How to choose a solar storage tank: volume, heat exchangers, and system type
How to choose a solar storage tank: volume, heat exchangers and system type
The solar storage tank is the heart of every thermal solar system. It is the part of the installation that customers often underestimate – they choose excellent collectors, a solar station, an expansion tank – and then buy a tank "by price" or "based on what fits in the boiler room". The result? The system works, but it utilizes less than 60% of the potential of the panels. Or, on the contrary – the tank is oversized, the water overheats in it during the summer and the vent valve releases steam every other day.
This article is here to help you choose the tank correctly – with an understanding of why volume matters, how heat exchangers work, in what ways the different types of systems differ, and what factors influence specific parameters. We will focus on real numbers and practical scenarios, not marketing phrases.
Why choosing a storage tank is critical for the whole system
A thermal solar system works by collectors capturing heat from the sun and transferring it via a solar medium (a mixture of water and glycol) to the tank, where the energy is stored. The tank thus performs two basic functions: it stores the collected heat and it delivers it to the hot water (TÚV), or possibly to the heating circuit.
If the tank is too small, the collectors become saturated quickly – the tank reaches maximum temperature before midday and the solar pump turns off. The remaining solar energy remains unused. In summer months, this can mean a loss of 30–50% of the system's real output.
If the tank is too large, the water in it never reaches a sufficient temperature for comfortable use of TÚV – even in summer months. The boiler has to heat up the tank almost every day, which undermines the economic sense of the whole installation.
Choosing a storage tank is therefore not a trivial decision. It depends on the collector area, the number of people in the household, the way heat is used, the type of boiler, and whether the system is used only for TÚV or also for supporting heating.
Storage tank volume: how to calculate it and what to avoid
The simplest recommendation you will find in every catalog is: 50–75 liters per person for a system intended only for TÚV. In practice, however, it depends on several factors that the basic rule does not take into account.
Volume calculation based on collector area
A more accurate approach is to base the tank volume on the installed collector area. The recommendation is: 50–70 liters of tank per each m² of collector aperture area. For a flat collector with an aperture area of 2.3 m², you therefore need a tank capacity of 115–160 liters. Two such collectors (4.6 m²) require 230–320 liters.
For vacuum (tubular) collectors, the ratio is slightly different – their peak performance per m² is higher, so the tank must be able to absorb a higher thermal output in a shorter time. Here, 60–80 liters per m² is recommended.
Why is this relationship important? Because collectors and the tank form communicating vessels in terms of thermal balance. If the tank is too small in relation to the collector area, the temperature in the tank rises very quickly, the solar regulation disconnects the pump, and the collectors overheat. In systems with glycol (and thus without the possibility of flushing cooling), this leads to stagnation – the glycol overheats, degrades, and the system becomes clogged with deposits.
Influence of the number of people and TÚV consumption
On the consumption side, it holds: an average Slovak household consumes 40–60 liters of hot water per person and day (at a temperature of 45–50 °C). A family of four therefore needs a tank that can hold at least 200 liters of usable TÚV – which, in the case of tanks with indirect heating (heat exchangers), corresponds to tanks of 200 to 300 liters nominal volume, since part of the volume is occupied by the heat exchanger and not every liter is available at the given temperature.
For practical decision-making:
- 1–2 people, 2–3 collectors: 150–200 liter tank
- 3–4 people, 3–4 collectors: 250–300 liter tank
- 4–6 people, 4–6 collectors: 300–400 liter tank
- Family house with solar heating support: 500–1000 liters and more
For a family of four with average TÚV consumption and 4 collectors (approx. 9–10 m²), I recommend our 300-liter tank – it is a proven combination where the system operates efficiently even in transitional periods (spring, autumn), when solar yield is lower and the tank retains the stored heat through the night.
If you have a larger family or a larger collector area, look at our 400-liter tank, which is suitable for installations combining solar with a heat pump or a solid fuel boiler.
Types of heat exchangers: one or two?
The heat exchanger in a solar storage tank is a coil or spiral heat exchanger placed directly inside the tank. The solar medium (glycol) flows through the exchanger and transfers heat to the water in the tank without mixing with it – this is indirect heating.
Storage tank with one heat exchanger
A storage tank with one heat exchanger has a single circuit – the solar one. It is suitable for simple systems where there is only one heat source: solar collectors. If you want to use an electric immersion heater as a backup or if the boiler heats the tank only via direct connection to the heating circuit (so-called direct heating via inlet/outlet), one heat exchanger will be sufficient.
