What power of a flow water heater do I need: 3.5 kW, 4.5 kW or 5.5 kW?

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What water heater power do you need: 3.5 kW, 4.5 kW or 5.5 kW?

When a customer chooses a water heater, the first question that usually arises is the power. On the shelf – or in the online shop – models with 3.5 kW, 4.5 kW and sometimes even 5.5 kW stand next to each other, and at first glance they look exactly the same. The price differs by just a few euros, the dimensions are also not dramatically different, and yet the choice of power determines whether you will be satisfied every morning while washing your hands, or whether you will stand under the shower and wait for the running water to stop being warm.

This article will step by step explain the physics behind the calculation, show concrete numbers for common practical situations, help you navigate the limitations of your home's electrical installation, and finally tell you which model from the available range makes sense for your specific use case.

How a flow water heater actually works and why power matters

A flow water heater does not work like a tank – it does not preheat a large volume of water and wait for you to take it. Instead, it heats the water directly on the fly, in the moment when you open the tap. This means that the power of the heater directly determines how much water per minute it can heat by the required number of degrees. Nothing more, nothing less.

The formula is simple, but the consequences are far-reaching:

Power (kW) = flow (l/min) × temperature difference (°C) × 0.07

The constant 0.07 comes from the specific heat capacity of water (4 186 J/kg·K) converted into practical units. If you want to know how many liters per minute you can heat by 30 °C at a power of 4.5 kW:

Flow = 4.5 / (30 × 0.07) = 4.5 / 2.1 ≈ 2.14 l/min

This is an important basis – we will return to it in each scenario below.

What affects the actual temperature difference – i.e., what to plug into the formula

The key point that most customers underestimate: the inlet water temperature is not the same all year round. In summer, water comes into the pipe at a temperature of 15–18 °C, in the middle of winter it can drop to 6–8 °C, especially if you live in a house with an older piping system or a longer connection. On the outlet side, you want warm water – for hand washing, 35–38 °C is sufficient, for a shower, a comfortable temperature is 38–42 °C.

This means that the temperature difference (ΔT) actually varies in practice as follows:

Summer: ΔT ≈ 20–25 °C (inlet 16 °C, outlet 38 °C)
Transition period (spring/autumn): ΔT ≈ 28–32 °C
Winter: ΔT ≈ 33–38 °C (inlet 7 °C, outlet 42 °C)

You must dimension the flow heater for the worst case – i.e., for winter operation with the coldest inlet water. If you don't do this, you will get a lukewarm shower in January, because the heater simply cannot keep up.

Three power classes – what they can realistically handle

3.5 kW – sink, kitchen sink, one draw point

A 3.5 kW heater is the weakest of the commonly available electric flow water heaters. Its capabilities are as follows:

At ΔT = 25 °C (summer): flow ≈ 2.0 l/min
At ΔT = 30 °C (transition period): flow ≈ 1.65 l/min
At ΔT = 35 °C (winter): flow ≈ 1.4 l/min

What does this mean in practice? A flow of 1.4–2.0 l/min is sufficient for comfortable hand washing at a sink. A standard faucet at a sink is set to a flow of 4–6 l/min, but you will only open the tap to a third to half – and that is completely sufficient to wash your hands with warm water. For dishes in a kitchen sink, it is also acceptable – the water flows slowly, but it is warm.

Where a 3.5 kW heater cannot be used: for showering. Not even in summer. A flow of 2 l/min is such a thin stream of water that most people would describe it as "dripping, but not showering". A standard shower head requires at least 5–7 l/min for acceptable comfort, which is three times what a 3.5 kW heater can achieve at a cold network temperature in winter.

Typical use of a 3.5 kW heater: a bathroom with a sink (WC without access to a central hot water supply), a kitchen sink in a shop or workshop, a sink in a garage, a cabin with a single draw point. The model electric flow water heater MK1 3.5 kW is a classic representative of this category in a pressure version with electronic switching, which allows for smooth switching without shocks.

Advantage of 3.5 kW: a single-phase connection of 16 A is sufficient (or even 10 A depending on the specific model), installation is simple, and the power consumption is low – it places the lowest demands on the electrical connection of all three power classes.

