. In a UK catalogue, 18 kW starts the three-phase multipoint class, not a plug-in basin unit. On a normal single-phase house the same 18 kW at 230 V is about 78 A. This page sizes the outlets and the supply before any model. The three amazon.co.uk listings we kept are on the [18kW instantaneous water heater comparison](/hub/hot-water/18kw-instantaneous-water-heater).

You have an 18 kW figure in a title, and you still need to know whether that heater can run the shower without taking the consumer unit with it.

Our top picks

WINTEMP WINTEMP YQ-WN18K 18 kW

Best overall

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
View offer
Rheem Rheem RTEX-18

Rank 2

Rheem Rheem RTEX-18
8.4/10about £364.17
View offer
Bosch Bosch Tronic 4000 18 EB

Rank 3

Bosch Bosch Tronic 4000 18 EB
7.0/10about £428.72
View offer
On this page
  1. 1.What 18 kW can heat
  2. 2.Single-phase or three-phase
  3. 3.Fixed plate or a switchable rating
  4. 4.When 18 kW is the right band

What 18 kW can heat

Stiebel Eltron UK publishes a rule of thumb, not a test of your house: divide the kilowatts by two for litres per minute at about 38°C, and by three at about 50°C. At 18 kW that is about 9 litres per minute and about 6 litres per minute. Flexiheat prints 8.7 litres per minute at a 30°C rise. CLAGE’s DEX 18 Next sheet lists 9.2 litres per minute at a 28 K rise. A standard shower is about 7–8 litres per minute. A bath mixer near 12 litres per minute, or a drench head, asks for more than that band. Do not use the Bosch amazon.co.uk attribute that says 6 litres per second. That unit is not a credible heated flow.

Single-phase or three-phase

UK Power Networks describes a check at the service head: one fuse is typically single-phase, and three 100 A fuses are three-phase. Most homes in Britain are single-phase. You do not guess from the age of the house. Stiebel Eltron UK says convenience heaters for a bathroom and a kitchen are 400 V, and that its US Tempra range is not suitable for UK electrical connections. In this Amazon set the Bosch Tronic 4000 18 EB is listed at 400 V. The Rheem RTEX-18 and the WINTEMP YQ-WN18K are listed at 240 V. Eighteen kilowatts at 230 V is about 78 A on one phase. On 400 V the same power is about 26 A per phase, as on Zip’s DBX18. If the service head has one fuse, stop. A 240 V listing is not a shortcut. The split is on the three-phase supply page.
WINTEMP WINTEMP YQ-WN18K 18 kW

Testsieger

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
View offer

Fixed plate or a switchable rating

A fixed 18 kW plate is one output. A switchable 18/21/24 kW chassis, such as the Stiebel Eltron DHE 18/21/24, lets the installer match the rating to the supply. Heater Shop lists that DHE at £808.62 including VAT. Zip’s DBX18, a fixed 18 kW 400 V unit, was £564 including VAT in the figures we saw and was shown unavailable at one UK seller. Those are catalogue peers, not Amazon rows. The kept Bosch is fixed 18 kW, energy class A, at about £428.72, with £385.10 plus £77 delivery on the live page on 19 October 2026. Class A is an ErP letter, not the 99% claim on the 240 V listings, and neither figure is a UK laboratory ranking.

When 18 kW is the right band

Choose 18 kW when you already have three-phase, you want about one standard shower, and a second outlet is a basin rather than another full shower. Step toward 21 or 24 kW when the installer expects two showers, or a bath plus basins, and the fuses allow the higher step. This page does not name a winner, because the right kilowatts are still a property of your service and your taps. Peter F. checked the listings and public UK sources on 19 October 2026. This is not a laboratory test. A purchase through our links can earn a commission at no extra cost to you.

Common questions

What current is 18 kW at 230 V?
About 78 A. That is 18,000 watts divided by 230 volts. On a normal single-phase consumer unit that load takes most of the service while the heater runs. On 400 V three-phase the same power is about 26 A per phase. Cable size is the electrician’s calculation under BS 7671, not a millimetre figure from this page.
Is 18 kW enough for a kitchen and a shower together?
It can cover a standard shower, about 7–8 litres per minute, with a modest second outlet if you accept a shared flow. It is tight once a bath mixer near 12 litres per minute, or a second full shower, opens at the same time. Stiebel’s 50°C rule of thumb is only about 6 litres per minute at 18 kW, which is a hot basin rather than two showers running together.
Why does the same heater feel weaker in January?
The element still draws 18 kW, but the mains inlet is colder, so the temperature rise is larger. Flow has to drop, or the spray arrives cooler, to stay inside that power. A 30°C rise is the Flexiheat case of about 8.7 litres per minute. A rise nearer 35°C in winter is a smaller flow. That is the heat balance, not a fault code on the heater.
What water pressure does an 18 kW unit need?
The kept Amazon listings do not state a minimum bar. Smaller single-phase manuals from Triton and Redring quote about 0.4–1.8 bar depending on the model, and a Zip sheet notes about 2 bar for optimum on that InLine unit. Copy the figure from the manual of the heater you buy. Do not paste those other models onto the Bosch Tronic or the Rheem RTEX-18.
Should I buy a switchable 18/21/24 kW unit instead of a fixed 18 kW plate?
Choose the switchable chassis when the electrician may need to land on 21 or 24 kW to match the fuses and the outlet list. A fixed 18 kW plate is enough when the supply and the shower are already known to sit in that band. The Stiebel DHE 18/21/24 is the catalogue example, at £808.62 including VAT at Heater Shop, and it is not one of the three kept Amazon rows.

Related pages

18kW instantaneous water heater for a shower

18kW instantaneous water heater

Best 18kW instantaneous water heater on three-phase

How to install an 18kW instantaneous heater in the UK

240V tankless water heater versus a UK 400 V supply

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. Part G FAQ, bath temperature limit (retrieved 19 October 2026)
  3. Approved Document P, 2013 (retrieved 19 October 2026)
  4. UK Power Networks, single-phase and three-phase (retrieved 19 October 2026)
  5. Stiebel Eltron UK, what size instantaneous water heater (retrieved 19 October 2026)
  6. Stiebel Eltron UK, Tempra products in the UK (retrieved 19 October 2026)
  7. Zip DBX18 installation sheet (retrieved 19 October 2026)
  8. Rheem RTEX product sheet (retrieved 19 October 2026)
  9. Flexiheat UK, electric multipoint water heaters (retrieved 19 October 2026)
  10. amazon.co.uk, Bosch Tronic 4000 18 EB (retrieved 19 October 2026)