What capacity of a flow water heater do I need for my apartment
Why the power of a flow water heater is a key decision, not a detail
When a customer comes into the store and says "I want a flow water heater for the bathroom", the first question I ask them is not "what color do you want?" or "what is your budget?". The first question is: how many kilowatts does your electrical connection allow and for what exactly do you need the heater? Because it is precisely here that most people make a mistake – they buy either too weak a device and then complain about cold water, or too powerful and find out that they cannot connect it because the home electrical installation simply cannot handle it.
A flow water heater is a device that heats water instantly – at the moment you open the tap. It has no tank in which it would store hot water for later. All the energy it consumes goes directly and immediately into heating the flowing water. This leads to one essential point: the power must be sufficiently high to be able to heat enough water to the desired temperature at a real flow rate. And this is exactly the physical relationship we will now examine in detail.
If you are still deciding whether you want a flow or a storage water heater, I recommend first reading the article How to choose an electric water heater: storage vs. flow in our Knowledge Center. Here we will focus exclusively on flow water heaters and the calculation of the correct power.
Basic physics: how power relates to temperature and flow
You don't have to be a physicist, but you need to understand the basic relationship. The power of the heater determines by how many degrees it can warm the water flowing at a given flow rate. The relationship looks like this:
P [kW] = Q [l/min] × ΔT [°C] × 0.07
where P is the power in kilowatts, Q is the water flow in liters per minute and ΔT is the desired temperature difference between the cold incoming water and the hot outgoing water. The constant 0.07 contains the specific heat of water and the unit conversion.
Example: If you want to have water at 38 °C in the bath and the cold water comes from the network at 10 °C (a typical winter value), the temperature difference is 28 °C. If you want a flow rate of 8 liters per minute (a realistic comfortable shower flow rate), you need: 8 × 28 × 0.07 = 15.68 kW. This is the power without which you simply cannot comfortably shower in winter.
From the graph it is clear what winter does to the requirements for power. In summer, when the cold water is 18–20 °C, you need significantly less for a comfortable shower. But in January and February, when the water from the network comes at 6–10 °C, the situation is completely different. And it is precisely this winter maximum that you need to dimension the heater for – not for the summer optimum.
What is the temperature of the incoming water and why it depends on the season
The temperature of the water in the water pipe is not constant. In Slovak conditions, it moves approximately like this:
- June – August (summer): 16–22 °C, in some areas even more
- May, September – October (transition periods): 12–16 °C
- November – April (winter and pre-season): 5–12 °C, in extreme frost even less
For the calculation of the power of a flow water heater, always use the winter value of 8–10 °C as the input. If you dimension for summer, you will complain all winter that the water is not warm enough. This is the number one mistake I see in DIY calculations.
Flow rate: how many liters per minute do you actually need
The second key parameter is the flow rate – how many liters of hot water you need per minute at the point of use. Here are realistic values for typical points of use:
- Sink (hands, face washing): 3–5 l/min – comfortable, hygienic minimum
- Shower – saving head: 5–7 l/min
- Shower – standard head: 7–10 l/min
- Shower – rain head or wellness: 12–20 l/min
- Bathtub – filling: 12–18 l/min (depends on the filling speed)
- Kitchen sink – normal dishwashing: 4–6 l/min
Important: a flow water heater heats only the water that passes through it at that moment. You cannot open the tap fully and expect the heater to maintain the desired temperature. Flow rate and temperature are connected in a container – if you increase the flow rate, the temperature of the outgoing water will drop. Therefore, most flow water heaters in practice work with a slightly lower flow rate than you would expect.
Typical scenarios in practice: how many kW for which case
Scenario 1: Small sink in the bathroom or toilet
The simplest case. You need hot water only for handwashing, brushing teeth, or shaving. Flow rate 3–4 l/min, temperature rise in winter approx. 28 °C. Calculation: 4 × 28 × 0,07 = 7,84 kW. In practice, 3,5 to 6 kW is sufficient, since you don't need full shower comfort at the sink. You can accept a slightly lower flow rate or even a slightly lower output temperature.
For this purpose, the electric flow water heater MK1 3,5 kW is suitable – a compact device with electronic switching, which can handle one sink without problems and does not require any special electrical wiring.
Scenario 2: Shower – one outlet, one person
This is the most common case. Flow rate 6–8 l/min, comfortable shower temperature 38 °C, cold inlet temperature 8 °C. ΔT = 30 °C. Calculation: 7 × 30 × 0,07 = 14,7 kW. The real minimum for a comfortable shower throughout the winter is 12–18 kW.
For this scenario, for example, the electric flow water heater with adjustable power 12 kW is ideal. Adjustable power is a big advantage here – in summer you can switch the heater to a lower power and save energy, in winter you can fully utilize all 12 kW.
Scenario 3: Shower + sink in an apartment, medium-sized bathroom
If you want to supply the shower and sink simultaneously with a heater (which is less common, but real in smaller apartments), or if you have a larger rain shower, you need higher power. We are talking about 18–21 kW here. At the same time, be careful – not every home electrical installation can handle this. Usually, a three-phase connection and appropriate circuit breakers are required.