4.5 kW – warm water for a shower (with limitations) or two draw points

A power of 4.5 kW is the most popular compromise. Here are the numbers:

At ΔT = 25 °C (summer): flow ≈ 2.57 l/min
At ΔT = 30 °C (transition period): flow ≈ 2.14 l/min
At ΔT = 35 °C (winter): flow ≈ 1.93 l/min

It is still not 6–8 l/min, which most people imagine when they think of a "normal shower". But there is one trick that plumbers commonly use in practice: a water-saving shower head with a flow of 4–5 l/min can create an acceptable shower with a 4.5 kW heater during the summer and transition period. In winter, when the water from the network is really cold, the flow must be reduced even more, or the temperature of the outlet water must be increased, which compensates for the lower flow.

From practice: customers who install a 4.5 kW heater with a water-saving head and learn to slightly restrict the flow with the tap are usually satisfied in a bathroom with a shower for one person. When two people want to take turns quickly in the morning and each wants to stand under a strong shower for 10 minutes – then 4.5 kW is not enough. For such a scenario, a storage water heater is a much better solution (you will read about it in the article How to choose an electric water heater: storage vs. flow-through).

Electric flow-through water heater MK1 4.5 kW with electronic switching and pressure operation is ideal for bathrooms with a full cold water supply under pressure and where simultaneous water draw from multiple locations is not expected.

5.5 kW – more substantial shower, more options

At a power of 5.5 kW, the numbers start to approach what most people consider a normal shower:

At ΔT = 25 °C (summer): flow ≈ 3.14 l/min
At ΔT = 30 °C (transition period): flow ≈ 2.62 l/min
At ΔT = 35 °C (winter): flow ≈ 2.25 l/min

With 5.5 kW and a 5 l/min water-saving head in summer, you get a flow that allows for a real comfortable shower. In winter, a water-saving head and patience are still necessary. Compared to 4.5 kW, this is a noticeable improvement, but at the cost of higher demands on the electrical installation – more on that in the next section.

Electrical installation: the hidden limit few ask about before purchase

Selecting the power is not only about how much hot water you want – it also depends on what your electrical connection allows. In practice, this is the most common reason why customers return the product or have to reconsider their choice.

Basic rule: an electric flow-through water heater requires a dedicated single-phase line (230 V) with appropriate circuit protection. Never connect it to an extension cord or a socket shared with other appliances.

3.5 kW: current draw ≈ 15.2 A → 16 A circuit breaker, CYY-J 3×2.5 mm² cable
4.5 kW: current draw ≈ 19.6 A → 20 A (or 25 A) circuit breaker, CYY-J 3×4 mm² cable
5.5 kW: current draw ≈ 23.9 A → 25 A circuit breaker, CYY-J 3×4 mm² cable

If your apartment has an old electrical installation with aluminum conductors or 1.5 mm² cross-sections, you cannot install even a 3.5 kW heater without upgrading the electrical system. In practice, we often encounter situations where a customer buys a 4.5 kW heater, an electrician arrives and finds that the existing line has a 2.5 mm² cross-section connected to a 10 A circuit breaker – and the entire line has to be replaced. This is an additional cost that should be considered in advance.

A more detailed description of electrical requirements can be found in the article Installation of an electric flow-through water heater: procedure, connection and electrical installation requirements in this Knowledge Center.

Pressure vs. non-pressure heater – how power interacts with the type of operation

When choosing power, we must not forget the type of operation, as it affects where the power can realistically be used. This topic is discussed in detail in the article Pressure vs. non-pressure water heater: what is the difference and which one to choose?, but briefly:

Pressure heater is connected directly to the water supply under pressure and delivers water through a standard faucet. Suitable for installations where there is an existing cold water supply and where you want to use standard faucets. Models such as MK1 4.5 kW are exactly pressure type.

Non-pressure heater operates without back pressure, water flows freely. It requires a special faucet with an open outlet (no shut-off on the hot side). It is easier to install because it does not require an expansion tank or safety valve. Models PM 3.5 kW and PM 4.5 kW are typical non-pressure heaters with hydraulic switching.

Hydraulic vs. electronic switching also affects the minimum flow required to activate the heater. Hydraulic switching usually requires a higher minimum flow (≈ 2 l/min), while electronic switching reacts more sensitively (from ≈ 0.5–1 l/min). More on this in the article Hydraulic vs. electronic switching of flow-through water heaters: what it means and which is better?

Specific scenarios from practice – which power for which situation

Scenario 1: Toilet in an old apartment building without hot water supply

Panel building, the toilet is separated from the bathroom, there is no hot water supply to the sink in the toilet. You need hot water only for hand washing. Flow rate of the faucet during regular hand washing: 2–3 l/min, desired output temperature: 35–37 °C.

