How to choose an electric water heater: storage vs. tankless

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Storage or Instantaneous Water Heater? A Practical Guide to Choosing

When you decide to heat hot water using electricity, sooner or later you will face a basic question: storage or instantaneous water heater? Both technologies have their place in the market and both work reliably – as long as they are chosen correctly for a specific situation. Problems arise when someone buys an instantaneous water heater with too low a power, or conversely, installs a fifty-liter storage tank in a small apartment where a compact instantaneous unit would have been sufficient. From experience, I know that people make such decisions quite often, and then they wonder why they get lukewarm water or why their monthly electricity bill has risen dramatically.

In this article, we will go through both technologies in depth – how they work, where they have strong points, where they fall short, and on what criteria to make a decision. I will also add specific numbers, dimensions, examples from typical orders and direct comparisons that will help you make an informed decision even before you reach for your wallet.

How a storage electric water heater works

A storage water heater (commonly called a "boiler") is essentially a thermally insulated tank made of stainless steel or steel with an enamel interior, in which an electric heating element is built in – usually a rod-type resistance heater with a power of 1.5 to 3 kW. The water is heated to a set temperature (typically 60–75 °C) and remains ready in the tank until you need it.

The principle is simple: cold water enters through a lower inlet, hot water is drawn off through a pipe at the top. A thermostat monitors the temperature in the tank and, when it drops below the set value (for example, after a large draw), automatically turns on the heating element to reheat the water. The whole cycle takes – depending on the tank volume and the power of the heating element – from 30 minutes to several hours.

An important part of the tank is the magnesium anode – a metal rod that sacrifices itself to protect the interior of the tank from corrosion. This anode is gradually consumed and must be replaced after several years. Neglecting to replace the anode is one of the most common causes of premature failure of the tank. More about preventive maintenance can be found in the article Maintenance and cleaning of electric water heaters: how to extend their lifespan.

Types of storage water heaters according to mounting position

Tanks are divided into vertical (standing or wall-mounted) and horizontal. Vertical wall-mounted models are most common for small volumes (from 10 to 80 liters) in apartment buildings – they are hung above the toilet, under the sink, or in the bathroom. Horizontal tanks are mounted horizontally, which is suitable, for example, in a ceiling cavity or where there is not enough height. Note: with a storage water heater, we distinguish between upper and lower water supply connections, which relates to where the drain valve and the safety valve are mounted. For example, the electric storage water heater BD 2.0 kW 30 l upper is designed for upper water connection, which must be respected when planning the piping.

How an instantaneous electric water heater works

An instantaneous water heater does not work with a water reservoir – it heats the water immediately, at the moment it starts to flow through it. Inside the device is a heating coil or resistance block with a power of usually 3.5 to 27 kW (most commonly 3.5 – 21 kW for households). When you turn on the tap, the water flow activates a switch (hydraulic or electronic), which turns on the heating element. The water flows around (or through) the heating element and at the outlet you get hot water – without waiting, without a reserve, without standing losses.

The key parameter of a flow heater is power – it determines what water temperature you will get at a given flow rate. Basic physical relationship: the higher the power and the lower the flow, the warmer the water. For regular hand washing (flow 4–5 l/min) 3.5 to 4.5 kW is sufficient. For a shower (6–8 l/min) you will need at least 12 kW, ideally more. For a bathtub or multiple outlets simultaneously, we reach 18–21 kW and more. A detailed calculation of power for specific situations can be found in the article What power of flow water heater do I need for my apartment.

Hydraulic vs. electronic switching

Flow heaters differ in the way of switching. Cheaper models use a hydraulic switch – a simple membrane valve that opens at sufficient flow. It is reliable, but may fail at low water pressure. Electronic switching is more accurate, reacts even to low flow and allows power setting. The differences between these technologies are discussed in detail in the article Hydraulic vs. electronic switching of flow heater: what to choose.

In practice: flow heater MK1 3.5 kW with electronic switching is a typical representative for installation under a sink – single-phase, compact, reliable at low flows. Its "older brother" MK1 4.5 kW can handle a bit more, but it is still suitable for a single outlet. For a shower or the whole bathroom you need powerful models – for example 12 kW version with adjustable power or directly 21 kW model for demanding consumption or multiple outlets at once.