The advantage is a simpler installation scheme, lower cost and easier installation. The disadvantage is lower flexibility – if you add a second heat source (boiler, heat pump), you will have to solve the hydraulic connection differently, for example via an external heat exchanger or a mixing unit.
Storage tank with two heat exchangers
A storage tank with two heat exchangers has a lower heat exchanger for the solar circuit and a upper heat exchanger for the boiler or other auxiliary heat source. This arrangement is physically optimal: the solar circuit heats the water from the bottom (where it is coldest – ideal temperature for collectors) and the boiler heats only the upper zone when solar energy is insufficient.
From a practical point of view, this is the standard solution for 90% of single-family homes. A solar storage tank with two heat exchangers including insulation is ready for immediate connection of both circuits – no external heat exchanger or complicated hydraulics is needed.
With storage tanks with two heat exchangers, the rule of thermal stratification applies: distinct temperature layers form in the tank. Cold water enters from the bottom, passes through the lower solar heat exchanger and gradually rises. When the temperature in the upper part is insufficient (during periods without sun), the boiler heat exchanger heats it quickly and efficiently. The result is that the boiler consumes minimal energy, as it heats only what has actually been used.
A more detailed comparison can be found in the article Storage tank with one or two heat exchangers: which is more suitable in our Knowledge Centre.
Heat exchanger surface area: a parameter that is often overlooked
Along with the number of heat exchangers, their surface area (in m²) is also crucial. The heat exchanger must be large enough to transfer heat from the collectors to the water efficiently – the larger the surface area, the smaller the temperature difference between the media is needed to transfer a given amount of power.
As a general rule: the solar heat exchanger should have a surface area of at least 0.1 m² for each m² of collector area. For a system with 6 m² of collectors, you need a heat exchanger with a minimum surface area of 0.6 m². Most quality 300-litre tanks have a lower solar heat exchanger with a surface area of 1.2–2.0 m², which is sufficient for standard residential installations with 3–6 collectors.
If the heat exchanger is insufficient (undersized surface area), the solar pump must work with a higher flow rate or increased temperature, which puts a strain on the system and reduces the lifespan of the collector medium. I also focus on this in the article Common faults of solar storage tanks: overheating, corrosion and pressure loss.
Types of solar systems and their impact on the choice of storage tank
A solar storage tank does not operate in isolation – it is always part of a larger system. The type of system significantly influences which storage tank you need.
System for hot water only (DHW)
The simplest and most common type. Solar collectors cover 50–70% of the annual DHW demand, the rest is heated by the boiler or an electric heating element. The tank must have a volume corresponding to the daily DHW consumption – no more, no less.
For a family of four (consumption of approx. 160–200 l DHW/day), an ideal tank is 200–300 litres. If you are unsure about the volume, read the article What size solar storage tank do I need for my home, where you will find detailed tables for different types of households.
System with solar support for heating
A more ambitious and investment-intensive solution. Collectors (usually 8–20 m²) provide heat not only for DHW, but also for the heating circuit – underfloor heating or low-temperature radiators. In this case, the tank is significantly larger: 500–1 500 litres, in some cases even a combined tank (buffer + DHW zone).
The tank in such a system serves as a daily accumulator – it is charged during the day and supplies the house with heat for several hours after sunset. Good insulation (at least 100 mm PUR foam) is important, as the tank must maintain the temperature for as long as possible.
Bivalent system (solar + heat pump or solid fuel boiler)
An increasingly popular solution, where the solar storage tank combines heat from collectors with another renewable source. The requirements for the tank are the highest here – you need a tank with two heat exchangers and sufficient storage capacity so that both sources can operate efficiently without interfering with each other.
We also recommend suitable hydraulic accessories for these systems – for example, a set of industrial stainless steel manifold with ball valves 6/4"×1", which allows for clean hydraulic separation of circuits in boiler rooms with multiple heat sources.
Solar tank with an integrated TÚV tank (so-called tank-in-tank)
A special category where a stainless steel tank for potable TÚV is placed inside the solar tank. The advantage is excellent heat exchange (the water is surrounded by the solar medium on all sides) and the hygienic reliability of TÚV. The principle and application are discussed in more detail in the article Solar tank with an integrated TÚV tank: how it works and when it is worth it.
Material and construction of the tank
Solar tanks are mainly made of steel with a protective enamel layer (enamelled tanks) or of stainless steel (stainless steel tanks). Each variant has its advantages and limitations.
