A higher η0 does not automatically make one collector better than another, and a lower a1 proves very little once it is lifted out of its testing context. Learning how to read a Solar Keymark datasheet in the right order is what separates a defensible collector selection from a spreadsheet exercise.

The order matters: confirm the certified model and its reference area, identify the test method, interpret η0, a1 and a2 as one curve, and only then compare annual thermal yield at the climate and operating temperature the project actually runs at.

A Solar Keymark datasheet is not a certificate attachment. It carries a performance model that describes how the collector behaves as irradiance and operating temperature change. Solar Keymark itself frames the certified performance measurements and calculations as a way for designers, planners and developers to compare products and find the better fit for specific conditions — not as a scoreboard.

The Soletks STK-FPC series is used below as a worked example, because reading real numbers is more instructive than defining symbols.

1. Start Here: Five Checks Before You Compare Two Solar Keymark Datasheets

Before any coefficient goes into a comparison table, establish whether the two datasheets are comparable at all.

Verify the exact certified collector model

Certification should be verified at product level, not inferred from a supplier's general claim. The official Solar Keymark database lists certified products together with links to their performance datasheets, which gives a buyer a direct way to confirm that the model in the quotation is the model that was tested.

So the procurement question is not "does your factory have Solar Keymark?"

It is: "send the Solar Keymark certificate and the performance datasheet for the exact collector model in this quotation."

That distinction gets sharper when a manufacturer offers several collector sizes, absorber constructions, or OEM variants under one family name. Warning signs include a certificate that covers a different area size than the one quoted, a datasheet with no traceable model designation, or a supplier who sends a certificate image but avoids sending the annex with the performance data.

If certification basics are still unclear internally, review the Solar Keymark certification basics before the technical review meeting so everyone is reading the same document structure.

Check the reference area

This is the detail most often skipped. Performance coefficients belong to a defined collector area, and Solar Keymark documentation is explicit that the area and the performance parameters must be used together. Aperture area — the area through which radiation enters the collector — is commonly the basis.

Do not copy η0, a1 and a2 into a comparison sheet until you have confirmed both datasheets report on the same area basis. Gross-area and aperture-area figures for the same physical collector are not interchangeable.

Identify the test method

Look for whether thermal performance was determined by the steady-state method or by quasi-dynamic testing. ISO 9806 recognises both. They are treated as valid characterisation routes, but they identify coefficients through different models, which is why a raw coefficient-to-coefficient comparison across methods is weaker than it looks. This point returns in Section 4.

Check the operating condition behind the number

Collector performance moves with the difference between mean fluid temperature and ambient air temperature:

A collector delivering water a few degrees above ambient is in a completely different thermal state from one producing usable hot water in cold weather. A performance figure quoted without its ΔT is close to meaningless for project work.

Compare output under identical assumptions

Annual yield numbers are only comparable when the inputs behind them match. At minimum, confirm: location and climate dataset, collector operating temperature, reference area, orientation and tilt assumptions, and calculation method.

This is why yield belongs at the end of the review sequence, not the beginning.

2. Anatomy of a Solar Keymark Collector Datasheet

The document looks dense, but almost everything a project buyer needs falls into four blocks.

Worked example: reading the Soletks STK-FPC-2.00 header values

Neither of the first two figures should be read alone. The useful question is not whether 0.774 counts as "high efficiency." It is what the combination of optical efficiency and heat-loss behaviour produces at the temperature the project's system actually operates at.

That leads directly to the three coefficients most comparisons hinge on.

3. η0, a1 and a2 Describe One Efficiency Curve, Not Three Rankings

Solar Keymark expresses collector power output as a function of irradiance, collector area, and the temperature difference between the collector fluid and ambient air, in the general form:

Read plainly: η0 sets the starting point, and a1 and a2 describe how quickly that starting advantage is eroded as the collector works above ambient temperature.

η0: optical efficiency and its limits as a selection criterion

η0 is the optical efficiency — the combined effect of the transparent cover and absorber in converting incoming radiation to heat before temperature-dependent losses are counted.

