Undersink storage or instant water heater
Updated: 17.10.2026
You can picture the cupboard and you still cannot choose between a small tank and an element that heats as the water passes. An undersink storage or instant water heater refuses a different thing in each case. Storage runs at a proper flow until the litres are gone. Instant heat does not run out, and it goes lukewarm when the flow is too high for the kilowatts. Prices are the amazon.co.uk dump of 15 October 2026 on the [point-of-use water heater comparison](/hub/hot-water/point-of-use-water-heater). We did not meter a tap. A qualifying purchase can earn a commission at no extra cost to you.
You can picture the cupboard and you still cannot choose between a small tank and an element that heats as the water passes. An undersink storage or instant water heater refuses a different thing in each case.
You can picture the cupboard and you still cannot choose between a small tank and an element that heats as the water passes. An undersink storage or instant water heater refuses a different thing in each case. Storage runs at a proper flow until the litres are gone. Instant heat does not run out, and it goes lukewarm when the flow is too high for the kilowatts. Prices are the amazon.co.uk dump of 15 October 2026 on the point-of-use water heater comparison. We did not meter a tap. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Type | While the tap is open | Afterwards | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  COMFEE' COMFEE' 10 L undersink | 10 litre store, 2 kW | A stored bowl | Recovery wait | 9.6 | about £79.99 | View offer |
| Preis-Tipp |  Triton Showers Triton T30i handwash | 3 kW instant | Short warm wash | No tank to refill | 9.0 | about £55.99 | View offer |
| Alternative |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 Premium | 10/12 kW instant | Set flow if supplied | Draw only while open | 9.0 | about £412.86 | View offer |
On this page
What you get while the tap is open
While the tap is open, a store delivers water it has already heated, until the hot volume is finished. Stiebel Eltron’s UK FAQ states the trade-off: an instant heater is smaller and heats only when needed, but flow and temperature are limited by the element; a stored heater flows harder until it is empty. In this set the coherent UK stores are the COMFEE' 10 litre, 2 kW unit at about £79.99 and the Ariston Andris R 15 litre, 3 kW unit at about £133.19. COMFEE' reviewers describe one washing-up session. That is a bowl, not an endless supply.
An instant heater’s limit shows up as temperature. A 3 kW element adds about 21°C at 2 litres a minute and about 5°C at 8 litres a minute. A 6 kW element adds about 29°C at 3 litres a minute. A 12 kW element adds about 34°C at 5 litres a minute. Those are calculations, not tests of these ASINs. They explain the Builrand reviews, where a 3,000 W unit only takes the chill off, and why the Triton T30i’s stated 8 litres a minute is not hot water at that flow. The Stiebel DCE-X lists 5 litres a minute and 55°C, and it still depends on the inlet and the circuit. Basin against kitchen is on the handwash or kitchen page.
What you wait for afterwards
After a store is emptied you wait for recovery, and you pay a little to keep it hot in between. Heathfield’s rule of thumb, a retail planning aid rather than a test, is about 12 minutes for 5 litres and about 25 minutes for 10 litres from cold. Ariston’s catalogue figure for the 15 litre undersink 3 kW model is about 18 minutes. Hyco’s theoretical note for a different 5 litre, 2 kW tank is about 6 minutes to lift 70 percent of the contents from 10°C to 60°C. Use the Ariston figure for the Andris. Treat the other minutes as a gap between bowls.
An instant heater’s wait is the moment until the element catches the flow. The cost then sits in the minutes the tap is open, and in the pipe between unit and spout. The Stiebel is about 109 mm deep. The COMFEE' is 30 cm deep. The Andris is 38.5 cm high, with depth not stated. A tank held at 50–60°C, the HSE range for a store of 15 litres or less, loses heat all day. An instant heater avoids that store. It does not avoid heat lost in a long pipe. You are only in instant territory if the electrics match. Litre and amp thresholds are on the buying guide.
What the electrics and the tap demand
A 2 kW or 3 kW store can live on a modest circuit. The IET’s September 2025 note still says a long 3 kW immersion is a poor match for a BS 1363 plug. A 6 kW instant heater is about 26 A at 230 V, and 12 kW is about 52 A. Neither is a 13 A plug. The 6 kW Normodic and laloohome listings mention thicker cable or a large fuse, and they have no reviews. The install guide is where that circuit becomes a point to stop.