Enamelled tanks
An enamelled tank is a steel container with an inner layer of glass-like enamel that protects the steel from corrosion and prevents contact between metal and potable water. This is currently the most common solution for domestic TÚV tanks. Enamel is chemically inert, does not affect the taste or quality of water and is cost-effective.
The disadvantage of enamelled tanks is their sensitivity to mechanical damage to the enamel (during installation) and the need for regular replacement of the magnesium protective anode (usually every 2–4 years). Without the anode, corrosion would attack the steel in places where the enamel is not perfect. The topic of maintenance is also covered in our article Maintenance of a solar tank: cleaning, checking the anode and the heat exchanger.
Stainless steel tanks
Tanks made of stainless steel are more durable, do not require an anode and have a longer service life. They are suitable for areas with more aggressive water (high chlorine content, soft water with a low pH value). The price is higher, but in the long run, the overall operating costs can be lower.
For installations with higher reliability requirements (accommodation, guest houses, larger family homes), we recommend considering the stainless steel solution.
Insulation of the tank: unobtrusive, but important
Heat losses from the tank are a factor that most customers ignore when choosing – and yet they can amount to hundreds of kWh of unnecessary heat loss per year. A good tank should have insulation made of rigid PUR foam with a minimum thickness of 80–100 mm, ideally 100–120 mm for tanks over 300 litres.
What does this mean in practice? A tank with 50 mm insulation can lose 2–4 kWh per day at a temperature difference of 40 °C. With 100 mm insulation, the losses are halved. Over a year, this makes a difference of 350–700 kWh – at an electricity price of 0.20 €/kWh, this is 70–140 € per year. Over a 15-year tank lifespan, this is a difference of 1,050–2,100 € between a good and a poor tank – and this is without even mentioning the environmental footprint.
Our offer of a solar tank with two heat exchangers including insulation includes high-quality PUR insulation directly in the packaging, which eliminates the need to install it separately on site.
Dimensions, connections and installation requirements
The tank must physically pass through the door, stairs and fit into the boiler room. This may seem trivial, but from experience I know that failure at this stage is surprisingly common. A 300-litre tank typically has the following dimensions: diameter 650–720 mm, height 1,650–1,800 mm (without legs and nozzles). A 400-litre tank: diameter 750–800 mm, height 1,800–2,000 mm.
Standard doors have a clear width of 800–900 mm. The tank usually passes through the door horizontally, so height and weight are key (an empty 300 L tank weighs 80–120 kg). For detailed dimensions and connection diagrams, we recommend reading the article Dimensions and connections of solar tanks: what to check before purchase.
The tank connections must be compatible with the pipe dimensions in your installation. Most tanks have G 1" or G 6/4" nozzles for heat exchangers and G 3/4" for TÚV inlet/outlet. For hydraulic separation of circuits in more complex systems, a distributor is needed – for this purpose, for example, the industrial stainless steel distributor/collector assembly with ball valves can be used, which allows for clean connection of multiple circuits.
The tank must stand on a flat, load-bearing floor (a 300 L tank when full weighs about 400 kg), in a well-ventilated boiler room and within reach of the anode replacement. A more detailed installation procedure can be found in the article Installation of a solar storage tank: procedure, placement and installation requirements.
Connecting the tank to the boiler and other circuits
Correct hydraulic connection of the tank to the boiler is key to the efficiency of the entire system. The most common mistakes from practice:
- Pipe between the boiler and the tank is too short – causes thermal bridges, losses and sometimes boiler thermal cycling
- Incorrect placement of temperature sensors – regulation does not work properly, the boiler turns on even when the tank has enough heat
- Missing check valve in the solar circuit – night thermosiphoning cools the tank
- Excessively large pipe volume between the tank and the outlets – long waiting time for hot water, unnecessary losses
The topic of correct hydraulic connection is covered in a separate article Connecting a solar storage tank to the boiler and circuit distributor, where you will find diagrams for various combinations of sources.
Practical scenarios from practice: three typical jobs
Scenario 1: Family house, 3 people, only DHW
Customer: new build, 3 people, low-energy house, gas. Installation: 2 flat collectors (approx. 4.6 m² total area), gas condensing boiler with indirect tank connection. Suitable tank: 250-liter tank with two heat exchangers – solar (lower) and boiler (upper). Result: 60–65 % DHW coverage by solar energy on an annual average, the boiler intervenes only in winter months and during long periods without sun.