Pressurized vs. unpressurized operation – a crucial difference that should not be overlooked

Before choosing a specific model, it is important to understand the difference between pressurized and unpressurized operation. It is a technical detail that directly affects how and where you can connect the heater.

Unpressurized (local) operation means that the heater is not connected to the pressurized cold water supply – water flows through it freely, without back pressure. The faucet is special (with a hollow valve), installed directly on the heater's outlet. Unpressurized models are simpler, cheaper, but always supply only one outlet. Typical use: a countertop heater above the sink in the kitchen or a bathroom sink.

Pressurized operation means that the heater is connected to the standard pressurized water supply (water connection, pressure 2–6 bar) and supplies hot water to one or more faucets via standard fixtures. Most modern flow and storage heaters for apartments and houses operate in pressurized mode. Pressurized operation allows connection of multiple outlets at once. More about this division can be read in the article Pressurized vs. unpressurized operation of water heater: what is the difference.

Comparison of storage and flow heater: a clear table

Criterion	Storage heater	Flow heater
Installed power	1.5 – 3 kW	3.5 – 27 kW
Time to first hot water	30 min – 3 hours (depends on volume)	Immediately (2–3 seconds)
Dependence on grid voltage	230 V, single-phase	230 V (low power) / 400 V three-phase (from ~12 kW)
Electrical installation requirements	Standard socket 16 A or fixed supply	Fixed supply, high-power models three-phase
Space requirement	Larger (tank 30–200 l)	Compact (a few kg, book size)
Standing losses	Yes (heat losses from the tank, approx. 0.5–2 kWh/day)	No (heats only when drawing)
Volume of hot water	Limited (tank size)	Unlimited (as long as cold water is flowing)
Temperature of output water	Stable, adjustable by thermostat	Depends on flow and power
Device price	Medium to high (depends on volume)	Lower (small models), medium (powerful)
Installation complexity	Medium (higher weight, safety valve)	Lower (compact, light)

When to choose a storage heater

A storage heater makes sense in situations where multiple outlets need hot water either simultaneously or in quick succession – and where the electrical connection does not have the capacity for a powerful flow heater.

Typical scenarios from practice:

Apartment without three-phase connection: An older apartment building has only single-phase electricity in the apartment, with protection or measurement at 40–50 A. In such a case, you have no technical possibility to connect an 18 or 21 kW flow heater. A 2 kW storage tank easily fits into the existing installation.
Bathroom and kitchen from one source: If you want to supply hot water to the bathroom and kitchen sink from one point, a storage tank with a volume of 80–120 liters is a standard solution.
Cottage or garden house: Hot water consumption is irregular, but sometimes you need a larger amount at once (washing dishes, shower). A 50–80 liter tank is ideal here.
Old family house with weak connection: If an old family house is being renovated without the possibility of immediately increasing the circuit breaker, the storage tank takes the load by slow heating during low consumption periods.
Night tariff (low tariff NT): A storage tank can be programmed to heat up during the night when the low tariff is in effect, significantly reducing the cost of water heating.

Approximate dimensions of storage tanks according to the number of people: for 1–2 people, 30–50 liters is sufficient; for 3–4 people, 80–120 liters; for 5 or more people, 150–200 liters. More detailed calculations can be found in the article What volume of a storage water heater do I need for my household?.

When to choose a flow water heater

A flow water heater is ideal wherever it is not possible or desired to sacrifice space for a tank, where there is a sufficient electrical connection, or where you need hot water immediately and continuously.

Typical situations from practice:

Renovation without space for a tank: In a small bathroom or WC with dimensions of 2 × 1.5 m, it simply is not possible to hang an 80-liter tank. A compact flow heater fits easily above the ceiling or behind a mirror.
Rented apartment or temporary accommodation: A flow heater is much easier to uninstall and move – no need to drain dozens of liters of water.
Garden cabin with one tap: A small 3.5 kW flow heater is sufficient for hand and dishwashing in the garden.
New construction with a three-phase connection and sufficient power: If you have a 3 × 25 A circuit breaker available, an 18–21 kW flow heater can handle a shower and sink simultaneously, without the need to plan the heating in advance.
Supplementary source to central heating: If the central hot water system is far from the bathroom and the water runs cold for a long time, a small flow heater installed directly under the sink elegantly solves this issue.

