How to choose an electric water heater: storage or instantaneous?

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Electric water heater: tank or flow type? Complete guide to choosing

This is probably one of the most common questions customers ask salespeople in the heating equipment industry. And honestly – there is no need to be surprised. At first glance, these are simple devices, but in practice, the right choice depends on a whole range of factors: apartment or house layout, number of residents, condition of the electrical installation, available space, and also on how and where hot water is actually used. The decision "tank or flow" has long-term consequences – a wrong choice will be reflected either in insufficient comfort or in unnecessarily high costs.

In this article, we will examine both systems in depth. Not just as marketing brochures describing pros and cons, but as we see it after dozens of real customer cases – with specific numbers, with typical situations from practice and with answers to the questions customers most often address when choosing.

Operating principle: how does it actually work?

Tank water heater

A tank water heater (in practice, you may also know it by the names boiler, storage heater or tank) works on the principle of heat accumulation. Inside the tank, there is an electric heating element (heating rod), which heats the entire volume of water to the set temperature – typically 60 to 75 °C. The thermostat monitors the water temperature and turns the heating on or off as needed. When you open the tap, hot water flows out of the tank and cold water from the water supply enters in its place.

The key feature of the tank: energy is consumed in advance, before you actually need the water. The tank thus "stores" the hot water and you just draw it off. The disadvantage is that when the supply is exhausted, you have to wait for further heating – so-called tank regeneration. The regeneration time depends on the power of the heating element and the volume of the tank.

Example from practice: a tank with a volume of 80 liters and a power of 2 kW takes about 3.5 to 4 hours to heat cold water (10 °C) to 65 °C. A 30-liter tank with the same power heats up in less than an hour and a half. These times are important when you consider whether the tank can "charge" during the night or during low electricity tariff periods.

Flow water heater

A flow water heater works completely differently – it heats the water instantly, directly during the flow. Cold water enters the heater body, passes around a powerful heating element and hot water flows out of the tap. No pre-storage, no waiting for heating. The water is practically available immediately.

A flow water heater has one fundamental technical consequence: in order to be able to heat the water during the short passage through the device, it requires much higher power than a tank. Tanks usually manage with 1.5 to 2 kW, flow heaters for normal use start at 3.5 kW and in practice, for a full shower you need at least 5–6 kW, and for a bath even 18–24 kW. This is one of the most important facts that determines whether a flow heater is even realistic in your property.

Overview of the main differences: tank vs. flow

Property	Tank water heater	Flow water heater
Power	1.5 – 3 kW	3.5 – 27 kW
Electrical installation requirements	Standard socket 230 V / 10 A	Separate circuit, for >3.5 kW 400V
Dependency on water pressure	Low (pressurized or non-pressurized)	Higher (flow sensor requires minimum pressure)
Installation space	Larger (depending on tank volume)	Small, compact
Temperature of output water	Constant, adjustable	Depends on flow and power
Unlimited hot water	No – limited by tank volume	Yes – as long as water is flowing, it is heated
Standby heat losses	Yes – the tank gradually cools down	No – no consumption without flow
Device cost	Medium to high	Lower for smaller powers
Installation costs	Medium	Lower (if the electrical installation is suitable)

When to choose a storage water heater?

A storage water heater is the right choice in many more situations than it may seem at first glance. In practice, we recommend it whenever at least one of the following conditions is met:

More than one point of use: If you want hot water in the bathroom and kitchen from one unit, a storage heater is almost always more efficient. A flow heater serving two locations either cannot keep up or requires an excessively high power consumption.
Household with 2 or more people: With higher hot water consumption – showering, dishwashing, possibly a bath – a storage heater with an appropriate capacity (80, 100 or 120 liters) will easily meet the needs of a family without any issues.
Older electrical installation: If your apartment does not have dedicated circuits for high power, a 2 kW storage heater can be easily plugged into a standard socket. A flow heater with a power of 5 kW or more would require reconstruction of the electrical installation.
Opportunity to use night/low electricity tariff: A storage heater can be programmed to heat only during the time of cheap electricity (HDO) and maintain temperature during the day. This results in significant savings in long-term operation.
Use with a bathtub: To fill a bathtub (140–200 liters of water), you need a storage heater with sufficient capacity. A flow heater with a standard power of 5–6 kW simply cannot do it – the water will be lukewarm.
Low water pressure in the building: A flow heater requires a minimum pressure (typically 0.5–1 bar) to function properly. In older buildings or at the end of a water supply network, a storage heater eliminates these problems.