Conclusion: 3.5 kW is completely sufficient. Do not invest in 4.5 kW – you would unnecessarily pay for power you do not use and would also have to deal with a stronger electrical installation.

Scenario 2: Small bathroom with a sink and a shower, apartment without central hot water supply

This is a typical situation in a cottage or a small apartment, where the customer buys a flow heater instead of a storage heater to save space. The shower is used once a day, one person at a time, the shower head is water-saving (flow ≈ 4–5 l/min).

Conclusion: 4.5 kW is a realistic choice for the transitional period and summer, in winter the flow or temperature is reduced by 2–3 °C. Customers who take this limitation into account are satisfied. Those who do not want to make compromises should consider a storage heater (see electric storage heater BD 2.0 kW 30 l) or a 5.5 kW flow heater.

Scenario 3: Kitchen sink in a restaurant or workshop

Work-related hand washing, possibly quick filling of buckets – the incoming water can be extremely cold (workshop without a heated area), but the flow is low and the output temperature of 35–38 °C is sufficient.

Conclusion: 3.5 kW is sufficient for one point of use, 4.5 kW for two parallel points (but not simultaneously!) If you need to quickly heat a large volume of water, a flow heater is not the right choice – in such cases a storage heater with a larger capacity would be better.

Scenario 4: Cottage in the mountains, used only on weekends

Cold water at the cottage from a source with a temperature of 8–10 °C even in summer (mountain spring). ΔT to reach 40 °C at the shower is therefore 30–32 °C. The shower lasts 5–10 minutes.

Conclusion: 4.5 kW is on the edge, 5.5 kW is a better choice. If the electrical connection at the cottage cannot handle a 25 A circuit breaker, a storage solution should be considered, where the water is preheated overnight at a lower power.

Scenario 5: Garage, workshop – hand washing during mechanical work

One sink, need for hot water several times a day, temperature requirements are lower (30–35 °C is sufficient). The electrical connection in the garage is limited to 16 A.

Conclusion: 3.5 kW is ideal. Nothing more is needed and nothing more can be handled by the circuit breaker anyway.

What impact does the choice of power have on operating costs and consumption

A flow heater consumes electricity only during water withdrawal – that is, only when hot water is flowing. Unlike a storage heater, it has no heat losses when standing. This is a big advantage for customers who need hot water only occasionally.

Let's compare the costs per month under the assumption that you take a shower once a day, 7 minutes, with a flow of 3 l/min and an output temperature of 40 °C (ΔT = 30 °C):

Energy consumption for a shower = 0.07 × 3 l/min × 30 °C × 7 min = 44.1 Wh ≈ 0.044 kWh
Per month (30 days): 30 × 0.044 = 1.32 kWh
At an electricity price of 0.20 €/kWh: 1.32 × 0.20 = 0.26 € per month

These are negligible costs compared to the heat losses of a storage heater. Practical conclusion: choosing between 3.5 kW and 5.5 kW does not affect monthly costs as much as many people think – the difference is only in how much water you can heat in the same time and how much energy you consume. A longer shower with 3.5 kW consumes the same as a shorter shower with 5.5 kW, if the same amount of water is heated.

Most common mistakes when choosing power

1. "I'll take a higher power just in case there is some reserve." – This makes sense only if the electrical installation can handle it and you will actually use the higher flow. An unnecessarily powerful heater does not break, but it does place higher demands on the circuit breaker and wiring.

2. "I looked at the power, but forgot about the temperature of the incoming water." – A classic mistake. The customer tries the heater in September and is satisfied. In December, the water barely flows and is only lukewarm. The heater must be sized for winter operation.

3. "I will install it in the same outlet where I have my washing machine." – Not acceptable. A flow heater must have its own dedicated line from the distribution board.

4. "I want a heater for a bathtub." – A bathtub (150–200 liters) requires either a large storage heater or a three-phase flow heater with a power of 18–27 kW. Single-phase versions up to 5.5 kW are not sufficient for a bathtub in any configuration.

5. "I chose a pressure model, but installed a standard faucet without an expansion tank." – Without an expansion tank and a safety valve, overpressure can occur. A safety component is not optional, it is mandatory.

