What solar thermal is and why choose it

Solar thermal is the technology that uses solar radiation to produce domestic hot water (DHW) and, in some configurations, contribute to space heating. It is one of the most space-efficient renewable technologies: a well-oriented flat-plate collector converts 45-60% of incident solar energy into useful heat, compared to 18-22% for photovoltaics. For a typical family of 4 in southern Europe, a 2-3 m² collector array covers 60-70% of yearly DHW demand (~2500 kWh/year), saving 150-350 €/year on gas and cutting 500 kg of yearly CO₂ emissions. Italian regulation makes it mandatory for new buildings (D.Lgs. 28/2011 – at least 50% of DHW from renewables) and incentivises with tax rebates (50% Ecobonus, up to 65% in condominiums).

The fundamental formula: yearly energy balance

Sizing starts from the daily energy demand to raise cold mains water to use temperature:

Q_day = n · q · c_p · ΔT

where n is the number of people, q per-capita daily consumption in L/day (typically 40-60 L/person at 40 °C for domestic use), c_p = 4.186 kJ/(kg·K) water specific heat, ΔT = T_use − T_mains temperature difference. Example: 4 people, 50 L/day at 40 °C, T_mains 12 °C → Q_day = 4·50·4.186·28 = 23,442 kJ = 6.51 kWh/day. Yearly: Q_year = 6.51·365 = 2,377 kWh/year. Required collector area:

A_coll = f_sol · Q_year / (H_year · η_coll)

with f_sol desired solar fraction (0.50-0.70 typical, higher risks summer overheating), H_year yearly solar irradiation on the collector plane (kWh/m²/year), η_coll yearly average collector efficiency.

Solar irradiation across Europe (H_year)

Yearly irradiation on a 30-40° south-facing tilted collector (kWh/m²/year, per PVGIS/JRC data):

• Northern Europe (UK, Germany, Poland, Baltic): 900-1150 kWh/m²/year.
• France, Alps, Northern Italy: 1200-1450 kWh/m²/year.
• Central Italy, Southern France, Slovenia: 1500-1700 kWh/m²/year.
• Southern Italy, Spain, Portugal, Greece: 1700-1900 kWh/m²/year.
• North Africa, Malta, Cyprus: 1900-2200 kWh/m²/year.

Deviations from optimal south-facing 35° tilt: up to ±20° azimuth or ±15° tilt reduce H_year by only 3-5%; beyond that the drop is steeper (pure east/west at 90° loses 20-25%). Partial shading (tree, chimney) on part of the collector can cut H_useful by 40-70% because shaded cells lower the whole panel's efficiency.

Collector types and efficiencies

• Flat plate non-selective (black paint): cheap (150-250 €/m²), optical η_0 = 0.70-0.75, loss coefficient a_1 = 5-7 W/(m²·K). Yearly average DHW efficiency ~40-45%. Suitable for warm climates.
• Flat plate selective (TiNOx, PVD coating): market standard (250-450 €/m²), η_0 = 0.80-0.85, a_1 = 3-4 W/(m²·K). Yearly efficiency 45-55%. Best cost-performance for DHW in temperate climates.
• Evacuated tube heat-pipe collector: premium (400-700 €/m²), η_0 = 0.75, a_1 = 1-2 W/(m²·K) thanks to vacuum. Yearly efficiency 55-70%, better in cold climates or high temperature (60-90 °C). Ideal for heating integration.
• Integrated in-roof glazed collector: aesthetic solution, integrated as glazed tiles. Price 400-600 €/m², efficiency similar to selective flat.
• Air collector: for space air preheating, not DHW.

Solar Keymark certification (European) validates performance measured per EN 12975/12977 and should always be requested from the supplier.

Storage tank: sizing and stratification

The storage tank (boiler) is essential because solar energy arrives mostly during the day while DHW demand concentrates morning and evening. Sizing (UNI-CTI 8477, EN 12977):

• Rule of thumb: 50-80 L storage per m² flat collector; 30-50 L/m² for evacuated tubes.
• Residential minimum: 50 L per person for at least one day of autonomy.
• For a family of 4 with 3 m² flat: 180-240 L; 200 L is market standard.
• Thermal stratification in the tank is crucial: cold at bottom, hot at top. Well-stratified tanks work at full charge (60-80 °C top) even when bottom is cold (30-40 °C, still receiving heat from collector).
• Double coil: lower coil for solar input, upper for gas boiler backup. Most efficient hybrid configuration.
• Legionella proliferates at 25-45 °C. Weekly "thermal shock" to 60 °C prevents it. Public/hospital use requires T ≥ 55 °C continuously.

Backup energy: gas or electric?

No solar system covers 100% of demand: typical solar fraction f_sol for DHW is 60-70% (lower in winter, near 100% in summer). The rest is covered by:

• Natural gas boiler: most common. Energy cost 0.10-0.12 €/kWh. Condensing boiler efficiency 92-98%. Integrated via separate coil in tank or thermostatic mixer.
• Electric resistance: emergency solution, inefficient. Cost 0.25-0.30 €/kWh, 2.5-3× more expensive than gas. Only for houses without gas or small integrators (5-15 L).
• Air-water heat pump: modern sustainable solution. Typical COP 3-3.5 for DHW → effective cost 0.08 €/kWh even at domestic tariff. High upfront investment (2500-4500 €), but Superbonus incentives cover 65-110%.
• Biomass (pellet stove/boiler): common in rural areas. Integrated with 500-1500 L buffer tank, also contributes to space heating.

Regulation, maintenance and lifetime

A well-installed solar thermal system lasts 25-30 years with ordinary maintenance:

• Circulation pump: low-flow pump (2-4 W electrical) moving heat carrier fluid (water-propylene glycol anti-freeze mix, replaced every 5-7 years).
• Expansion vessel: absorbs fluid volume variation with temperature (from 20 to 180 °C in summer stagnation). Sized for total expansion volume, typically 10-25 L for residential.
• Control unit: activates the pump when collector T exceeds tank bottom T by 5-10 °C. Also controls backup.
• Safety valves: for pressure (6-10 bar) and temperature (95 °C). Fluid can boil in summer stagnation → design for stagnation risk.
• Maintenance: yearly antifreeze check (40-50% concentration), collector cleaning (dust cuts η 5-15%), valve check. Typical cost 80-150 €/year.
• Summer overheating: if f_sol is too high (>75%) the system stagnates repeatedly in summer → fluid degradation. Better f_sol = 60-65% and use summer excess to preheat a pool.

How to use the calculator

Enter the number of people using DHW and the per-capita daily consumption in L at use temperature (default 50 L/person; 30-40 L for saving habits, 60-80 L for intensive use with baths). Specify use temperature (40 °C standard; 55 °C for legionella prevention without thermostatic mixer) and mains water temperature (10-12 °C north, 14-18 °C south). Enter yearly irradiation on the collector plane in kWh/m²/year (use PVGIS: 1000-1200 UK/Germany, 1200-1400 France/N.Italy, 1500-1700 central Italy/Spain, 1700-2000 southern Europe). Choose yearly collector efficiency (0.45 economic flat; 0.55 selective flat; 0.65 evacuated tube) and desired solar fraction (0.50-0.70 optimal). The calculator returns daily water consumption, daily and yearly energy demand, required collector area, recommended tank volume, yearly backup energy and an assessment with system type classification.