Typical example: a family house with 3–4 people, where there is no central heating. A storage heater with a capacity of 100–150 liters and a power of 2 kW, heated mainly at night. In the morning, the family has enough hot water for showers, and during the day the storage heater is recharged. Monthly costs are predictable and relatively low. We have seen dozens of such installations and they have been working reliably for years.

A practical solution for a bathroom with a single point of use is, for example, the electric storage water heater BD 2.0 kW 30 l top-mounted – a compact storage heater with a power of 2 kW and a capacity of 30 liters, ideal for a separate bathroom or a small household with 1–2 people. Thermostatic switching ensures that the water is heated once and maintained at the set temperature without unnecessary overheating.

When to choose a flow water heater?

A flow heater has its irreplaceable place – but it is really a specific situation. The main reasons for choosing it are:

Limited space: A flow heater is compact. It can fit under a sink, behind a panel, or in a small bathroom without a cabinet. A 80-liter storage tank takes up significantly more space and not every bathroom can accommodate it.
Seasonal or occasional use: A cottage, a garden house, a weekend cabin – places where hot water is not needed all the time. A flow heater does not consume energy in standby mode. A storage heater would cool down and reheat in the meantime.
Low consumption – one location, one person: For a single-person household with a single point of use (only a shower, not a bath), a flow heater may be sufficient if the electrical installation allows the required power.
Supplementary heating for a remote bathroom: In a large house, where hot water from a central storage tank would have to travel a long pipe run, a flow heater at the point of use can elegantly solve the problem.
Instant heating without waiting: With a flow heater, you do not have to wait for the tank to heat up. You turn on the water and soon it is hot. This is an advantage that some customers truly appreciate.

Practical example: a customer renovated an old cottage where there was no hot water at all before. Minimal space in the bathroom, partially renovated electrical installation with one larger circuit breaker. We installed the flow heater MK1 3.5 kW with electronic switching and pressure operation – installation in half a day, without special construction work. Ideal solution for cottage needs (2 people, only a shower, occasional weekend visits).

For a situation where a higher power is available, but it is still a flow heater for one point of use (bathroom, shower cubicle), the flow heater MK1 4.5 kW comes into consideration – with the same compactness, it provides a higher thermal output – at the same water flow, it heats the water to a higher temperature, which is advantageous especially in winter, when cold water (6–8 °C) enters the system.

Important detail: pressure vs. non-pressure flow heater

This is a subtle detail that many customers overlook and later regret. Flow heaters are divided into pressure and non-pressure types – and this difference determines what you can connect behind them.

Pressure flow heater is a closed system – you can have a standard tap or shower head behind it. The water is under pressure, you can close or open the valve on any side. This type is suitable wherever you want full control of temperature and flow with a tap.

Non-pressure flow heater works differently – it has an open outlet nozzle (or a special rotating shower head without a shut-off valve) and water always flows freely from it, without pressure. The tap is before the heater – by controlling the flow of cold water, you regulate the heating. A shut-off valve must not be behind a non-pressure heater – there is a risk of the device bursting due to pressure.

Non-pressure heaters – for example, the PM 3.5 kW with hydraulic switching or the PM 4.5 kW – are most often installed as a replacement for an old electric storage tank with its own outlet nozzle, or in buildings where a pressure hot water system is not available (e.g., in older apartments, where hot water was originally only from a central source and now local heating is being addressed). Their advantage is that the switching is mechanical (hydraulic), without electronics – simpler and more durable in dusty or humid environments.

We will discuss this topic in more detail in a separate article "Pressure vs. non-pressure water heater: what is the difference and what to choose?" – we recommend reading it before making a final decision.

Electric installation: the most common barrier to selection

This is a topic that influences the decision more than many customers realize. In practice, the electrical connection is the number one limiting factor – especially for flow water heaters.

A storage water heater with a power of 2 kW works without problems on a standard 230 V socket with a 10 A fuse. Ideally, it should be on its own circuit (not shared with other appliances), but it usually works fine even on a shared circuit.

A flow water heater with a power of 3.5 kW requires its own circuit with a 16 A fuse at 230 V. At 4.5 kW, we are talking about a 20 A fuse at 230 V – and in practice, we often encounter the fact that such a circuit is not even available in older apartment buildings. In most cases, for powers above 5 kW, we are talking about three-phase power supply at 400 V – and most apartments simply do not have this installed in the bathroom.

