Instantaneous or stored hot water: which is the better choice for your home?

When it comes to heating domestic hot water, most people sooner or later face the same question: an instantaneous water heater or a storage one? It looks like a simple choice, but in reality a lot of variables lie behind it – the size of the household, the type of heating system, daily habits, energy prices and the technical constraints of the particular flat or house. In 2026, with electricity and gas prices still at historically higher levels than before 2021, this choice has a direct impact on your monthly budget.

In this article we go through the whole subject really thoroughly – from the basic operating principle of both systems, through real consumption calculations, to specific recommendations for different types of household. If you came here searching for an "instantaneous water heater" and want to make a genuinely wise decision, you are in the right place.

How does an instantaneous water heater work?

An instantaneous water heater (sometimes also called a tankless water heater or an on-demand water heater) works on a simple principle: the water is heated at the very moment you run it. The appliance has no tank – cold water enters the heater body, passes through a heat exchanger (electric, gas or gas condensing) and leaves the appliance at the outlet as hot water at the required temperature.

The advantage is that the appliance uses no energy when you are not using water. There are no heat losses from a cylinder and no "waiting" for a tank to heat up again. On the other hand, the appliance must have enough output to cover instantaneous demand – typically 6 to 27 kW for electric versions, or 18 to 28 kW for gas condensing combi boilers.

Electric vs. gas instantaneous water heater

There are two main categories of instantaneous water heater on the market, depending on the energy source:

• Electric instantaneous water heaters – typically have an output of 3.5 kW (hand washing only) to 27 kW (showering + washing up at the same time). They are compact and cheap to buy (from €60 to €400), but at higher outputs they require a 400 V three-phase supply and suitable circuit breakers. Running costs are higher – the price of electricity in Slovakia in 2026 is around €0.18–0.25/kWh (including distribution), which is roughly 3 times the cost of the same amount of heat from natural gas.
• Gas instantaneous water heaters / combi boilers – gas condensing combi boilers can provide instantaneous domestic hot water (DHW) heating at an output of 18–28 kW, and the thermal efficiency of modern condensing boilers exceeds 108% (based on net calorific value). The price of natural gas in 2026 is around €0.065–0.085/kWh (for households in Slovakia), which makes gas heating considerably cheaper.

How does a storage water heater work?

A storage water heater (a hot water cylinder) works differently: the water is heated in advance and stored in an insulated tank with a capacity of 50 to 500+ litres, where it waits for you to draw it off. When you turn on the tap you do not have to wait – hot water is available immediately (as long as the cylinder is big enough and you do not exhaust its capacity).

Stored hot water can be connected to various heat sources: an electric immersion heater (directly in the tank), a heat pump, solar collectors, or the secondary circuit of a gas condensing boiler via a heat exchanger. A boiler + hot water cylinder combination is still the most widespread way of heating water in houses in Slovakia.

Cylinder heat losses – what does "permanent standby" cost?

This is a key point that many customers underestimate. Every cylinder loses heat through the walls of the tank, even when you use no water at all. Modern cylinders with good-quality polyurethane insulation have heat losses of approx. 0.8–1.5 kWh per 24 hours (for a 100–150 l cylinder). Older cylinders with poorer insulation can lose as much as 2.5–4 kWh/day.

Over a year, with an electric cylinder (price €0.22/kWh), that means:

• Modern 150 l cylinder: 1.2 kWh/day × 365 × €0.22 = ~€96/year on heat losses alone
• Old 150 l cylinder: 3.0 kWh/day × 365 × €0.22 = ~€241/year on heat losses alone

That is exactly why choosing a good-quality cylinder (and maintaining it regularly, including a working temperature sensor) matters. NTC temperature sensors are used to control the cylinder temperature correctly – for example the NTC sensor kit for cylinder 10 kΩ, which ensures accurate temperature control and prevents the cylinder from being overheated unnecessarily or, conversely, underheated.

Instantaneous and stored water heating compared – at-a-glance table

When is an instantaneous water heater the better choice?

Instantaneous heating makes sense in clearly defined situations. It is not a solution for everyone, but where it fits, it is genuinely efficient and economical.

Small households and flats with one bathroom

If you live alone or are a couple without children, daily hot water consumption is relatively low. Standard STN 06 0320 gives a reference DHW consumption of 40–50 litres per person per day (at 55 °C) for a dwelling with one bathroom and a kitchen. For two people that is 80–100 litres a day – which a gas condensing combi boiler with an output of 18–24 kW will handle with ease, as will a decent electric instantaneous water heater.

In a flat with no gas and no room for a cylinder, an electric instantaneous water heater with an output of 18–21 kW (three-phase) is practically the ideal solution – it takes up minimal space, requires no cylinder installation, and if the house/flat has an adequate circuit breaker, fitting it is quick and cheap.

