What capacity of hot water storage tank do I need for my household

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What volume of a hot water storage tank do I need for my household?

This question is among the most common ones we encounter when designing hot water supply systems. And the answer is never simple – the correct volume of a storage tank depends on many factors: how many people live in the household, what heat source you use, when the water is heated, and how the entire system is designed. Underestimating can lead to cold water arriving just when you're enjoying a long shower. Overestimating, on the other hand, unnecessarily increases the investment and increases heat losses.

In this article, we will show you how to correctly calculate the required volume, what are typical scenarios for different types of households, and where mistakes are most commonly made. If you are also interested in selection based on other criteria (type of heat exchanger, material, insulation), see the related article How to choose a hot water storage tank – volume, heat exchanger and other criteria in the Knowledge Center.

What is a hot water storage tank and why does the volume matter?

A hot water storage tank (TUV) serves as a reservoir – pressurized or unpressurized – in which preheated water is ready for immediate use. Unlike flow-through heaters, which heat the water at the time of withdrawal, the tank stores thermal energy and delivers water at a constant temperature without waiting for heating.

Why is the volume so critical? Because the tank must cover the peak water consumption in the household without being depleted before the heat source can heat another batch. At the same time, it shouldn't be unnecessarily large, because every liter of cold water that you have to heat costs energy and time. And every liter of hot water that doesn't reach the appliance in time cools down in the tank and represents heat losses.

Typically, if a tank is oversized by 50%, heat losses increase disproportionately – not by 50%, but depending on the quality of insulation, even more. We have written in detail about this topic in the article Insulation of a hot water storage tank – why it is important and what to look for when choosing.

Basic calculation: TUV consumption per person and day

Professional standards (STN EN 15316 and others) define standard hot water consumption per person and day. In practice, these values vary according to the equipment and habits of the household, but as a basis for calculation, the following approximate figures are used:

Minimum standard (shower, no dishwasher): 40–50 liters TUV/person/day (at 45 °C)
Standard (shower, dishwasher, regular dishwashing): 60–70 liters TUV/person/day
Comfort standard (bath, dishwasher, shower, bidet): 80–120 liters TUV/person/day
Luxury standard (multiple bathrooms, large bath, hydromassage): 120–160+ liters TUV/person/day

Important note: these values refer to a temperature of 45 °C. If you heat the water to 60 °C (which is standard for legionella prevention), more "thermal energy" fits into the tank and you mix it with cold water when drawing – effectively increasing the tank volume. Most tanks in the standard price range are also calibrated for pressure tests at 60 °C.

Conversion: how much water do you actually need?

The calculation is simple: total daily consumption = number of people × consumption per person/day. But that's not enough. You also need to consider peak consumption – what happens in the morning when the whole family showers one after another, or in the evening before bedtime. If the whole family showers within a 60–90 minute window, the tank must cover all this consumption at once, without the heat source having time to reheat.

Therefore, in practice, a so-called peak consumption factor is used, which ranges from 1.5 to 2.5 – meaning the tank should have a capacity of 1.5 to 2.5 times the average hourly consumption during the peak. For a typical family, this means the tank volume should cover at least the entire evening peak plus a reserve for reheating.

Typical scenarios according to household size

Two-person household (2 people)

A couple without children, an apartment or a small family house. Average daily hot water consumption is around 80–140 liters at 45 °C. When heated to 60 °C and mixed with cold water (1:0.5), a tank with a volume of 200–300 liters is sufficient in practice. Most of these households can manage with a conventional storage heater, not a large-volume accumulation tank.

A large-volume accumulation tank (500+ liters) makes sense for a two-person household only if the heat source is a heat pump with a night-time electricity rate or a solar system, where you need to accumulate the entire day's energy gain and distribute the consumption.

Four-person household (4 people)

This is the most common scenario in family houses. A family with two children, one bathroom, possibly a bathroom + shower room. Daily consumption: 240–320 liters at 45 °C. Peak consumption (morning + evening): 150–200 liters within a 60–90 minute window.

Recommended tank volume: 500–800 liters. With a heat pump, where heating is slower, the upper limit is recommended. With a gas boiler, which heats quickly, 500 liters with a sufficient heat exchanger is sufficient. For example, the ZGIB 850-liter accumulation tank is a popular choice in this category, as it provides sufficient reserve even for higher comfort consumption or occasional visits.

