Aperture area is the opening through which solar radiation enters a collector — for a flat plate, the visible glass area inside the frame, typically 90–93% of gross area. Since the 2013 revision of ISO 9806, collector performance and Solar Keymark datasheets are referenced to gross area instead, so always check which area a quoted efficiency figure uses before comparing collectors.
What is the difference between gross, aperture and absorber area?
Every solar collector has three areas, and they can differ by more than 10%. Gross area is the outer envelope — length times width including the frame. Aperture area is the opening that admits light: the glazing inside the frame of a flat plate, or the projected clear openings of an evacuated tube array. Absorber area is the physical absorber sheet or fin surface itself.
| Area | Definition | Example flat plate |
|---|---|---|
| Gross | Outer dimensions including frame | 2.40 m² |
| Aperture | Light-admitting opening | 2.23 m² |
| Absorber | Absorber sheet surface | 2.20 m² |
The same physical collector therefore has three possible denominators for its efficiency — which is exactly why the reference area must be stated with every number.
Which area do ISO 9806 and Solar Keymark use?
Gross area is the current reference. EN 12975 and early Solar Keymark practice reported efficiency per aperture area, but the international test standard ISO 9806 harmonised reporting on gross area from its 2013 edition, and Solar Keymark datasheets followed, completing the switch in 2016. Gross area was chosen because it is unambiguous — anyone with a tape measure gets the same value — and because it reflects the roof space the collector actually occupies. Older datasheets and brochures still circulate with aperture-based values, which look systematically better for the identical product.
Why does the reference area change the efficiency numbers?
Because the energy output stays the same while the denominator grows. A flat plate delivering 1,600 W from 2.23 m² of aperture under 900 W/m² of irradiance shows η0 ≈ 0.80 on aperture but ≈ 0.75 on its 2.40 m² gross area. The effect is far larger for evacuated tube collectors: their gross rectangle includes the empty gaps between tubes, so a tube collector with a headline aperture efficiency of 0.75 may certify at only 0.45–0.65 per gross area. Comparing one manufacturer's aperture figure against another's gross figure is the single most common mistake in collector selection — the collector efficiency curve parameters η0, a1 and a2 only compare fairly on the same area basis.
How is collector area used in sizing and subsidies?
Subsidy schemes and sizing rules attach real money to these definitions. Italy's Conto Termico 3.0 computes its grant per square metre of collector; the scheme now admits solar thermal installations up to 2,500 m² of collector area. France's MaPrimeRénov' sets its own area and certification conditions for PVT collectors. Sizing rules of thumb — such as 1–1.5 m² of flat-plate collector per person for domestic hot water — traditionally refer to aperture area; the practical arithmetic is covered in How many solar collectors do I need? For any application or subsidy file, quote the area exactly as the scheme defines it, from the Solar Keymark datasheet — Solimpeks lists gross and aperture areas on the datasheet of every Wunder collector model, so planners can read the correct value directly rather than deriving it.
Frequently asked questions
Which area should I use when comparing two collectors?
Use the same reference for both — gross area, since that is what current Solar Keymark datasheets certify. Better still, compare the datasheet's annual energy output per collector module at your reference climate, which removes the area question entirely.
Why do evacuated tube collectors show low efficiency per gross area?
Their gross rectangle includes the empty space between tubes, which collects nothing. Per aperture the tubes test well; per gross area the gaps drag the figure down to typically 0.45–0.65. Neither number is wrong — they answer different questions.
Is aperture area the same as absorber area?
No. Aperture is the light-admitting opening; the absorber is the metal surface behind it. In a good flat plate they differ by only a few percent, but in tube collectors with internal fins the difference can be substantial.
Does subsidy paperwork use gross or aperture area?
It varies by scheme, so check the definition in the application rules. Italy's Conto Termico pays per square metre of installed collector, while France's MaPrimeRénov' sets its own conditions for PVT — the certified datasheet states both values.