Cottages, garden houses and occasional use

Here the instantaneous water heater is almost unbeatable. At a cottage where you stay only at weekends or in summer, a cylinder would be an absurd solution – it loses heat all week while you are away, the risk of scale and legionella is higher, and on top of that it has to be drained for the winter. A small 6–9 kW electric instantaneous water heater for washing hands and dishes is ideal here.

When you want supplementary heating at a particular draw-off point

Even in houses with stored hot water, a small instantaneous water heater is often installed at a remote draw-off point (for example a garden shower, garage or outdoor sink). Instead of running a long hot water pipe from the boiler room, a simple small instantaneous water heater right at the point of use is enough.

When is stored hot water the better choice?

Stored hot water is irreplaceable in certain conditions where instantaneous heating simply cannot cope or does not add up financially.

Larger families (3 or more people)

A family of four needs 160–200 litres of DHW (55 °C) a day. If three people take turns in the shower one after another in the morning, you need a cylinder that will cover this. A correctly sized 150–200 litre cylinder combined with a condensing boiler handles this situation easily – and you do not have to worry about whether a combi boiler will hold the temperature at full flow.

Houses with two bathrooms or several outlets in use at once

When water is running in the bathroom and the kitchen at the same time, or when the family uses a bath and a shower enclosure in two bathrooms simultaneously, the demand on instantaneous flow is extreme. A combi boiler would need an output of 30+ kW to cope without a drop in temperature – and most gas condensing boilers on the market are sized precisely for a cylinder as the main source of DHW.

Combination with renewable energy sources

A cylinder is a key element in any system with a heat pump, solar collectors or photovoltaics. These sources do not produce heat on demand – they produce it when the sun is shining or at a set time. The cylinder serves as a thermal store that evens out this mismatch between production and consumption. Without a cylinder, solar collectors would make no sense.

Gas condensing boiler + hot water cylinder: the gold standard for houses

In practice, the combination most often installed in house renovations and new builds in Slovakia is a condensing gas boiler + external hot water cylinder. It is a proven, efficient and flexible solution that handles both heating and DHW.

The top products in this category include packages from Vaillant – for example a condensing boiler combined with external cylinders of various capacities:

Vaillant ecoTEC pro VU 246/5-3 + VIH Q 75 B cylinder – a compact package for smaller households or flats

The Vaillant VU 246/5-3 ecoTEC pro + VIH Q75 B package is sized for smaller households or flats, where a 75-litre cylinder covers the needs of 2–3 people. The boiler has an output of 6.5–24 kW, which is more than enough for most flats, and combined with multiMATIC 450 control you can manage all your heating and hot water conveniently from one place.

Vaillant ecoTEC pro VU 246/5-3 + VIH R 120 B cylinder – a popular package for families of 3–4

For families of 3–4, the Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B package is more suitable. In practice a 120-litre cylinder is the standard for four people with one bathroom and a shower cubicle – it covers the morning showers without the cylinder "running out" before the last member of the family.

Vaillant ecoTEC pro VU 246/5-3 + VIH R 150 B cylinder – ideal for larger houses and two bathrooms

If you have a larger house, two bathrooms, or a family of 5 or more, go for the Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B package with a 150-litre cylinder and multiMATIC 700 control. This controller offers not only intelligent heating time programmes but also connection to smart home systems and provision for integrating solar collectors in the future.

Protherm Gepard Condens – instantaneous DHW in a single compact boiler

Not everyone wants a cylinder. For households with no room for an external cylinder, or where instantaneous DHW is sufficient for the daily routine, condensing combi boilers are the ideal solution – a single compact unit provides the heating and heats water instantaneously as well.

Protherm Gepard Condens 18/25 MKV – a condensing combi boiler with instantaneous DHW, ideal for flats

A typical example is the Protherm Gepard Condens 18/25 MKV. This condensing combi boiler has a heating output of 4.5–18 kW and up to 25 kW for DHW, which gives a hot water flow rate of around 12–14 litres per minute at 40 °C. For a flat with one bathroom and a kitchen that is the optimum – a shower and the washing up at the same time without any problem.

If your household is smaller still and the heated area does not exceed 100–120 m², or if you want the most energy-efficient solution for a small flat, take a look at the Protherm Gepard Condens 12 MKO – a lower-output version intended for low-temperature systems and well-insulated flats.

Sizing a hot water cylinder – how to choose the right capacity

One of the most common mistakes in renovations is sizing the cylinder incorrectly – either too small (the cylinder "runs out" and the last person in the shower freezes) or too large (an unnecessarily big cylinder means higher heat losses and a longer heat-up time after a fault or a holiday).

