Tankless water heater buying guide
Updated: 17.10.2026
A sink that takes too long to run hot makes an electric instant heater look simple. This tankless water heater buying guide checks the outlet, the supply and the flow before you pay. Tankless only means water is heated as it passes. It does not mean the unit can fill a washing-up bowl or run from a 13 A socket. The seven listings are in our [tankless water heater comparison](/hub/hot-water/tankless-water-heater). Start with the job at the tap, not the brand. A qualifying purchase can earn a commission at no extra cost to you.
A sink that takes too long to run hot makes an electric instant heater look simple. This tankless water heater buying guide checks the outlet, the supply and the flow before you pay.
A sink that takes too long to run hot makes an electric instant heater look simple. This tankless water heater buying guide checks the outlet, the supply and the flow before you pay. Tankless only means water is heated as it passes. It does not mean the unit can fill a washing-up bowl or run from a 13 A socket. The seven listings are in our tankless water heater comparison. Start with the job at the tap, not the brand. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Power | Supply | Stated flow | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 | 12 kW | High-current circuit | 5 L/min | 8.8 | about £412.86 | View offer |
| Preis-Tipp |  ThermoMate ThermoMate ET055 5.5 kW | 5.5 kW | 25 A, no plug | 3 L/min | 8.0 | about £109.74 | View offer |
| Alternative |  TopSer TopSer BS-18DC-3 Pro tap | 3 kW | Corded, 220 V | 3 L/min | 8.0 | about £49.99 | View offer |
On this page
Start with the outlet, not the brand
A cloakroom basin, a kitchen sink and a shower ask different things of the same heater. Hand washing can work with a narrow stream for 20 or 30 seconds. Washing up needs several litres before the first water cools. A shower needs continuous flow at a comfortable temperature, and a bath is generally a poor fit for a compact single-phase unit.
At the low end, a 3 kW heater is principally a hand-wash appliance. Hyco's selection chart puts 3 kW light hand washing at roughly 1.6 litres per minute. That is a manufacturer guide, not a promise for every installation. Listings may state a higher flow, but hotter water usually means throttling the tap. If the test is filling a washing-up bowl at normal mixer speed, a small store may make more sense than a plug-region instant model.
More kilowatts buy rise or flow, not every job. The 12 kW Stiebel listing states 5 litres per minute and 55°C, and reviewers still do not recommend it for a bath. Measure with a jug, then use the kilowatts and flow guide. The band only helps if the consumer unit can supply it.
Match kilowatts to the electrical supply
Around 3 kW is the region in which a heater may use a UK 13 A connection, subject to its instructions and the circuit. A genuine 5.5 or 6 kW load is not a normal plug-in appliance. The ThermoMate listing asks for 25 A or more and says no plug is included. The Delousiber 6 kW bullet calls for a dedicated circuit and a fuse of at least 40 A. A picture of a regular plug beside a 6 kW claim does not make that combination safe.
Stiebel's data for the DCE-X class gives high current figures and calls for a qualified contractor. A new circuit covers cable route, protective devices, earthing and the consumer unit. In England and Wales it is notifiable under Part P, and fixed electrical work must meet BS 7671. Requirements differ across the UK, so use a competent local electrician rather than a breaker size copied from a review.
Price the installation before a £45 high-power listing looks cheaper than a £110 product or a small store. The data plate must agree with the manual and the circuit. If it is missing or different, pause. The installation guide is where plumbing ends and the electrical work begins.
Check outlet type, pressure and winter performance
Some small instantaneous heaters are open-outlet designs. They need the manufacturer's tap, nozzle or aerator, and an ordinary mixer can prevent safe operation. A pressure-rated model can use a compatible tap, but not every mixer. Identify the arrangement in that model's manual.
Winter exposes optimistic flow claims. The heater adds a rise to the incoming water rather than holding one outlet temperature. Hyco's guide is temperature rise = kW × 14.3 ÷ litres per minute. Colder inlet water leaves colder at the same power and flow. Check the winter chart, not only the largest figure on the amazon.co.uk page.
Resolve listing conflicts before buying
Treat contradictions as missing information. In this set, the laloohome title says 6000 W while its technical details say 5500 W. One Mulcort page mixes gallons of capacity with gallons per hour, despite describing a tankless unit. The ThermoMate table prints a 25°C maximum although reviews describe warmer outlet water. The Delousiber combines a 40 A instruction with reports of a fitted plug. These are reasons to request the manual or inspect the data plate.
Before checkout, write down the outlet, target temperature, measured flow, available circuit, heater type and pressure range. Confirm who will install it and whether the supplied tap is mandatory. If the answers point towards stored hot water, compare instantaneous and storage heaters rather than forcing a 3 kW unit into a kitchen job.
Common questions
Is 3 kW enough for a kitchen sink?
It can provide warm water at a restricted flow, but it is usually better suited to hand washing than quickly filling a washing-up bowl. Hyco's guide places a 3 kW hand-wash application around 1.6 litres per minute. If you need several litres of properly hot water quickly from a 13 A supply, consider a small storage heater.
Do I need a dedicated circuit for 5.5 kW or 6 kW?
Yes. These ratings are beyond a normal 13 A socket load. The exact circuit, protection and notification route depend on the heater, the property and the UK nation. Use the delivered data plate and manual, then have a competent electrician design and test the installation. Do not rely on a plug shown in an online listing.
What is an open-outlet water heater?
An open-outlet heater is a non-pressurised design that normally works with a specific tap or outlet arrangement supplied or approved by its manufacturer. Closing or restricting it with an unsuitable mixer can be unsafe. Do not assume every compact undersink heater is open-outlet. Identify the model's type in its manual.
Can one instant heater feed a shower and basin together?
Some pressure-rated models can be connected to more than one outlet, but their fixed kilowatts are shared. Opening a second tap usually reduces temperature or flow, and manufacturers may intend only one outlet to run at a time. Size for the simultaneous demand, or choose separate point-of-use heaters.
What should I check on the data plate?
Check rated voltage, wattage, current, pressure range, protection rating and the exact model against the manual. Those values must match the planned electrical and plumbing design. If the plate disagrees with the amazon.co.uk title, use the plate as the appliance identity and stop until the supplier or installer resolves the discrepancy.
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)