What kW instantaneous water heater do you need?
Updated: 17.10.2026
You open a normal tap and expect an instant heater to keep that flow hot. The useful answer to what kW instantaneous water heater you need is a trade-off: fixed power gives a large temperature rise at low flow, or a smaller rise at high flow, not both. A 3 kW unit can suit hand washing. A kitchen sink or shower usually needs more power or less flow. Use the arithmetic below before the models in our [tankless water heater comparison](/hub/hot-water/tankless-water-heater). A listing flow is incomplete unless it names inlet and outlet temperatures. A qualifying purchase can earn a commission at no extra cost to you.
You open a normal tap and expect an instant heater to keep that flow hot.
You open a normal tap and expect an instant heater to keep that flow hot. The useful answer to what kW instantaneous water heater you need is a trade-off: fixed power gives a large temperature rise at low flow, or a smaller rise at high flow, not both. A 3 kW unit can suit hand washing. A kitchen sink or shower usually needs more power or less flow. Use the arithmetic below before the models in our tankless water heater comparison. A listing flow is incomplete unless it names inlet and outlet temperatures. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Power | Stated flow | Supply | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 | 12 kW | 5 L/min | High-current circuit | 8.8 | about £412.86 | View offer |
| Preis-Tipp |  ThermoMate ThermoMate ET055 5.5 kW | 5.5 kW | 3 L/min | 25 A, no plug | 8.0 | about £109.74 | View offer |
| Alternative |  TopSer TopSer BS-18DC-3 Pro tap | 3 kW | 3 L/min | Corded, 220 V | 8.0 | about £49.99 | View offer |
On this page
Temperature rise is not the number at the tap
An instantaneous heater adds heat to the water entering it. A 25°C rise on a 10°C inlet leaves about 35°C. The same rise on a 4°C inlet leaves about 29°C, which is why a heater can feel adequate in mild weather and weak in winter. Use the manufacturer's chart for the coldest inlet at the property.
Hyco's relationship is temperature rise in degrees Celsius ≈ kW × 14.3 ÷ litres per minute. At 6 kW and 3 L/min the rise is about 28.6°C, so a 10°C inlet leaves at roughly 38.6°C. At 4 L/min the rise falls to about 21.5°C. Opening the tap further can cool the outlet.
Around 38°C may suit hand washing or a shower. A hotter kitchen target cuts the flow from the same kilowatts. The buying guide puts that choice next to the circuit.
Use two rules of thumb, then measure
Stiebel Eltron's UK sizing guide divides kW by two for litres per minute at about 38°C, or by three for about 50°C. On that basis, 6 kW suggests roughly 3 L/min at 38°C or 2 L/min at 50°C, and 12 kW suggests roughly 6 L/min or 4 L/min. Check the shortcut against the model's own chart.
For 3 kW the same shorthand gives around 1.5 L/min at 38°C and 1 L/min at 50°C. Hyco's table puts a 3 kW light hand wash around 1.6 L/min. That can cover a short basin use. It fills a 10-litre bowl slowly, and a kitchen label does not add energy.
Measure the tap with a jug and a stopwatch. A 1-litre jug in 10 seconds is 6 L/min. Compare that with the chart at your target temperature, not with a bare maximum flow.
Match 3 kW, 6 kW and 12 kW to the job
A 3 kW heater belongs at a low-flow hand-wash basin. TopSer states 3 L/min at 3 kW, but reviews say the flow must fall before the water is hotter. Read 3 L/min as a stated figure, not as 3 L/min at 50°C.
At 5.5 to 6 kW, hand-wash flow improves and a modest sink or shower may be possible, but the target temperature still decides. The ThermoMate ET055 field says 3 LPM at 5.5 kW. Critical reviews report about 1 L/min for genuinely warm water. A 6 kW example gives a 28.6°C rise at 3 L/min, so a cold inlet can stay only warm, and these heaters need a dedicated circuit.
Around 10 to 12 kW, a single-phase unit can provide more credible basin, kitchen or modest shower service. The Stiebel listing states 5 L/min and 55°C for its 10/12 kW-class model. Treat that line as the listing's specification, not a measurement we reproduced. Reviews say some shower installations work. Bath filling remains a poor use. If a high-current circuit is impractical, compare instantaneous and storage water heaters.
One heater cannot hide a second open tap
Two outlets on one instantaneous heater share its fixed power. If flow doubles, the temperature rise roughly halves. Electronic control can restrict flow. It cannot create extra kilowatts. Two outlets can still mean one-at-a-time use.
A 12 kW heater at 4 L/min, plus a basin at 2 L/min, is 6 L/min combined. The rise drops unless the controller limits one outlet. That is not whole-house performance. Size for the overlap if two people draw hot water together.
If the calculated band fits the property, see the conditional top picks. If it needs an uncomfortably narrow stream, reject the listing before you pay for a circuit.
Common questions
What is temperature rise?
Temperature rise is the difference between water entering and leaving the heater. An outlet at 38°C from a 10°C inlet represents a 28°C rise. Instantaneous units add a rise determined mainly by power and flow, so the same setting can produce colder outlet water when the inlet becomes colder.
How many litres per minute can 3 kW provide?
Hyco's selection table suggests about 1.6 L/min for light hand washing at 3 kW. Stiebel's broader rule of thumb gives roughly 1.5 L/min at 38°C or 1 L/min at 50°C. These are sizing guides. Check the exact heater chart and your coldest expected inlet temperature.
Is 6 kW enough for a kitchen sink?
It can support modest kitchen flow, but not necessarily normal mixer-tap flow at a high washing-up temperature. Using Hyco's formula, 6 kW at 3 L/min gives about a 28.6°C rise. It also requires a dedicated electrical circuit, so compare the installed cost with a small storage heater.
What kW do I need for a shower?
The answer depends on measured shower flow, inlet temperature and target temperature. Manufacturer guidance places light showers around the upper single-digit or 10/12 kW region, while lower power means reduced flow. Have the model and circuit professionally assessed. Do not infer shower suitability from a generic icon on a listing.
Does a stated 5 L/min mean 5 L/min at 50°C?
Not unless the specification states the inlet temperature, outlet temperature and test conditions. A bare flow figure may describe maximum throughput rather than hot output. Ask for the performance chart and check the temperature rise at 5 L/min under winter conditions before you rely on it.
Related pages
Sources
Public UK sources used for this comparison, last checked on 17 October 2026.
- Approved Document G (2024 amendments) (retrieved 17.10.2026)
- Approved Document P (retrieved 17.10.2026)
- IET Part P frequently asked questions (retrieved 17.10.2026)
- HSE HSG274 Part 2 (retrieved 17.10.2026)
- Hyco instantaneous water heater chart (retrieved 17.10.2026)
- Stiebel Eltron: what size instantaneous water heater do I need? (retrieved 17.10.2026)
- Stiebel Eltron: instantaneous versus stored water heaters (retrieved 17.10.2026)
- Stiebel Eltron DCE-X 10/12 technical data (retrieved 17.10.2026)
- amazon.co.uk listings for the seven ASINs in this comparison (retrieved 17.10.2026)