Six-person and larger household (6+ people)

A larger family, or a house with two generations. Two bathrooms, a dishwasher, possibly a small home laundry room. Daily consumption: 400–700 liters at 45 °C. The peak can be significant, especially if several family members shower at the same time.

Recommended volume: 800–1500 liters. Here we enter the category where accurate dimensioning is really important. A tank that is too small in combination with a heat pump can cause the pump to run in short cycles (so-called cycling), which damages it and reduces efficiency. The ZGIB 1000-liter accumulation tank or the ZGIB 1500-liter accumulation tank are standard solutions for such households.

Large households, recreational properties, guesthouses

A recreational house with a capacity of 8–12 people, a small guesthouse, a farm with several permanent residents. The needs are significantly higher and irregular – high peak consumption in the season, minimum outside the season. For such properties, tanks from 1500 to 3000 liters are standard. The ZGIB 2200-liter accumulation tanks or special solutions such as the HPWB3000 accumulation tank are suitable choices here, with HPWB3000 specifically designed for systems with a heat pump and offering an optimized input/output for this type of heat source.

How the heat source affects the required tank volume

This is one of the most underestimated aspects. The tank volume is not only about the number of people – it is equally about the performance and reaction speed of the heat source.

Heat pump (air-water, ground-water)

A heat pump is a slow but economical source. A typical performance of an air-to-water heat pump for hot water preparation is 3–10 kW. To heat 1000 liters of water from 15 °C to 55 °C at a performance of 5 kW, you need:

Energy: 1000 l × 4.186 kJ/kg·K × 40 K ÷ 3600 = 46.5 kWh
Time: 46.5 kWh ÷ 5 kW = ~9.3 hours

This means that if a 1000-liter tank is emptied in the morning, refilling takes almost the whole day. Therefore, with a heat pump, the rule applies: the tank must cover the entire daily consumption, not just the peak, because the pump cannot replenish the supply continuously. The recommended volume with a heat pump is therefore higher – 800–1000 liters for a 4-person family, 1500+ liters for a 6-person family.

More details on the connection can be found in the article Connecting an accumulation tank for hot water preparation to a heat pump or boiler – how to do it.

Gas or oil boiler

A boiler is more powerful. A typical performance for hot water preparation via an indirect storage tank: 15–30 kW. For the same 1000 liters at a performance of 20 kW, you need:

Time: 46.5 kWh ÷ 20 kW = ~2.3 hours

A boiler can quickly replenish the supply – therefore, smaller tanks are sufficient with a boiler with a good storage heat exchanger. For a 4-person family with a gas boiler: 300–500 liters is usually sufficient.

Solar collectors

A solar system depends on weather and daily irradiation – the performance is uneven and unpredictable. Therefore, with solar systems, the opposite logic applies compared to a boiler: you want the largest possible accumulation volume to fully utilize every sunny day and not waste excess energy. A typical recommendation is 50–80 liters per 1 m² of collector. A family with 8 m² of collectors should have 400–640 liters of capacity. Solar hot water preparation is, however, combined with a heat pump in modern systems, where the volume logic is a bit different.

Factors that change the calculation: what else to consider

Heating time and electricity tariff

With a heat pump and a low night tariff (HDO), the entire heating is shifted to the night hours. This fundamentally changes the dimensioning logic: the tank must hold the entire daily consumption plus a reserve, because nothing (or only occasionally) is heated during the day. For a 4-person family with a night tariff, this can mean the need for a 800–1000 liter tank instead of the usual 500–600.

Comfort level and type of appliances

A bathtub consumes 120–200 liters of hot water. A shower: 40–80 liters (depends on duration and flow rate). A dishwasher: 15–25 liters. A washing machine for hot washing: 30–60 liters. If you have a bathtub in your household that you use regularly, increase the tank volume by at least 150 liters compared to the basic calculation.

Thermal loss level of the tank

A well-insulated tank loses 1–2 kWh/24 hours. A poorly insulated (or non-insulated) one can lose 4–6 kWh/24 hours. This means that in a 24-hour cycle, the temperature of water in a 1000-liter tank without insulation can drop by 4–5 °C, which can be insufficient for peak consumption. Good insulation reduces losses and extends the time the temperature is maintained – this directly affects how large a tank you actually need.