In such a case, the electric flow water heater with adjustable power 21 kW comes into consideration – the most powerful heater in this category, capable of supplying multiple outlets or ensuring real comfort flow throughout the year.
Scenario 4: Kitchen sink – only for dishes and cooking
The situation in the kitchen is a bit simpler, because the temperatures are not so high and the flow is lower. Flow rate 4–5 l/min, temperature 40 °C, ΔT in winter 30 °C. Calculation: 5 × 30 × 0,07 = 10,5 kW. In practice, 6–12 kW will be sufficient for you here, because the consumption in the kitchen does not last longer than a few minutes and you can slightly reduce the flow.
For a kitchen, where you do not want to deal with three-phase power supply, the electric flow water heater MK1 4,5 kW may be suitable – a single-phase device that ensures hot water for handwashing and light dishwashing. For longer dishwashing in winter, the flow will be limited, but for normal kitchen use, it is a realistic and energy-saving choice.
Electrical installation: a limitation that is not talked about loudly enough
Calculating the correct power is one thing. But if the home electrical installation does not support such power, no calculation will help. This is a very common bottleneck in practice – the customer buys a powerful heater, and then finds out that the electrician refuses to connect it because the installation does not allow it.
Here is a basic overview according to phase and circuit breaker:
| Electrical supply | Circuit breaker [A] | Max. power [kW] | Typical use |
|---|---|---|---|
| 1-phase 230 V | 16 A | 3,5 kW | Sink, kitchen (limited) |
| 1-phase 230 V | 20 A | 4,5 kW | Sink, kitchen |
| 1-phase 230 V | 32 A | 7 kW | Kitchen, small bathroom |
| 3-phase 400 V | 16 A | 11 kW | Shower – basic comfort |
| 3-phase 400 V | 20 A | 13–14 kW | Shower – good comfort |
| 3-phase 400 V | 32 A | 18–21 kW | Shower – full comfort, multiple outlets |
The golden rule: Before you buy any flow heater with a power of more than 7 kW, call an electrician. Have the bathroom circuit and the type of power supply checked. The cost of a consultation is just a few euros. The cost of unnecessary warranty claims or the need to return the product is much higher.
More about the technical installation can be found in the article Installation of an electric flow water heater step by step in our Knowledge Center.
Pressurized vs. non-pressurized operation: how it relates to power
Flow heaters are divided into pressurized and non-pressurized. This is not just a formal category – it directly affects how many outlets you can connect and what power you need.
Non-pressurized heaters have an open output – water flows out through a special mounted faucet, not through a closed water supply. Their power is usually lower (3.5–6 kW), they serve one outlet and are suitable mainly for sinks and kitchen basins. They are not suitable for showers (where the tap is closed and a regular head is used).
Pressurized heaters are connected to a closed plumbing system, work with regular showers, taps and faucets. They have higher power and are significantly more versatile. All heaters mentioned in this article are pressurized.
This difference is discussed in detail in the article Pressurized vs. non-pressurized operation of water heaters: what is the difference.
Adjustable power: why it is an advantage, not a luxury
Heaters with adjustable power – such as the mentioned 12 kW and 21 kW models – allow switching between two or three power levels. In practice, this means that in summer you switch to a lower power (and save on electricity), and in winter you switch to a higher one. In addition, you can regulate the flow of hot water without having to lower the outlet water temperature below a comfortable level.
This is an advantage in terms of longevity as well – the appliance does not operate at full power constantly, which reduces thermal stress on the heating element. From experience, I know that heaters with adjustable power are significantly more appreciated by customers after a few months of use than at the time of purchase – precisely because of this flexibility.
Hydraulic vs. electronic switching: what it affects
Flow heaters can switch the heating element in two ways. Hydraulic switching is mechanical – it is activated by water pressure when you open the tap. Electronic switching is more sophisticated – it is controlled by electronics, reacts faster, maintains the set temperature more accurately and usually works better at low flow rates.
This has a direct consequence for power calculation: electronically switched heaters usually perform better at lower flow rates and do not have a problem with so-called "minimum switching flow" – in other words, you don't have to open the tap fully for the heater to start. This is important in practical operation, as the actual flow during a shower is usually lower than the maximum flow of the heater.
You can read more about this topic in the article Hydraulic vs. electronic switching of flow heaters: what to choose.
When a flow heater is not enough and a storage heater is better
There are situations where a flow heater is simply not the right choice – and it is honest to say it directly. If you have an old apartment with the original single-phase electrical installation without the possibility of reinforcement, a flow heater with sufficient power for a comfortable shower simply cannot be installed. In such a case, a storage heater, e.g. electric storage heater BD 2.0 kW 30 l top, is a much better choice. A storage heater only needs 2 kW – it heats water slowly, but has a 30-liter supply, which is enough for a comfortable shower (5–6 minutes at a flow rate of 5 l/min).
