Instantaneous vs storage water heater
Updated: 17.10.2026
Search results mix slim tankless boxes with 5, 10 and 15 litre cylinders when you only need hot water at one sink. The instantaneous vs storage water heater choice is what you will limit. An instant unit runs for as long as the tap is open, but kilowatts cap the flow. A small store releases several litres quickly from a modest supply, then needs recovery time and loses some heat while it waits. Our [tankless water heater comparison](/hub/hot-water/tankless-water-heater) lists only instantaneous products. This page decides whether you belong there. A qualifying purchase can earn a commission at no extra cost to you.
Search results mix slim tankless boxes with 5, 10 and 15 litre cylinders when you only need hot water at one sink. The instantaneous vs storage water heater choice is what you will limit.
Search results mix slim tankless boxes with 5, 10 and 15 litre cylinders when you only need hot water at one sink. The instantaneous vs storage water heater choice is what you will limit. An instant unit runs for as long as the tap is open, but kilowatts cap the flow. A small store releases several litres quickly from a modest supply, then needs recovery time and loses some heat while it waits. Our tankless water heater comparison lists only instantaneous products. This page decides whether you belong there. 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
Instant limits flow; storage limits volume
An instantaneous heater holds no useful reserve. It adds a temperature rise set by power, litres per minute and inlet temperature. A 3 kW model can suit a short hand wash, but reviewers in this set often turn the tap down for hotter water. That narrow stream can run on. A washing-up bowl may still take a long time.
A store heats a finite volume first, commonly 5 to 15 litres, then releases it at a stronger short flow. Mixed with cold, a 10 litre store may fill a bowl, depending on set temperature and losses. When the reserve is gone, you wait while the element reheats.
Frequent 20-second hand washes favour instant heat. A bowl filled in one go often favours storage on a modest supply. ThermoMate reviews split the same way: adequate at a cloakroom, too little warm flow for a bowl.
Let the circuit and the cupboard decide
A small store commonly uses an element around 2 to 3 kW and may suit a fused or 13 A-region supply when its instructions allow it. It heats slowly, then delivers the saved water faster than the element could heat that flow in real time. That is why a bowl that disappoints a 3 kW instant heater can suit a store.
Instant power rises with the flow you want. Around 3 kW is mainly a hand-wash band. A 5.5 or 6 kW unit needs a dedicated circuit and still compromises on flow. A 10/12 kW unit can support a basin or a modest shower, but only on a high-current installation. A cheap high-wattage box does not fix a consumer unit that cannot take the circuit.
Choose instant for short repeated draws when the circuit can carry the flow and cupboard space is scarce. Choose a small store for several litres quickly from a modest supply, plus a wait after heavy use. The tankless heater buying guide covers the instant path after this test.
Compare standing loss, recovery and a second tap
Instantaneous heaters use energy only while water flows, so there is no cylinder standing loss. That does not prove a lower bill. A poorly sized heater can encourage longer draws, and tariff, pipe loss and usage still matter. The clear advantage is that no tank is kept warm between uses.
A store loses heat through its insulation and reheats to replace it. Better insulation reduces the loss. It does not remove it. In return you get a burst of flow the element could not heat in real time. Heathfield's point-of-use explainer gives about 12 minutes for 5 litres and 25 minutes for 10 litres. Treat that as the retailer's rule of thumb, not a timing for every product.
Measure litres per minute with a jug and note whether two taps overlap. The kilowatts, flow and temperature guide shows why a bare 5 L/min claim is not the hot flow. If that sum fails, storage fits the demand better.
Do not copy safety fittings between types
Instantaneous and unvented storage heaters do not share a safety layout. A pressure-rated instant unit heats water as it passes and holds very little volume. An unvented cylinder holds heated water under pressure and may need expansion control, pressure relief and a discharge route. An open-outlet small store is a third arrangement, often with its own tap.
Guidance treats unvented storage over 15 litres as a controlled installation, and carves instantaneous heaters out of that storage package. A 10 or 15 litre unit is still not a casual fit. Follow the manufacturer's scheme rather than copying valves from the other type.
Confirm vented or unvented, the discharge route and whether the tap is mandatory before you order a store. For an instant model, confirm pressure type and electrical load. The instantaneous heater installation guide covers water before power, and where an electrician takes over.
Common questions
Which is better, an instant heater or a 10-litre store?
Choose instant for short, frequent draws when restricted flow is acceptable and the circuit supports the rated power. Choose a 10 litre store when you need several litres quickly, such as a washing-up bowl, from a modest electrical supply. The store needs cupboard space and recovery time. The instant unit needs sufficient kilowatts.
Will a 3 kW instant heater fill a kitchen sink?
It will eventually fill one, but reviews and manufacturer charts suggest properly hot output may be around hand-wash flow rather than normal kitchen flow. Hyco places 3 kW light hand washing near 1.6 L/min. If speed matters, a small store can accumulate heat and release the water faster.
Does a storage heater waste electricity while it sits?
It has standing heat loss because stored water and the cylinder remain warmer than their surroundings. Insulation limits that loss, and the practical cost depends on temperature, usage and tariff. Instantaneous heaters avoid cylinder standing loss, but that fact alone does not prove a lower total cost for every household.
Can a small store use a 13 A supply?
Many point-of-use stores have elements in the 2 to 3 kW region and are designed for a fused or plug-region connection, but you must follow the exact manual. Circuit condition, location and fixed-wiring requirements still matter. Do not assume every storage heater includes a plug or suits an existing socket.
Why did storage cylinders appear in a tankless search?
Retail search results often group undersink and point-of-use water heaters together because they solve a similar problem. A capacity field can also be misleading on marketplace listings. A genuine tankless heater does not store a useful hot-water reserve. A 5, 10 or 15 litre cylinder belongs to the storage category.
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)