Scenario 2: Older family villa, 5 people, DHW + partial heating
Customer: renovation, 5-person family, stove insert + solar, underfloor heating in part of the house. Installation: 6 vacuum collectors (approx. 12 m²), 400-liter tank with two heat exchangers (solar + boiler for stove insert with heat exchanger). Result: the system covers 70 % DHW and 15–20 % heating in the transitional period, the stove insert works efficiently in winter as the primary heat source.
Scenario 3: Guesthouse, 12 rooms, only DHW
Customer: mountain guesthouse, seasonal operation (May–September), DHW consumption 800–1 200 l/day. Installation: 10 flat collectors (23 m²), 1 000-liter tank (combination of two 500-liter tanks connected in series). Result: in the season, 85–95 % DHW coverage by solar energy, electric heating rods used only during prolonged periods of low sunlight.
Most frequently asked questions (FAQ)
Can I use a regular DHW tank (boiler) instead of a solar tank?
This is not recommended and usually not possible without modifications. A standard tank (boiler) does not have a solar heat exchanger – it does not have a pipe or coil heat exchanger for the solar circuit. In addition, it is not designed for the temperatures that the solar circuit reaches (90–120 °C at stagnation). A solar tank is specifically designed for higher temperatures, has a test pressure for higher overpressure and contains the necessary nozzles for all circuits.
What happens if the tank overheats to 95 °C in summer?
A tank with a glycol-based solar medium starts to discharge the medium through the safety valve at temperatures above 95 °C. Glycol degrades when repeatedly overheated, loses its anti-foaming function and clogs the heat exchanger. The result is increased service costs and reduced system lifespan. A properly oversized tank (sufficient volume) in relation to the collector area eliminates this risk. More in the article Common solar tank faults: overheating, corrosion and pressure loss.
What is the difference between a 300 and 400-liter tank with the same number of collectors?
A larger tank can accumulate more heat – it can "charge" more solar energy and retain it for longer. In the summer period, this means that a 400-liter tank will be fully charged in one day and the remaining solar energy will be lost (the system will disconnect). In the transitional period (spring, autumn), a larger tank is more advantageous – it can accumulate heat over 2–3 sunny days and supply the household with DHW even in poor weather. For smaller households with 3 collectors, a 400-liter tank is oversized and may reduce system efficiency.
Do I need to service the tank regularly?
Yes. At least once every 2–4 years, it is necessary to check and possibly replace the protective magnesium anode (enamelled tanks). Once every 5–10 years, it is recommended to inspect and flush the heat exchangers, check the safety valve and pressure in the solar circuit. A detailed procedure is described in the article Maintenance of a solar tank: cleaning, anode and heat exchanger check.
Can I connect the tank to two boilers at the same time?
A tank with two heat exchangers can have one heat exchanger connected to boiler No. 1 and the other to boiler No. 2 (or to solar). If you want to connect more than two sources, you will need a tank with more than two heat exchangers, or solve the hydraulics via an external heat exchanger and distributor. A hydraulic diagram for more complex systems is the subject of the article Connecting a solar storage tank to the boiler and circuit distributor.
Is a solar tank suitable for a heat pump?
Yes, but with an important condition: the heat pump operates at low temperatures (45–55 °C), so its heat exchanger in the tank should be as large as possible (for efficient transfer with a small temperature difference). Some tanks are specially designed for the combination of solar + heat pump. A standard solar tank with two heat exchangers is fully suitable for most bivalent systems, provided that the heat pump heat exchanger has sufficient surface area.
Conclusion: a system approach as the basis for the correct choice
Choosing a solar tank is not about buying the "biggest" or "cheapest" one. It is about understanding the relationship between all system components: collectors, tank, heating source and DHW consumption must be in balance.
For a typical 4-person family with 3–4 collectors and one boiler, the answer is relatively simple: a 250–300-liter tank with two heat exchangers and good insulation. For larger and more complex systems, it is worth investing time in calculation and consultation. Answers to many other questions can be found in the sections Common questions about solar tanks and What volume of solar tank do I need for my home in our Knowledge Center.
If you are unsure about the choice, take a look at our products with an evaluation of specific parameters: for standard family homes we recommend starting with the 250-liter tank or 300-liter tank – both are suitable for solar systems with 2–4 collectors and meet the requirements for safety, pressure resistance and heat losses according to current standards. Choosing the right tank is an investment that pays off in the form of lower energy costs and a reliable system for 15–25 years.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your household? Write to us – we are happy to advise you.