Therefore, we recommend the following procedure: always check the condition of the electrical installation with an electrician before selecting a specific device. The cost of a new circuit can be 150–400 €, which can significantly change the economic calculation in favor of the storage heater.

Detailed requirements for the electrical installation for flow water heaters can be found in the article "Installation of an electric flow water heater: procedure, connection and requirements for electrical installation".

Operating economy: what is cheaper to operate?

This is another area where customers often make mistakes in their thinking. They compare only the power of the devices, not the actual consumption for a specific usage scenario.

Storage water heater has heat losses – the tank radiates heat to the surroundings, even when you are not drawing water from it. Modern tanks with good polyurethane insulation have losses of around 0.5–1 kWh per 24 hours. That is an additional 15–30 kWh per month just to maintain the temperature. On the other hand, you can heat it during off-peak hours and maintain it during the day – or let it cool down partially during the day.

Flow water heater has zero standby losses – if no water is flowing, it consumes nothing. Every watt goes directly into heating the water being used, with an efficiency close to 100 %. The disadvantage is that you always heat at the higher (more expensive) rate – you cannot use the night rate, because you heat exactly when you are using the water.

In practice: for a household with higher hot water consumption (a family, daily showers, dishwashing), a storage heater with a night rate is more economically advantageous. For occasional seasonal use (cottage, garden house), a flow heater is cheaper – even if the cost per kWh is the same, the storage heater would lose heat in the meantime.

Note on the chart: the values are approximate and depend on actual consumption, the set temperature of the tank, the quality of insulation, and the number of people. The chart illustrates the basic logic – for a family, a storage heater with a night rate saves money, while for seasonal use, a flow heater clearly leads.

Tank capacity: how to calculate it correctly?

If you have decided on a storage heater, the next question is: what capacity? The most common customer mistake is choosing a tank that is too small, with the idea that "we can always reheat". In practice, it takes hours – not minutes – to heat a tank from cold water to operating temperature.

Approximate values for selecting the tank capacity:

30–50 liters: One person, or one point of use (just a sink / small shower). A 30-liter tank is, for example, suitable above a sink in a separate toilet or for a smaller bathroom with one person.
80–100 liters: 2–3 people, shower and sink. An 80-liter tank easily covers morning showers for two people without problems, provided there is good night heating.
120–150 liters: 3–4 people, or a household where a bathtub is regularly used. A 120-liter tank can fill a standard bathtub (140 l) with heated water at a mixed temperature.
150–200 liters: 5 or more people, larger house, higher comfort. For a family with three children and a daily use bathroom, a 150–200 liter tank is a reasonable choice.

The calculation is simplified as follows: a shower lasts on average 5–8 minutes, at a flow rate of 8–10 l/min, which is 40–80 liters of hot water (mixed). A bathtub holds 130–200 liters. Dishwashing: 10–20 liters. Calculate your daily needs and a tank with a slight reserve will meet your needs.

Hydraulic vs. electronic switching: the difference in practice

Flow water heaters are also divided according to the method of switching the heating – into hydraulic and electronic. This topic is discussed in more detail in the article "Hydraulic vs. electronic switching of flow water heaters: what it means and which is better?", but in short:

Hydraulic switching uses a mechanical valve driven by the pressure of the flowing water. When water starts to flow with sufficient flow rate, the membrane of the valve is compressed and the heating is activated. It does not require electronics, is more resistant to power outages, and is more reliable in simpler conditions. You typically find it on non-pressurized heaters.

Electronic switching uses a flow sensor (flow meter) and an electronic control unit. It is more precise, reacts faster, allows for smoother temperature regulation, and is suitable for pressurized systems. The disadvantage is the higher price and greater sensitivity to water pressure fluctuations.

Installation and mounting: what you need to know in advance

A storage water heater is mounted on the wall and connected to the water supply and an electrical socket (or fixed connection). The installation is not technically complex, but the tank must be securely fastened to a load-bearing wall capable of supporting its full weight – a 80-liter tank weighs about 95 kg when full. Special mounting brackets and concrete anchors are used for installation.

Storage tanks are produced in two versions depending on the position of the heating element: top heating (heating element at the top, the tank is installed below the water connection point, hot water exits from the bottom) and bottom heating (heating element at the bottom, classic installation under the sink). More information can be found in the article "Installation of storage water heaters: top vs. bottom heating, installation and wiring."

A flow water heater is easier to install – it is light, compact, and connected using standard hoses. However, the electrical connection is critical – it must be on a properly dimensioned circuit. If you have an older house, it is almost always necessary to have an electrician verify the capacity of the fuses and wire cross-sections.