Summary table: power, flow and recommendation

Power	Flow (summer, ΔT=25°C)	Flow (winter, ΔT=35°C)	Circuit breaker / cable	Recommended use
3.5 kW	≈ 2.0 l/min	≈ 1.4 l/min	16 A / 3×2.5 mm²	Basin, sink, WC, garage, workshop
4.5 kW	≈ 2.6 l/min	≈ 1.9 l/min	20–25 A / 3×4 mm²	Shower (with water-saving head), bathroom, cottage (summer/spring)
5.5 kW	≈ 3.1 l/min	≈ 2.2 l/min	25 A / 3×4 mm²	Shower (with water-saving head), cottage with cold water, year-round

When not to choose a flow water heater

A flow water heater is a great solution for specific situations, but it is not a universal answer. There are cases where you should choose a different solution:

Large family with morning peak usage: A storage water heater (e.g., storage heater BD 2.0 kW 30 l) heats 30 liters of water overnight and is ready for the whole family in the morning. A flow heater up to 5.5 kW cannot fill a bathtub or serve multiple people at the same time.
Bathtub as the primary way of bathing: As we have mentioned – a flow heater up to 5.5 kW cannot handle a bathtub.
Electrical supply limited to 6 A or 10 A: Some older buildings, garden cottages, kiosks – there is simply not enough power available for a flow heater.
Hard water with high calcium content: Flow heaters have a small heat exchanger volume and quickly clog with deposits. A storage heater is easier to clean and maintain. More information in the article Maintenance and descaling of electric water heaters: how to extend their lifespan.

Frequently asked questions (FAQ)

Can I connect a 4.5 kW heater to a 16 A circuit breaker that I have in my distribution board?

Technically, a 16 A circuit breaker should handle 4.5 kW (at 230 V, the current is ≈ 19.6 A), but it is on the edge and the circuit breaker may repeatedly trip during prolonged use. The correct procedure is to use a 20 A or 25 A circuit breaker and 4 mm² wires. The installation must be assessed and carried out by a qualified electrician.

Why does my 4.5 kW heater give only lukewarm water in January, even though it worked well in September?

Exactly for the reason we described above: the inlet water temperature dropped from 16–18 °C to 7–9 °C. The temperature difference the heater must overcome increased by 10 °C, so with the same power, it heats less water. Solution: reduce the flow from the tap, or set a lower output temperature and stay under the shower a bit longer. If this is not satisfactory, you need to increase the power to 5.5 kW or switch to a storage heater.

Is a non-pressurized heater PM less powerful than a pressurized heater MK1 with the same kilowatts?

No, the power in kW is the same. The difference is in the connection method, not in the power. A non-pressurized PM can be installed more easily (without an expansion tank), but it requires a special open faucet. A pressurized MK1 works with standard valves, but installation is a bit more complex. The power – and thus the flow of heated water – is identical for the same kW.

Can I connect two flow heaters in parallel to get a higher flow?

Theoretically yes, but it is a rare and complicated solution. Each heater needs its own line, its own protection, and you must solve hydraulic balancing. In practice, it is much simpler and cheaper to buy a more powerful three-phase heater (11–27 kW), which elegantly solves the problem. Connecting two single-phase heaters in parallel is not recommended for normal households.

Does water pressure in the network affect the performance of the heater?

Water pressure does not directly affect the electrical power of the heater (which depends only on the electrical connection). But it does affect the flow – at low pressure in the network (under 0.5 bar), a pressurized heater may not even start, because it cannot overcome the minimum pressure drop required to activate the switch. At very low water pressure, a non-pressurized heater with hydraulic switching (PM) is a more reliable choice, because its minimum activation flow is lower and does not depend on back pressure.

Do I have to change the entire electrical installation in my apartment after buying a more powerful heater?

It depends on the condition of the existing installation. If you have a modern apartment distribution board with free spaces and wires of the appropriate cross-section, it is enough to add one line from the distribution board. If the distribution board is full or the wires are undersized, more work is needed. Always have the installation checked and carried out by an electrician with the appropriate qualification according to Ordinance No. 508/2009 Coll.

Conclusion: the right choice is not about the bigger, the better

Selecting the power of a flow water heater is a compromise between your needs, the limitations of the electrical installation, and the real physical possibilities of the technology. It is not true that 5.5 kW is always better than 3.5 kW – it is true that each power has its place and its application, where it makes the most sense.

If you need hot water only for a basin or sink: 3.5 kW is the right and economical choice. If you want a heater for showering and can work with a water-saving head: 4.5 kW is a reasonable compromise. If you have cold water from a source or want maximum comfort without compromises within the single-phase power: 5.5 kW is what you are looking for – but check your electrical installation in advance.

And if you are still unsure whether a flow heater is the right path for you, read the comparison article How to choose an electric water heater: storage or flow? – it will help you make an informed decision before you invest in a purchase.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.