A storage heater is also more suitable if:
- You need to fill a bathtub (volume 100–150 l – a flow heater would have to be extremely powerful)
- The electrical installation cannot handle power above 6–7 kW
- You use a large amount of hot water at once (family of 4+ people)
- You want a comfortable flow regardless of the season without the need for strong electricity
A detailed comparison can be found in the article What volume of storage heater do I need for my household.
Practical calculation process step by step
Let's summarize the entire process into five steps that anyone can handle:
Step 1: Find out what you need the heater for (sink, shower, bathtub, kitchen).
Step 2: Determine the required flow rate in l/min (see the table above – 7 l/min for a shower as a basis).
Step 3: Estimate the temperature rise – in winter, calculate with an inlet temperature of 8 °C and an outlet temperature of 38 °C → ΔT = 30 °C.
Step 4: Calculate: P = Q × ΔT × 0,07.
Step 5: Check whether the electrical installation can handle the calculated power. If not, reduce the power and accept a limited flow, or consider a storage heater.
Example of the entire process for a family of three in a panel apartment with a shower: flow rate 7 l/min, ΔT = 30 °C → P = 7 × 30 × 0,07 = 14,7 kW. Check whether the apartment has a three-phase connection – if yes and the circuit breaker is at least 20 A, a 12 kW heater will work (with slightly reduced flow), or a 21 kW model at 32 A circuit breaker will ensure full comfort.
Most common mistakes when selecting power
From dozens of customer orders, I know that people repeatedly make the same mistakes. Here are the most common ones:
- Dimensioning for summer conditions: Buying a heater that works great in June, but barely heats the water to 30 °C in January.
- Ignoring the electrical installation: Buying a 21 kW heater for an apartment with a single-phase electrical installation.
- Thinking like a storage heater when using a flow heater: Expecting the heater to fill a bathtub at full power. A 12 kW flow heater needs about 25 minutes to fill a 120-liter bathtub.
- Underestimating the flow rate: Calculating power for 5 l/min, but in practice using a head with a flow rate of 9 l/min.
- Ignoring water pressure: Low pressure in the pipes (under 1 bar) can prevent proper operation of pressure flow heaters.
Frequently asked questions (FAQ)
Can I connect a shower and a sink to one flow heater at the same time?
Technically yes, if the heater is powerful enough (usually 18–21 kW) and you have a three-phase connection. In practice, it is not ideal, because the total flow is divided between two points and the temperature at both will drop. A more common solution is the installation of two separate heaters – one for the shower, one for the sink.
Why is my flow heater not sufficient in winter, even though it worked well in summer?
Because in winter the temperature of the cold water is 10–15 °C lower than in summer. This means that the same power has to cover a larger temperature difference – and at the same flow rate, the outlet temperature will drop. The solution is to either reduce the flow (open the tap less), or to have a more powerful heater from the start, dimensioned for winter conditions.
Is electronic switching really better than hydraulic?
For most home applications, yes. Electronic switching reacts faster, works even at lower flow rates and controls the temperature more precisely. Hydraulic switching is mechanically simpler and more durable, but requires a minimum flow rate to activate – which can be a problem at the sink, where you want only a gentle water flow. The difference is less noticeable in the shower. More in the article Hydraulic vs. electronic switching of flow heaters: what to choose.
What is the difference between a 12 kW and a 21 kW heater in everyday use?
At 12 kW, you have a flow of about 5.7 l/min at a winter inlet temperature of 8 °C and an outlet temperature of 38 °C – which is acceptable, but not luxurious for a shower. At 21 kW, you have about 10 l/min – this is a full-comfort shower even with a standard head. You will notice the difference mainly in winter. In summer, 12 kW is completely sufficient for a typical household.
Do I need a permit to install a flow heater above 6 kW?
The device itself does not require a building permit, but the electrical connection must be done by a licensed electrician who issues an inspection report. Connecting to a three-phase grid without an inspection is not in line with standards and can affect insurance coverage in case of damage. Always insist on an inspection report.
Can a flow heater be used to preheat for a combination boiler?
No, it is counterproductive. A flow electric heater is designed as a standalone source of hot water, not as a preheater. Combining it with a boiler brings problems with temperature regulation and unnecessary energy losses. If you have a boiler, handle hot water through it or through a separate tank.
Conclusion: the power of a flow heater is not a number, it is a compromise
At the end, it is important to emphasize one thing that customers sometimes do not like to hear: choosing the power of a flow heater is always a compromise between what you want and what the electrical installation allows. An ideal world would be 21 kW with a three-phase connection and a 32 A circuit breaker in every apartment – but the reality of Slovak apartments is different. Older panel apartments mostly have a single-phase connection and a maximum allowed power of 3.5–6 kW for the bathroom.
Therefore, the process is simple: first find out what the electrical installation allows. Then calculate what you need. And find the optimum in that intersection. If you cannot achieve the power needed for a comfortable shower with a flow heater, consider a storage solution – it is not a step back, it is the right choice for the given situation.
If you are still unsure which model is right for your specific case despite this article, take a look at our full category of electric water heaters – the parameters are always listed directly on the product. And if you would appreciate advice before purchasing, further topics in our Knowledge Center – including articles on common faults, maintenance and installation – will help you make a decision with full information.
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 advise.