Important practical note: when choosing a flow heater, people often underestimate the requirements for the electrical installation. A model with a power of 12 kW requires a three-phase supply of 3 × 400 V and a circuit breaker of at least 3 × 20 A, with cabling of at least 2.5 mm² cross-section. A 21 kW model requires a circuit breaker of 3 × 32 A and cabling of at least 4 mm². All of this must be assessed and connected by a certified electrician. Details on installation can be found in the article Installation of an electric flow water heater step by step.

Energy efficiency and operating costs: the numbers that decide

Simplified: a flow heater is more efficient in the sense that it does not consume energy when you are not using water. A storage heater, on the other hand, keeps the water warm even during the night, even if you are away from home all day – and that takes some energy.

Concrete numbers: a modern storage tank with good thermal insulation (foam PUR jacket, thickness 25–35 mm) loses about 0.8–1.5 kWh per day when standing still, which at an electricity price of 0.20 €/kWh represents 58–110 € per year in just standing losses. Older storage tanks with thinner insulation can lose twice as much.

A flow heater does not have these losses – it consumes energy only during actual use. On the other hand, its instantaneous power is high (12–21 kW), which means a short but intense load on the electrical installation. For comparison: a 2 kW storage tank heats slowly and evenly – it is gentler on the fuse box and the metering device.

For households with an electricity tariff featuring a low night rate (dual-rate NT/VT), a storage tank with night heating can be more economically advantageous than a flow heater, despite the standing losses. In such cases, a specific calculation based on actual hot water consumption and the ratio of tariff prices is necessary.

Water pressure: what you need to know before buying

Flow heaters are sensitive to the pressure and flow of incoming cold water. Manufacturers specify the minimum pressure required for startup (usually 0.1–0.3 bar) and the recommended operating range (0.5–5 bar). If your household has low water pressure (for example, in a basement apartment or a cabin with a gravity-fed tank), the hydraulic switch of the flow heater may not start at all, or may switch irregularly. For such situations, models with electronic switching and a lower minimum pressure are suitable, or alternatively a storage heater, whose operation is independent of water pressure.

Storage heaters in pressure operation are equipped with a safety valve (usually at 8 bar), which protects the tank from excessive pressure. This valve must be connected to a drain – some water is released during heating (thermal expansion). If you do not see a drain pipe or a hardened limescale coating around the valve on the tank, something is wrong. More about typical faults can be found in the article Common faults of electric water heaters and how to fix them.

Installation requirements and what to consider when planning

A storage heater is relatively heavy – even a 30-liter model weighs around 40 kg when filled, and an 80-liter model over 90 kg. It must be anchored to a load-bearing wall, not to drywall without special anchors. At the same time, it requires a cold water supply, a hot water outlet, a drain for the safety valve, and an electrical supply (fixed, fused 10–16 A, not a plug with a pin – this is common but technically borderline for permanent appliances). The step-by-step installation process is described in the article Installation of a storage water heater: what you need to know before installation.

A flow heater is much lighter (3–8 kg), but its electrical requirements are crucial. Small models up to 4.5 kW (230 V) are connected to a fixed supply with a 20–25 A circuit breaker, while larger models from 12 kW require a three-phase connection. If three-phase power does not lead to the bathroom, its installation can be a more expensive investment than the tank itself. This is, by the way, a very common situation in renovations of older apartments – the electrical distribution board is designed for a different type of load, and the flow heater simply does not fit into it without major modifications.

Special situations: combined solutions and less common applications

In practice, I also encounter combinations of both technologies. For example, in a house with a central gas boiler or heat pump, no electric heater is needed for regular operation – but a small storage tank or flow heater serves as a backup or supplement for a remote bathroom, where hot water from the central source arrives slowly.

Another common scenario: a cabin with photovoltaic panels. Here, a storage tank makes perfect sense – you can set the control to heat only when the solar panels produce enough energy (so-called surplus dumping). The storage tank serves as a thermal accumulator. A flow heater is not suitable for this task, as the immediate energy demand is too high and unstable.

In commercial premises (cafés, restaurants, hair salons), medium-power flow heaters (12–21 kW) are used as a backup or as the main source for the sink, where hot water consumption is moderate but must be immediately available. Here you will also appreciate the optional power – for example, the ability to switch from 12 to 6 kW in summer, when cold water is warmer and less heating is needed.

