For domestic hot water, plan roughly 1–1.5 m² of flat-plate collector area per person in southern climates and 1.5–2.5 m² per person in central/northern Europe, with 40–60 litres of storage per m² of collector. A family of four typically needs 2 collectors (≈4–5 m²) and a 300 L tank in Turkey or Spain, or 3 collectors and 300–400 L in Germany.
The sizing logic, step by step
1. Start from hot water demand. Average European consumption is 30–50 litres of 45 °C water per person per day. A four-person household therefore needs roughly 5–8 kWh of water heating energy daily.
2. Match collector area to climate.
| Region (annual irradiation) | Area per person | Family of 4 |
|---|---|---|
| Southern Turkey, Spain, Greece (≥1,700 kWh/m²) | 1 – 1.5 m² | 4 – 5 m² (2 collectors) |
| Central Europe (1,100 – 1,400 kWh/m²) | 1.5 – 2.5 m² | 6 – 8 m² (3 collectors) |
| Northern Europe (≤1,100 kWh/m²) | 2 – 3 m² | 8 – 10 m² (3–4 collectors) |
A standard Solimpeks Wunder flat plate has ≈2.3–2.5 m² gross area, which is where the collector counts come from.
3. Size the storage to the collectors, not the other way round. The rule of 40–60 L per m² of collector area protects against both lukewarm evenings (tank too big) and midday stagnation (tank too small). Four collectors feeding a 200 L tank will stagnate all summer; two collectors on an 800 L tank will never reach temperature.
4. Target a sensible solar fraction. Design for 60–70% annual hot water coverage in central Europe and 80–90% in the south. Chasing 100% means gross summer oversizing and frequent stagnation — see stagnation temperature.
What changes the answer
- Orientation and tilt: south ±45° barely matters (≤10% loss); flat roofs use tilted frames at 30–45°.
- Combi systems (heating support): multiply the DHW area by 2–2.5 and use a 600–1,000 L combi store — a different design exercise entirely.
- Evacuated tubes deliver ~20–30% more per m² in cold, low-sun conditions; in sunny climates well-built flat plates match them at lower cost.
- Household rhythm: evening-shower families store; morning-shower families can run smaller tanks.
A worked example (Konya, family of 4)
Demand: 4 × 40 L at 45 °C ≈ 6.5 kWh/day. Two Wunder ALS collectors (4.6 m² net) at Konya's ≈1,750 kWh/m² annual irradiation yield ≈ 2,600 kWh/year after system losses — an ≈85% solar fraction against ≈3,000 kWh annual demand including storage and circulation losses, with a 300 L tank (65 L/m²) buffering the day-night shift. The same house in Munich would fit a third collector and accept ≈65% coverage.
Frequently asked questions
Is it better to oversize the collectors slightly?
No — oversizing shows up as summer stagnation, which ages fluid and components. If in doubt between two and three collectors, decide by winter target coverage, and manage summer with correct tank sizing.
Can I add collectors later?
Yes, if the tank has the exchange surface and volume headroom, and pipe dimensions allow the extra flow. Planning the tank one size up front makes later extension cheap.
Does the calculation change for a thermosiphon system?
The physics is identical; thermosiphon kits simply come pre-matched (e.g. 150 L/1 collector, 200 L/1–2, 300 L/2). Pick the kit whose per-person storage matches the household.