Allow 50–70 litres of storage per m² of collector aperture for domestic hot water, with 60 L/m² as the planning figure across most of Europe. A 4 m² flat plate array therefore pairs with a 240–280 litre cylinder. Evacuated tubes sit lower at 40–50 L/m²; combi systems that also support heating need 80–100 L/m².
How many litres of tank per m² of collector?
Sixty litres per m² of aperture area is the working rule for domestic hot water, within a sensible band of 50–70 L/m². The figure recurs across European design practice: German planning guidance settles near 60 L/m², general solar engineering references quote 50–70 L/m², and UK guidance sets a floor of 25–35 litres of dedicated solar volume per m² of net collector area — a minimum rather than an optimum.
Apply it to real arrays and the numbers are immediately recognisable:
| Collector aperture | Flat plate, DHW (60 L/m²) | Evacuated tube (45 L/m²) | Combi, heating support (80–100 L/m²) | Suits |
|---|---|---|---|---|
| 2 m² | 120 L | 90 L | — | 1–2 people |
| 3 m² | 180 L | 135 L | 240–300 L | 2–3 people |
| 4 m² | 240 L | 180 L | 320–400 L | 3–4 people |
| 6 m² | 360 L | 270 L | 480–600 L | 5–6 people |
| 10 m² | 600 L | 450 L | 800–1,000 L | Guesthouse, small apartment block |
Solimpeks pairs its TSM thermosiphon systems with 120–350 litre tanks and offers Solibuffer stores from 50 to 1,000 litres for exactly this range of arrays.
Why does the ratio matter?
Because it decides whether the collector's harvest is captured or thrown away — and both errors are expensive.
Too little volume and the store hits its temperature limit by early afternoon on a clear day. The pump stops, the collector climbs toward its stagnation temperature, and the glycol cooks. Undersized storage is a leading cause of premature fluid degradation in solar thermal systems, and it caps the achievable solar fraction no matter how many collectors are on the roof.
Too much volume and the store never reaches a usable temperature. Spreading a modest harvest through 800 litres yields a tank at 35 °C — useless for showers, and every extra litre adds standing loss 365 days a year. Oversizing also destroys stratification, the layering that lets the top of the tank be usable while the bottom stays cold enough to keep the collector efficient.
Should I size from the collector or from the household?
From both, and take the larger. The two calculations answer different questions:
- Household demand. Plan 30–50 litres per person per day. A family of four needs 120–200 litres, which is the volume required for comfort regardless of solar.
- Collector harvest. Multiply aperture area by 60 L/m². This is the volume required to absorb a sunny day without stagnating.
- Take the greater of the two, then round up to the next standard tank size.
A useful cross-check is the time-shift: a well-sized solar store holds 1.5 to 2 times daily demand, enough to carry a sunny day's harvest through 24–36 hours of cloud. That is what turns a 40% solar fraction into a 60–70% one. Collector sizing itself is covered in how many solar collectors do I need.
What changes for tubes, combi systems and thermosiphons?
- Evacuated tubes: 40–50 L/m². Their aperture harvests more per m² but at higher temperature, so the litres-per-m² figure quoted must state which area it refers to — aperture, absorber or gross. Quotes that mix these are not comparable, a point covered in flat plate vs evacuated tube collectors.
- Combi systems supporting space heating: 80–100 L/m², because the store now serves two loads with different temperature requirements and needs stratified zones for each.
- Thermosiphon systems: the ratio is fixed by the manufacturer's matched package, typically 60–80 L/m², and should not be altered in the field.
- Pre-heat configurations, where solar feeds an existing cylinder: size the solar volume on the collector rule and treat the existing cylinder purely as the final-stage heater.
How do I check a quotation?
Divide the proposed tank volume by the collector aperture area. Below 40 L/m², the system will stagnate on clear summer days and its glycol will age fast. Above 90 L/m² for a DHW-only system, the tank will rarely reach useful temperature. Anything between 50 and 70 is a system designed by someone who has done this before. Store performance itself is characterised under EN 12977-3, which is the standard a serious supplier will quote.
Frequently asked questions
How many litres of tank per m² of solar collector?
Sixty litres per m² of aperture area is the standard planning figure for domestic hot water, within a band of 50–70 L/m². Evacuated tubes sit at 40–50 L/m², and combi systems supporting space heating need 80–100 L/m².
What size tank for 4 m² of solar collectors?
A 240–280 litre cylinder for domestic hot water, which also matches a family of four's daily demand. If the system also supports space heating, size at 320–400 litres with stratified zones for the two loads.
What happens if my solar tank is too small?
The store reaches its temperature limit by early afternoon, the pump stops and the collector climbs toward stagnation. Repeated stagnation degrades the glycol quickly and caps the achievable solar fraction regardless of collector area.
Can a solar tank be too big?
Yes. Spreading a modest harvest through too many litres leaves the store at 35 °C — warm but not usable — while every extra litre adds standing loss all year. Above roughly 90 L/m² for hot water only, extra volume costs more than it returns.