How to decide step by step

Based on hundreds of customer orders and recurring questions, I have come up with this practical decision-making process:

Check the electrical connection: Single-phase (230 V, 40 A)? A storage tank or a small flow heater up to 4.5 kW. Three-phase (400 V, 3 × 20 A or more)? You can consider a high-performance flow heater.
Measure the available space: Is there room for a storage tank (wall height, load capacity)? If not, a flow heater is the clear choice.
Determine the water pressure: Below 0.3 bar? A flow heater may have problems with switching.
Estimate daily consumption: Only one wash point, 1–2 people? A small flow heater or a 30-liter storage tank will suffice. Shower + kitchen for 4 people? A storage tank of 80–100 liters or a high-performance flow heater.
Consider the tariff: Do you have a two-rate tariff and want to heat at night? A storage tank with a timer.
Compare investment and operating costs: A storage tank is sometimes cheaper to install (no need for new electrical installation) but more expensive to operate (standby losses). A flow heater is more expensive to install, cheaper to operate.

Most frequently asked questions (FAQ)

Can I connect a 30-liter storage heater to a regular socket?

Technically yes – most 2 kW storage heaters have a power cable with a plug. However, the standard STN 33 2130 recommends a fixed electrical supply for permanent appliances over 2 kW, not a socket. In practice, it is commonly done via a 16 A socket, but for safety and to prevent accidental unplugging, it is better to connect the storage heater to a fixed supply. Definitely do not use an extension cord or power strip – the heater must be directly connected to a wall socket or fixed supply.

Why is warm water coming out of my flow heater?

The most common cause is an undersized power. For example, if you have a 6 kW heater and you open the tap to full flow (8 l/min), it is physically impossible to heat the water sufficiently – the result is lukewarm water. The solution is either to reduce the flow (by adjusting the tap), or to replace the heater with a more powerful model. Another possible cause is a clogged filter at the inlet (limescale deposits limit flow and thus performance). Another possibility is a faulty power stage – for models with adjustable power, check that the switch is set correctly.

Is an 80-liter storage tank worth it for one person living alone?

For one person, 80 liters is oversized. A larger tank means more energy for heating and higher standby losses. For one person, the optimum is 30–50 liters. If you have an 80-liter tank inherited from the previous tenant, you can at least lower the set temperature to 55–60 °C (minimum to prevent legionella) and set the timer to heat only once a day – in the morning before morning hygiene.

Do I need to call an electrician to install a flow heater?

Definitely yes, and this applies even to small single-phase models up to 4.5 kW. All flow heaters are connected to a fixed supply (not a socket), which is a job that can only be performed by a qualified electrician according to Ordinance 508/2009 Coll. For three-phase models over 12 kW, this is even more important – incorrect wiring can be fatal. In addition, a maintenance report from the electrician is a condition for warranty recognition by most manufacturers.

How long does a storage heater last?

With regular maintenance (anode replacement every 2–4 years, descaling the heating element), 15–20 years is not uncommon. Without maintenance, a storage tank may fail after 5–8 years due to corrosion of the tank. The key is to monitor the condition of the magnesium anode – if you miss it, the tank itself will start to rust and no repair will save it. A detailed maintenance procedure can be found in the article Maintenance and cleaning of an electric water heater: how to extend its lifespan.

Is a flow heater suitable for heating a bathroom?

No – an electric flow heater is used exclusively for heating domestic water, not for heating a room. For heating a bathroom, electric panel radiators, infrared panels or underfloor heating are intended. These devices can be found in other categories of our range. At the same time: a flow heater is not a suitable replacement for a boiler with central heating – its power is used exclusively for flow water heating.

Conclusion: there is no universal answer, there is the right answer for your situation

After reading this article, it should be clear that the choice between a storage and a flow heater is not a matter of fashion trends or simple "this is better". It is a technical decision that depends on specific conditions: what is your electrical connection, what is your water pressure, how many people and where need hot water, what space do you have available, and how much do you want to invest in installation versus operating costs.

If you are still undecided, I recommend going through other articles in the Knowledge Center – especially Frequently asked questions about electric water heaters, where you will find answers to specific situations. And if you need advice on a specific model for your case, take a look directly at the products in the category electric water heaters – each model lists technical specifications from which you can directly assess whether it meets your conditions.

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Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.