Seasonality and variability of consumption

Hot water consumption is lower in summer months – people take shorter showers and use the bathtub less. In winter, it is higher. If you are dimensioning a tank for a cottage with irregular occupancy, consider maximum occupancy, not the average. A tank designed for 2 people, but used for 2 weeks a year by 10 people, will constantly be insufficient.

Practical table: quick overview of recommended volumes

Number of people	Boiler / direct heating	Heat pump	Solar + HP	Night rate HDO
2 people	150–300 l	300–500 l	400–600 l	500–700 l
3 people	200–400 l	500–700 l	600–800 l	700–900 l
4 people	300–500 l	700–1000 l	850–1100 l	900–1200 l
5 people	400–700 l	900–1200 l	1100–1500 l	1200–1500 l
6 people	500–800 l	1200–1500 l	1500–2200 l	1500–2200 l
8+ people / guest house	1000–1500 l	2000–3000 l	2500–3000+ l	2200–3000+ l

Concrete examples from practice: projects we've seen

Case 1: Family of 4, heat pump, new construction

A standard family house, 120 m², 4 people (2 adults, 2 children). The installer recommended an air-to-water heat pump of 8 kW. The customer originally wanted a 500-liter tank "to save money". After calculating the consumption (4 × 70 l/day = 280 l/day at 45 °C) and analyzing the night tariff, we convinced him to go for an 850-liter tank. Reason: the entire heating takes place in the night HDO period, and during the day the heat pump does not heat DHW. Result: the customer is satisfied, morning and evening peak demand is fully covered, no "cold surprises".

Case 2: Cottage, 8 people in season, gas boiler

A recreational cabin, 2 people in regular use, 6–10 people in summer and on holidays. Original installation: 300-liter tank, 20 kW gas boiler. Seasonal problem – after the first two showers, the water was cold. The boiler heats up quickly, but not quickly enough for simultaneous use by 8 people. Solution: replacement with an 850-liter tank with a simple plate heat exchanger. The boiler replenishes the supply in 2 hours, and in the meantime, there is sufficient reserve. The investment was repaid by comfort, not by savings – but that was his goal as well.

Case 3: Two-generation house, 6 people, heat pump + solar panels

A large family house, two apartment units, 6 people in total, two bathrooms, laundry room, utility room. System: 12 kW heat pump + 10 m² solar collectors. We solved the so-called combination boiler – a tank with two heat exchangers (one for the heat pump, one for solar). Volume: 1500 liters. The summer surplus of solar energy is fully stored, and the heat pump heats up only what the solar collectors could not cover during the night. DHW electricity consumption in summer dropped by 70%. Read more about this type of installation in the article Accumulation tank for DHW with one or two heat exchangers – what is the difference.

Case 4: Underestimated tank and its consequences

The installer installed an air-to-water heat pump and a 300-liter tank for a family of 5 – "it was enough with the previous boiler". Result: the heat pump was switched on 12–15 times a day in short cycles (so-called on-off cycling), which significantly shortened the compressor's lifespan and increased electricity consumption by about 15%. After one year of operation, the customer replaced the tank with a 1200-liter one and the number of cycles dropped to 2–3 per day. Lesson: when using a heat pump, overdimensioning the tank pays off.

Mistakes in selecting volume: what to avoid

Considering only a "typical" day, not the peak. The decisive factor is the peak situation – weekend, visitors, season.
Forgetting the heat source. A tank designed for a boiler is usually too small for a heat pump.
Ignoring HDO tariffs. If you have a night tariff, heating the entire volume overnight means a significantly higher demand for volume.
Overestimating the performance of the heat exchanger. A large tank with a small heat exchanger heats up slowly – the heat exchanger's performance must match the source's performance. More on this in the article How to choose an accumulation tank for DHW – volume, heat exchanger and other criteria.
Not accounting for insulation losses. In a cold space (basement, unheated boiler room), heat losses are higher and affect the tank's actual capacity.
Forgetting about room dimensions. A 1500-liter tank won't fit in every boiler room. Check the dimensions in advance – the article Dimensions and space requirements of DHW accumulation tanks 500–3000 liters will help you.