A rule of thumb for sizing a hot water cylinder:

• 1 person: 50–75 litre cylinder
• 2 people: 75–100 litre cylinder
• 3–4 people: 100–150 litre cylinder
• 4–5 people: 150–200 litre cylinder
• 5+ people or 2 bathrooms: 200+ litre cylinder, or 2 cylinders connected in series

These figures apply to standard European habits (a shower once a day, a bath approx. twice a week). If baths or long showers are the norm in your household, go one size up.

The importance of the right cylinder temperature and a working NTC sensor

A cylinder should be kept permanently at a temperature of at least 60 °C. Below this temperature the bacterium Legionella pneumophila, which causes Legionnaires' disease – a serious respiratory illness – can multiply in the cylinder. Many boiler controls therefore automatically run what is known as an anti-legionella cycle once a week, heating the cylinder to 70 °C.

For this function to work properly, a reliable and accurate NTC cylinder temperature sensor is essential. The NTC sensor kit for cylinder 10 kΩ is a spare part suitable for most common cylinder systems. If the boiler control indicates a cylinder fault, or if DHW heating is inaccurate (the boiler heats the cylinder for too long or, conversely, not enough), replacing the NTC sensor is the first diagnostic step.

Energy efficiency in 2026 – a cost calculation

Let us compare the real annual cost of heating DHW for a family of four (consumption of approx. 180 litres of hot water/day at 55 °C, i.e. heating from 10 °C to 55 °C).

Energy required for heating: Q = m × c × ΔT = 180 kg × 1.163 Wh/kg·K × 45 K ≈ 9.4 kWh/day, i.e. approx. 3,430 kWh/year.

The chart makes it clear that a gas condensing boiler combined with a hot water cylinder is by far the most economical solution in terms of running costs for a family household – despite the higher upfront costs. The difference compared with electric solutions comes to €380–580 a year, which over a 10-year horizon means a saving of €3,800–5,800.

An electric instantaneous water heater does come out better than an old cylinder (thanks to the absence of standby heat losses), but it is still considerably more expensive to run than a gas condensing system.

Legislation and standards relevant to 2026

When choosing and installing a water heater in Slovakia, there are several legislative and technical standard requirements to bear in mind:

• ErP Directive (Energy-related Products): Since 2017, electric storage water heaters with a capacity above 50 litres have had to meet at least energy efficiency class B (from an output of 2 kW). Heaters bought before 2017 may be considerably less efficient, and replacing them pays off not only environmentally but also financially.
• STN EN 806: The technical standard for water installations inside buildings, which sets out requirements for DHW temperature and measures to prevent Legionnaires' disease (minimum distribution temperature 55 °C, minimum cylinder temperature 60 °C).
• Slovak Government Regulation 401/2023 Z. z. (and related legislation): In line with European decarbonisation targets, support for renewable energy sources continues – in 2026, combining a hot water cylinder with a heat pump or solar collectors is also supported by subsidy schemes (ŠFRB, OP Slovensko, Zelená domácnostiam).
• Decree of the Slovak Ministry of the Environment 416/2012 Z. z.: Sets out the requirements for the energy certification of buildings. DHW preparation forms part of the calculation of a building's energy demand, which is relevant when selling or letting a property.

Practical scenarios from everyday experience

Scenario 1: Two-room flat, 1–2 people, no gas

A typical flat in a prefabricated block with no gas connection and no room for a cylinder. The right choice here is an electric instantaneous water heater with an output of 18–21 kW (three-phase). It takes up minimal space, installation is quick, and for one or two people with ordinary showering habits the flow rate is sufficient. Expect annual DHW costs of around €500–650 at an electricity price of €0.22/kWh.

If the flat has only a 230 V (single-phase) supply, the maximum output is approx. 3.5 kW – not enough even for a normal shower. In that case a small 80–100 litre storage water heater is better: it heats up overnight and provides hot water during the day.

Scenario 2: House, 4 people, gas connection, renovation

This is the most common scenario in practice. The original inefficient combination boiler with a cylinder is replaced by a modern condensing boiler with an external cylinder. Recommendation: a Vaillant or Protherm condensing boiler with an output of 18–24 kW and an external 120–150 litre cylinder. Annual DHW costs fall by 30–40% compared with the original set-up. The investment in a new package (boiler + cylinder + installation) is around €2,500–4,000, and the payback period compared with an electric solution is 5–7 years.

Scenario 3: New build with photovoltaics

An increasingly common scenario in 2026. The house has a 6–10 kWp photovoltaic installation, and electricity is generated free (or almost free) during the day. In that case an electric storage water heater (80–200 litres) makes sense as a so-called smart cylinder, heated exclusively by surplus photovoltaic electricity via a smart controller (e.g. Eastron, ABB, Shelly or dedicated PV controllers). The cylinder acts as a thermal store for the surplus electricity, and DHW running costs are practically zero during the sunny months.

