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What size instantaneous water heater do I need?

Updated: 21 October 2026

The cold tap in a British kitchen or cloakroom is usually the moment you start shopping. You want warm water at one outlet, and a remote looks like the easy answer. What size instantaneous water heater do I need comes first: the kilowatts set the litres, and a handset does not add any. This page uses a one-minute jug test and a rule of thumb for about 38°C and about 50°C, then what 3 kW, 6 kW and 8 kW can feed in winter. The seven amazon.co.uk listings stay in the [instantaneous water heater with remote control comparison](/hub/hot-water/instantaneous-water-heater-with-remote-control). A qualifying purchase through our links can earn us a commission at no extra cost to you.

The cold tap in a British kitchen or cloakroom is usually the moment you start shopping. You want warm water at one outlet, and a remote looks like the easy answer.

Our top picks

Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium

Best overall

Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium
6.9/10about £344.99
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Mulcort Mulcort LF-001 3000 W

Best value

Mulcort Mulcort LF-001 3000 W
5.1/10about £46.99
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Briwellna Briwellna 5.5 kW tankless

Rank 3

Briwellna Briwellna 5.5 kW tankless
6.1/10about £108.99
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On this page
  1. 1.Measure the flow you already have
  2. 2.What 3 kW, 6 kW and 8 kW can feed
  3. 3.Winter inlet and the capacity trap
  4. 4.What this page will not choose

Measure the flow you already have

Stiebel Eltron UK’s sizing guidance starts with the tap you already have. Run that outlet into a measuring jug for one minute and read the litres. An instantaneous heater has no stored reserve, so a faster tap stays lukewarm unless you slow it or step up the kilowatts. Their rule of thumb is kilowatts divided by 2 for litres per minute at about 38°C, and kilowatts divided by 3 at about 50°C. That pair is a rule of thumb, not a laboratory result for any listing here. A one-litre jug in a minute sits near 3 kW at about 38°C, because 3 divided by 2 is 1.5 litres a minute, and at about 50°C the same heater is 1 litre a minute. Two litres in the jug points nearer 4 kW on the 38°C rule. That jug result is what picks the kilowatt band you read next.

What 3 kW, 6 kW and 8 kW can feed

The YUCHENGTECH and both Mulcort listings are the 3 kW class, and reviews describe a trickle. A cloakroom rinse can cope. A long 3 kW load is a poor match for a 13 A BS 1363 plug, as the IET’s September 2025 Wiring Matters note sets out. Briwellna, VexNook and Delousiber are the 5.5–6 kW class. Six kilowatts is about 3 litres a minute at about 38°C and about 2 at about 50°C on the same rule of thumb. It needs more than a 13 A socket, since 6 kW at 230 V is about 26 A. The circuit sits on the installation guide. Stiebel’s DCE-X 6/8 Premium, model 238158, is about £344.99, with memory keys and no handheld remote. Amazon states 6 and 8 kW at 220–240 V. The 2026 counter mat lists that model as 7.2/9.6 kW at 240 V. On the rule of thumb, 8 kW is about 4 litres a minute at about 38°C. A bath, or a shower and a sink together, is more than one single-phase unit here can warm at once.
Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium

Testsieger

Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium
6.9/10about £344.99
View offer

Winter inlet and the capacity trap

Summer comments overstate a British January. Quality Heating’s winter rise is near 30°C, so an inlet around 8°C reaches about 38°C only if the flow slows to match. The Stiebel divisors mean water leaving at about 38°C or about 50°C, which is a different sum from adding 38 degrees to icy mains. A July review feels warmer at the same tap. That is how 3 kW sounds fine in summer and lukewarm in February. A remote only moves the setpoint. It leaves kilowatts, amperes and litres per minute unchanged. Prices on the amazon.co.uk files run about £47 to £60, then about £108.99, then about £344.99 for the Stiebel.

What this page will not choose

The seven listings stay on the main comparison, which is where the row is chosen. Who should skip which row is the job of the best instantaneous water heater with remote control page. If the jug test is well above 1.5 litres a minute and you will not add a higher-kilowatt circuit, a small storage heater can be the better kitchen tool. Peter F. checked this on 20 October 2026 from amazon.co.uk listings and public UK guides, without a lab test.

Common questions

What size instantaneous water heater do I need for one basin?
For one hand basin, start at 3 kW and expect a slow flow. Stiebel’s rule of thumb puts 3 kW at about 1.5 litres a minute near 38°C and about 1 litre a minute near 50°C. Hyco calls its own 3.0 kW inline unit light hand washing at about 1.6 litres a minute, which is not a Mulcort or YUCHENGTECH reading. If the basin fills faster than about 1.5 litres a minute, slow the tap or step up the kilowatts.
How many litres per minute does 3 kW give in winter?
Quality Heating’s table gives about 1.4 litres a minute from 3 kW at a rise near 30°C, and calls that a theoretical maximum. Stiebel’s rule of thumb is about 1.5 litres a minute when the outlet is about 38°C. January mains often need the larger rise, so the heater passes less water than a summer review suggests. Neither figure is a lab test of the 3 kW amazon.co.uk listings.
Why does the listing quote litres or gallons of capacity?
Pages in this set put 5 litres, or 43.2 gallons, in a capacity field. An instantaneous heater does not store that volume, and the number is not your litres per minute. Stiebel, Quality Heating and Hyco size from kilowatts, flow and temperature rise. A small storage heater is what holds a real few litres, and you then wait while it recovers.
Does a remote mean I can ignore the kilowatts?
A handheld remote changes the setpoint from across the room. It does not add litres per minute or cut the current. Handsets in this set sit on the 3 kW and 5.5–6 kW imports. The Stiebel DCE-X 6/8 Premium at about £344.99 has memory keys and no handheld remote on the UK page. The handset leaves you short if the tap needed 6 kW and a circuit above 13 A.
When is a small storage heater the better kitchen tool?
A small storage heater suits a short, faster burst at one sink when you can wait for recovery. A 3 kW instantaneous unit only heats water as it passes, so washing-up flow often outruns it. The store holds a few litres and releases them quicker than the element reheats, then the tank is empty. Choose it when the jug test is well above 1.5 litres a minute and you will not fit a higher-kilowatt circuit.

Related pages

Instantaneous water heater for shower

Under sink instant water heater

Instantaneous water heater with remote control

Best instantaneous water heater with remote control

How to install an instantaneous water heater in the UK

Sources

Public UK sources and manufacturer pages named below. amazon.co.uk prices, stars and review counts on these seven listings were read on 20 October 2026. Page updated 21 October 2026.

  1. Stiebel Eltron UK sizing FAQ (retrieved 20 October 2026)
  2. Stiebel Eltron UK, instantaneous versus stored (retrieved 20 October 2026)
  3. Stiebel Eltron UK counter mat (retrieved 20 October 2026)
  4. Stiebel DCE-X 6/8 Premium (retrieved 20 October 2026)
  5. Approved Document P (2013) (retrieved 20 October 2026)
  6. Approved Document G (retrieved 20 October 2026)
  7. IET Wiring Matters, September 2025, 3 kW and a 13 A connection (retrieved 20 October 2026)
  8. Water Fittings Regulations 1999, Schedule 2 paragraph 15 (retrieved 20 October 2026)