Special situations: when to choose an oversized tank

There are situations where a large tank is the right solution even with a small number of people:

Solar system with a large collector area: If you have 15–20 m² of solar panels, you need an accumulation volume of at least 1000–1500 liters to make full use of the daily gain and not lose it through the overflow valve.
Bathroom with a large whirlpool or hydro massage bath: Filling the whirlpool requires 400–700 liters at once. If you do this regularly, the tank must cover this one-time consumption.
Home SPA zone, saunas with shower rituals: Multiple short showers after the sauna, ice showers, contrast – consumption can reach 300+ liters per hour for 3–4 people.
Guesthouse or agrotourism accommodation: If you rent out rooms, don't calculate with average, but with maximum occupancy. Better to have an oversized tank than an unhappy guest without hot water.
Combined system DHW + floor heating: Some tanks also serve as an accumulator for heating. In such a case, the logic of volume is completely different and depends on the overall thermal project.

How the selection process works in practice: step by step

If you want to make an approximate calculation yourself, follow these steps:

Determine the number of people in the household (or maximum number for a recreational property).
Estimate daily consumption – standard 60 l/person/day as a base, adjust according to equipment (bathtub = +40 l, long showers = +20 l).
Identify the heat source and its capacity. For a heat pump, multiply daily consumption by 1.5–2 (heating overnight). For a boiler, 1.0–1.2 is sufficient.
Take into account special conditions – night tariff, large bathtub, solar, recreational use.
Round up to the nearest available volume (500, 850, 1000, 1500, 2200, 3000 liters).
Check dimensional limitations – height, diameter, accessibility of the installation site.

The result of this process will give you a good foundation. For a precise calculation, we recommend consulting an HVAC designer who will also consider the specifics of your installation, pressure conditions, water hardness, and other factors.

Overview of available volumes and who they are suitable for

In the category accumulation tanks for DHW, you will find tanks from small storage tanks to large-capacity solutions. Here is a brief overview for whom they are typical:

Up to 500 liters: Two- to three-person households with a gas boiler, apartments with a combination boiler.
500–1000 liters: Four- to five-person households, heat pumps, systems with a night tariff. Typical choices are ZGIB 850 liters or ZGIB 1000 liters.
1000–1500 liters: Six-person and larger families, comfortable systems with a heat pump, solar installations. Ideal choice: ZGIB 1500 liters.
2000–2200 liters: Large households, recreational properties, small guesthouses. ZGIB 2200 liters is a standard solution here.
3000 liters and more: Larger operations, complex energy systems, systems with multiple sources. Accumulation tank HPWB3000 is specially dimensioned for heat pump systems and covers demanding operations.

Frequently asked questions (FAQ)

Is it better to buy a larger tank than I think I need?

Generally yes – a slight overdimensioning (by 10–20%) is better than underdimensioning. A larger tank reduces the number of heating cycles, which saves the heat source (especially the heat pump), and provides a comfortable reserve during visits or increased consumption. A tank that is too large (by 50% or more) unnecessarily increases heat losses and installation costs. A reasonable compromise is key.

Can I connect two smaller tanks instead of one large one?

Yes, it is technically possible – this is called a cascading tank connection. Two 1000-liter tanks can replace one 2000-liter tank if there is not enough height in the room for a large tank. Disadvantages: higher cost (two tanks + double hydraulics), more complex installation, higher heat losses (larger surface area). More on installation can be found in the article Installation of DHW accumulation tank – procedure, connection and commissioning.

How many liters of hot water does one shower consume?

A regular shower (5–8 minutes, shower head 8–12 l/min) consumes 40–70 liters of hot water at a temperature of 38–42 °C. A luxury rain shower head can consume 80–120 liters. In contrast to a bath (120–200 liters), a shower is much more efficient. If you prefer showers over baths, you can slightly reduce the tank volume.

What temperature should the water in the storage tank be?

From a hygiene perspective, the water in the tank should be maintained at a minimum of 60 °C to eliminate the risk of Legionella pneumophila bacteria multiplying. At temperatures of 20–50 °C, Legionella multiplies rapidly. Practical consequence: you heat the tank to 60 °C and mix it with cold water to reach the desired temperature of 38–42 °C. This means that from a 1000-liter tank at 60 °C, you get approximately 1350–1500 liters of usable hot water after mixing (60 °C + cold 15 °C → 42 °C). A higher temperature = greater effective capacity.

How long will hot water stay hot in a well-insulated tank if not used?

Well-insulated tank (insulation thickness 100+ mm, PU foam)

Do you have a question about this topic?

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