Scenario 4: Older house, poor thermal insulation, higher DHW demand

An older house with poor insulation, a family of five, two bathrooms. The clear choice here is a powerful condensing boiler (24–28 kW) + 150–200 litre cylinder. Instantaneous heating would not cope with two bathrooms in use in parallel without a marked drop in temperature, whereas a 200-litre cylinder covers the morning rush without difficulty. For the future, being able to connect solar collectors to the cylinder via a second coil is an advantage.

Frequently asked questions (FAQ)

Is an electric instantaneous water heater cheaper than an electric hot water cylinder?

In terms of upfront cost, yes – you can buy an electric instantaneous water heater for €60 to €400, whereas a 100–150 litre cylinder costs €200–600. In terms of running costs they are much the same – the instantaneous heater has no standby heat losses, but the cylinder can be heated on a cheap night tariff. The outcome depends on your particular circumstances and electricity tariff. For a large family a cylinder is generally more convenient, though not necessarily cheaper.

Can I use an instantaneous water heater as the only source of hot water in a house?

Yes, if the output is sufficient. A gas condensing combi boiler with a DHW output of 25–28 kW will cope with a shower and the kitchen at the same time for a small family. With a large family or two bathrooms in use in parallel, however, the temperature may drop – in which case stored hot water is the safer bet. An electric instantaneous water heater with enough output for a whole house (at least 21–27 kW) requires a three-phase supply and heavy-duty circuit breakers.

What is the minimum cylinder temperature to prevent Legionnaires' disease?

According to the technical standard STN EN 806 and WHO recommendations, a hot water cylinder should be kept permanently at a temperature of at least 60 °C. The bacterium Legionella pneumophila survives at temperatures of 20–45 °C and dies within 2 minutes at temperatures above 60 °C, or practically instantly at 70 °C. That is why most modern boiler controls include an anti-legionella programme (heating the cylinder to 70 °C once a week). A properly working NTC temperature sensor is essential for this.

Is it worth replacing an old electric cylinder with a gas combi boiler?

If you have a gas connection, almost always yes. A modern condensing gas boiler with an external hot water cylinder is 3–4 times cheaper to run than an electric cylinder. The investment (boiler + cylinder + installation of the gas pipe and flue) ranges from €2,000 to €4,500 depending on the scope of the work. With a difference in running costs of €400–600 a year, the payback period is 4–8 years, which is very attractive financially given a boiler service life of 20 years.

What hot water cylinder do I need for a family of four?

For four people with one bathroom and ordinary showers, a 120–150 litre cylinder is enough. If you have two bathrooms, or the family likes long showers and baths, go for 150–200 litres. A rule of thumb is 30–40 litres of cylinder capacity per person, and the cylinder should be connected to a boiler with sufficient output (at least 18 kW for heating the cylinder).

What is a condensing combi boiler and how does it differ from a boiler with an external cylinder?

A condensing combi boiler (e.g. Protherm Gepard Condens 18/25 MKV) has instantaneous DHW heating built into the body of the boiler itself – it does not need an external cylinder. With a boiler and external cylinder (e.g. Vaillant ecoTEC pro + VIH R 120 B), the boiler heats the cylinder, from which DHW is then drawn. A combi boiler is more compact and cheaper to install, but where DHW consumption is higher (larger families, several outlets in use in parallel), the cylinder solution is more comfortable and more reliable.

Conclusion: how to decide?

There is no universal answer to the question "instantaneous or stored hot water?". There is only the right answer for your particular situation. Let us sum up the key decision criteria:

• Small household (1–2 people), flat with no gas, little space: An electric instantaneous water heater with an output of 18–21 kW (three-phase). Or a small 50–80 l cylinder on a night tariff.
• Medium-sized family (3–4 people), house with gas: Condensing boiler + 120–150 litre cylinder. The best balance of price and comfort in 2026.
• Large family (5+ people), two bathrooms: Condensing boiler + 150–200 litre cylinder, possibly pre-equipped for solar collectors.
• Flat with gas, limited space, one bathroom: A condensing combi boiler with instantaneous DHW – a neat, compact solution.
• New build with photovoltaics: An electric storage water heater connected to a smart PV surplus controller, possibly combined with an air-to-water heat pump.
• Cottage, garden, occasional use: A small 6–9 kW electric instantaneous water heater, single-phase.

If you are not sure, discuss your choice with a professional installer who knows your house, your gas and electricity supply and your habits. A correctly chosen and professionally installed DHW system will save you hundreds of euros a year and give you worry-free comfort for 15–20 years.

Do you have a question on this topic?

Can't decide, or dealing with a specific situation in your home? Write to us - we will be happy to advise.