Lifespan and maintenance

A storage water heater can last 10–15 years with proper maintenance, and some high-quality units even longer. The key is regular descaling of the heating element (every 1–2 years in hard water) and checking the magnesium anode, which protects the tank from corrosion. The anode should be replaced depending on its condition – usually every 2–5 years. Neglecting these tasks will significantly shorten the tank's lifespan. More details can be found in the article "Maintenance and descaling of electric water heaters: how to extend their lifespan."

A flow water heater has fewer moving and wear-prone parts, but limescale can still accumulate on the heating coil. In hard water conditions, we recommend checking the heating element once a year. Replacing the heating coil is a routine service task that can restore the device's functionality at a lower cost than buying a new one.

Most frequently asked questions (FAQ)

Can a 3.5 kW flow heater be connected to a standard 230 V socket?

Technically, it is possible – the current draw at 3.5 kW / 230 V is about 15.2 A. A standard socket is protected by a 10 or 16 A fuse, so with a 16 A fuse, it will technically work, but the socket is not designed for continuous use at this level. The correct solution is a dedicated circuit with a 2.5 mm² cable cross-section, a 16 A circuit breaker, and a socket or fixed connection certified for this power. Never connect a flow heater via an extension cord or power strip.

What size of storage tank is sufficient for two people?

For two people who shower in the morning (each for about 7 minutes, 50–60 liters of hot water in total), an 80-liter tank with night heating is completely sufficient. If both shower one after the other and the tank does not have enough time to reheat, a 100-liter tank may be more comfortable. A 30-liter tank would not be sufficient for two people with morning showers – it would be empty after the first shower.

Is a flow heater suitable for showering in winter?

It depends on the power. In winter, the water entering the system is at a temperature of 6–8 °C (compared to 12–14 °C in summer). To heat it to a comfortable shower temperature of 38 °C at a flow rate of 6 l/min, you need at least 4.5–5 kW. A 3.5 kW heater will heat the water to a lower temperature in winter – it is sufficient in summer, but the water may feel lukewarm in winter. Therefore, it is important to consider the season and the temperature of the incoming water when selecting the power. More information can be found in the article "What power of flow water heater do I need: 3.5 kW, 4.5 kW or 5.5 kW?"

Storage tank or flow heater – which is cheaper to operate?

For a household that uses hot water throughout the year with more than two people, a storage tank using night-time electricity tariffs is cheaper – the difference can be 20–40 % of monthly costs. For seasonal, occasional, or single-use applications, a flow heater is more economical, as it does not have standby heat losses. The exact savings depend on the electricity price in your tariff and your actual daily hot water consumption.

Can a storage water heater be used with low water pressure?

Yes, storage heaters are much less sensitive to water pressure than flow heaters. Pressure tanks operate with normal water supply pressure (0.2–6 bar). Non-pressure tanks (with an open outlet nozzle) work without pressure – they fill by gravity. In contrast, a flow heater requires a minimum pressure to activate the switch (usually 0.3–1 bar) – otherwise, it will simply not start.

What to do if the storage tank cannot heat in time?

You have three options: let the tank heat during the day (not just at night), increase the tank's set temperature to maximum (75 °C – hot water will then be mixed with cold water, effectively "extending" the tank's capacity), or replace the tank with a larger capacity. Note that the tank temperature should be set to at least 65 °C+ at least once a week to prevent legionella. More detailed solutions for problems and insufficient heating can be found in the article "Common problems with electric water heaters and how to fix them."

Conclusion: how to make a decision in practice?

After reading this article, the criteria for selection should be much clearer. Let's summarize it into a simple decision key:

If you have more than one person in the household or more than one point of use → storage tank, volume according to the number of people.
If you have limited electrical installation without the possibility of installing high power → storage tank with 2 kW on a standard circuit.
If you have limited space and are only one person or a seasonal user → flow heater, power selection according to the season and flow rate.
If you want unlimited hot water without storage and the electrical installation allows it → flow heater with sufficient power.
If you need supplementary heating for a remote bathroom or kitchen → a small flow heater directly at the point of use.

Do not save on the selection and do not choose the device based on the first glance price – a cheaper product with insufficient power or the wrong type can lead to discomfort, higher costs, or the need for a complete reinstallation. If you are unsure, consult a specialist – a well-chosen device will reliably serve you for 10–15 years.

Do you have a question on this topic?

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