Solar Thermal Hot Water (With or Without Space Heating Support)

Flat plate or evacuated tube collectors heat domestic hot water (typically 50-65 % solar coverage) and can support space heating.

How does solar thermal hot water production work?

Solar collectors on the roof convert solar radiation into heat. In a flat plate collector, an absorber heats up under a glass cover; in an evacuated tube collector, the absorber sits inside evacuated glass tubes, which minimises heat losses and enables higher yields at low irradiance. A solar fluid transports the heat via the primary circuit to a heat exchanger in the hot water cylinder. If the solar heat is insufficient, the existing boiler tops it up. As a rule of thumb, allow 1.5 m² of flat plate collector or 1 m² of evacuated tube per person plus 80 litres of storage volume. Peak yield is around 350 kWh/(m²·a) for flat plate collectors and 450 kWh/(m²·a) for evacuated tubes. Larger combi systems with a buffer cylinder additionally provide space heating support in the shoulder seasons.

Which buildings benefit from solar thermal?

The prerequisites are an unshaded roof area oriented south plus or minus 45°, a structurally sound roof and space for the buffer cylinder. Poor orientation or shading by neighbouring buildings and trees are exclusion criteria. Typical properties are detached and multi-family houses as well as hotels and care homes with high, evenly distributed year-round hot water demand. Precisely there, summer demand is high, so the solar yield is fully used and no stagnation occurs. Compared with photovoltaics plus a domestic hot water heat pump, solar thermal delivers more usable heat per square metre but is limited exclusively to heat applications.

What does solar thermal actually deliver?

Hot water demand is met 40 to 60 % by solar; with space heating support, a further 8 to 15 % of total heat demand is added. The saving is derived from collector area, specific yield and system efficiency, referred to the efficiency of the old heat generator. A detached house with 8 m² of flat plate collectors and a 500-litre cylinder saves around 2,000 kWh/a of natural gas and roughly 0.4 t CO2. A hot-water-only system with 5 to 8 m² costs €5,000 to €9,000; a combi system with 12 to 20 m² and a buffer cylinder costs €12,000 to €22,000. With a service life of 25 years, payback is 12 to 20 years.

Energy carrier

Erdgas, Heizöl, Pellet, Strom → Solar, Erdgas/Heizöl/Pellet (Nachheizung)

Savings potential

40-60 % WW-Bedarf

Worked example

A detached house with four occupants has a hot water demand of 2,500 kWh per year, so far met by an old gas boiler with an efficiency of 0.75 in hot water mode. 8 m² of flat plate collectors are mounted on the south-facing roof and connected to a 500-litre cylinder. With a specific yield of 350 kWh/(m²·a) and a system efficiency of 0.85, the system delivers Q_solar = 8 × 350 × 0.85 = 2,380 kWh per year. Of this, around 1,500 kWh is actually usable, because in summer more heat is produced than the house consumes; this corresponds to a solar fraction of about 60 %. Referred to the boiler efficiency, the final energy saving is 1,500 / 0.75 = 2,000 kWh of natural gas per year and roughly 0.4 t CO2. A heat meter in the solar circuit allows the yield per square metre to be checked.

Investment & payback

WW-Anlage (5-8 m²): 5.000-9.000 €; Kombianlage 12-20 m² inkl. Pufferspeicher: 12.000-22.000 € · Service life: 25 a · Payback: 12-20 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much hot water can solar thermal cover?

A solar thermal system in Austria and Germany typically covers 50 to 65 % of annual hot water demand, around 60 % with a correctly sized collector area. From May to September the solar heat is usually fully sufficient; in winter the boiler takes over most of the load. With space heating support, a further 8 to 15 % of total heat demand can be met by solar.

How much collector area do you need per person?

For hot water production, allow 1.5 m² of flat plate collector or 1 m² of evacuated tube collector per person plus 80 litres of storage volume. A four-person household therefore needs 6 to 8 m² of flat plate collector and a cylinder of 300 to 500 litres. With space heating support, the area rises to 12 to 20 m² with a larger buffer cylinder.

How much does a solar thermal system for hot water cost?

A hot-water-only system with 5 to 8 m² of collector area costs €5,000 to €9,000 including cylinder and installation. A combi system with 12 to 20 m² and a buffer cylinder for space heating support costs €12,000 to €22,000. With a service life of 25 years, the system pays back in 12 to 20 years, depending on energy prices and subsidies.

Which is better: flat plate or evacuated tube collector?

Evacuated tube collectors achieve a higher specific yield of around 450 kWh/(m²·a) than flat plate collectors at about 350 kWh/(m²·a), because the vacuum minimises heat losses. However, they are more expensive per square metre. Flat plate collectors are more robust and, given sufficient roof area, usually the more economical choice. Where space is limited or space heating support is planned, evacuated tubes are more worthwhile.

Which standards apply to solar thermal in Austria?

ÖNORM EN 12975 governs the testing and performance assessment of solar thermal collectors. ÖNORM M 7140 specifies in Austria the economic comparison method for energy systems and serves as the basis for profitability assessments, for example in subsidy applications. Collectors with Solar Keymark certification demonstrably meet the requirements of EN 12975.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Klassisches Submetering: Solar-Wärmemengenzähler im Primärkreis, Speichertemperaturen, ggf. Globalstrahlungssensor (LineMetrics Pyranometer-Anbindung). Solar-Ertrag pro m² als KPI.

All measures