Open-outlet stores need the vented tap and will drip as they heat. Unvented stores need the safety kit. Many instant heaters do not. A normal mixer on a vented tank is a mistake on either side of this comparison. The FVSTR title is a store at about £94.95, and the specification says 120 V and 1,500 W, so it is not the worked example of a UK tank.
A conditional recommendation
Choose a 10–15 litre store when the circuit is ordinary and the draw is a bowl. That is the COMFEE' if 10 litres and 30 cm of depth fit, or the Andris R if you want 15 litres. Choose a 3 kW instant heater when the draws are short and the outlet is a hand basin. That is the Triton T30i, not a 3 kW box with a 2.7 average. Choose the Stiebel DCE-X only when you need on-demand heat at a higher flow and an electrician can supply 10/12 kW. Skip it for a plug, a bath, or a shower the reviews do not agree on.
A kitchenette with room for a 30 cm tank is a store. A cloakroom with cold mains and no space for a cylinder is the Triton. A flat already quoted for a high-current circuit is the only place the Stiebel belongs. Who should skip each listing is on the best point of use water heater page.
Common questions
Will an instant heater run out?
The element does not empty a tank, so in that sense it does not run out. The water falls away as soon as you ask for more flow than the kilowatts can lift. At 3 kW that is about 21°C of rise at 2 litres a minute, and about 5°C at 8 litres a minute, as a calculation. A store runs out in litres instead, then needs a recovery wait.
How long does a 10 or 15 litre tank take to recover?
Ariston quotes about 18 minutes to heat the 15 litre undersink 3 kW model. A retail rule of thumb is about 25 minutes for 10 litres from cold, and about 12 minutes for 5 litres. Hyco’s theoretical figure for a different 5 litre, 2 kW tank is about 6 minutes for most of the contents from 10°C to 60°C. Plan a gap between bowls. Do not treat every marketplace tank as the Ariston catalogue.
Why is my instant heater only warm if I slow the tap?
Because the temperature rise is power divided by flow. Slowing the tap is how a 3 kW unit produces a hotter trickle. The Builrand page says that, and its reviews then call a normal flow only the chill off. A store stays hot at a wider flow until the litres are gone. More kilowatts, as on the Stiebel, move that limit, and they need a larger circuit.
Which one uses more electricity standing by?
The store does, because you keep 10 or 15 litres hot between draws. HSE guidance for a small store is 50–60°C, or a very high turnover. An instant heater draws heavily only while the tap is open, which is why a short handwash suits it. A long pipe wastes heat on both. Energy class A or B does not, by itself, tell you which bill is lower.
Which one needs the bigger circuit?
The high-power instant heater. A 2–3 kW store or a 3 kW handwash sits on a modest circuit, with the IET caution about a long 3 kW load on a 13 A plug. A 6 kW instant heater is about 26 A, and the Stiebel at 10/12 kW is a dedicated supply. The tank is the practical choice when you cannot add that circuit.
Can I use a normal mixer?
On many instant heaters, yes, if the sheet allows it. On an open-outlet store, no. The vented tap is the safety outlet, and a mixer closes it. An unvented store can use a normal tap with the safety kit. Match the tap to the vessel rather than to the technology you prefer.
Is a 6 kW instant a fair swap for a 15 litre tank?
Only if the circuit exists and you have evidence the heater works. On paper, 6 kW at 3 litres a minute is about a 29°C rise, which can beat a 3 kW trickle, and it is still not the stored bowl an Andris gives you at 3 kW. The 6 kW listings here have no reviews. Until they do, the fair alternative is another store, or the Stiebel if an electrician has already said yes.
Related pages
Sources
Public UK sources used for this comparison. Listing prices, stars, and review counts are from amazon.co.uk extracts dated 15 October 2026.
- Approved Document G, with 2024 amendments (retrieved 15 October 2026)
- Approved Document P, 2013 (retrieved 15 October 2026)
- Water Supply (Water Fittings) Regulations 1999 (retrieved 15 October 2026)
- HSE HSG274 Part 2 (retrieved 15 October 2026)
- IET Wiring Matters, September 2025 — 3 kW and BS 1363 (retrieved 15 October 2026)
- Retained regulation 812/2013, Annex II (retrieved 15 October 2026)
- Ariston Andris Lux, UK (retrieved 15 October 2026)
- Stiebel Eltron — instantaneous versus stored water heaters (retrieved 15 October 2026)