The first lesson from this table is not that the highest η0 wins. Vaillant sits slightly above Soletks here. A buyer who stops at this column has decided the case on the one condition — near-ambient operation — that most real DHW and heating systems spend the least time in.

a1: first-order heat loss

The a1 coefficient describes the first-order component of heat loss as ΔT increases. Lower generally means less temperature-dependent loss within the conditions the test model represents.

Against the GREENoneTEC range in this comparison set, the reported Soletks a1 is roughly 47 – 57% lower.

That is a large difference in the reported coefficient. It is not a licence to claim "47 – 57% less real-world heat loss." There is a second-order term still to account for, and a methodological issue underneath it.

a2: why the ranking shifts as operating temperature rises

The a2 coefficient carries the temperature-dependent second-order component of heat loss. Because it acts on ΔT², its weight grows as the collector operates further above ambient. Solar Keymark's own parameter guidance notes that a higher a2 indicates a stronger temperature influence on the efficiency curve.

No single coefficient ranks a collector for every application. Domestic hot water, space heating and higher-temperature process loads place the same collector at different points on its own curve. Where the required ΔT is consistently high, the question also widens into collector technology, which is covered separately in this comparison of flat plate versus evacuated tube collectors.

4. Why Some a1 Values Cannot Be Compared Directly

This is where the Soletks figures need honest framing.

Steady state vs quasi-dynamic: what changes

The 1.64 W/(m²K) value was obtained by the steady-state method. Some competing datasheets report coefficients derived from quasi-dynamic testing. Both are recognised under ISO 9806, but they fit collector behaviour with different models and different treatment of diffuse radiation and thermal capacity effects.

The claim buyers should refuse to make

A weak comparison sounds like this: Collector A has a1 = 1.64, Collector B has a1 = 3.5, therefore A loses less than half the heat.

That conclusion outruns the evidence. Coefficients should be interpreted inside their own performance equation, alongside the other terms, not extracted and ranked in isolation.

This is one of the reasons ScenoCalc exists. Solar Keymark's official calculation tool is designed to evaluate collectors under different weather conditions, and it can process performance data from both steady-state and quasi-dynamic tests — which makes it a fairer basis for cross-supplier comparison than any single coefficient.

5. IAM, Kd, Stagnation Temperature and Working Pressure as Project Requirements

Once the efficiency curve is understood, the remaining parameters should be mapped to specific project questions rather than filed as miscellaneous data.

These four values vary across models within a single flat plate collector range, so they have to be read per model rather than per manufacturer. A collector rated above the system requirement adds nothing if the weakest component in the circuit sets the real limit.

On larger commercial arrays, where array pressure drop, header sizing and mounting loads become design drivers, an engineered flat plate collector configuration is usually the more relevant reference point than a standard residential-scale model.

6. Annual Yield: Where the Real Comparison Happens

Once the datasheets are normalised, annual energy output becomes the most decision-relevant number on the page.

What ScenoCalc actually does

ScenoCalc — Solar Collector Energy Output Calculator — is a free spreadsheet-based tool published through Solar Keymark for calculating annual collector energy output, with monthly results at defined operating temperatures and climate datasets. It is used in solar thermal product certification, which is precisely why it works as neutral ground between two competing suppliers.

It replaces a weak question with a strong one. Not "which collector has the best η0?" but "which collector produces more modelled thermal energy in my climate, at the temperature my system needs?"

Worked example: Athens and Davos at ΔT = 25 K

The Davos figure represents a 20.5% improvement over the previous-generation BTE design, and it is not directly comparable with the Athens result: different location, different climate dataset, and in this case a different collector generation.

The transferable lesson is bigger than either number: compare collectors at the climate and operating temperature relevant to the project, not at one universal "efficiency" condition. A collector that leads in a Mediterranean DHW scenario will not necessarily lead in an Alpine or Northern European heating scenario, and vice versa.

This is also why a slightly lower η0 can still finish ahead on annual kWh when the loss curve is more favourable.

What annual collector yield does not promise

Annual collector yield is standardised modelled collector output. It is not a guarantee of delivered useful heat from the installed system.

Real output is also shaped by hydraulic design, store volume and stratification, pipe and connection losses, control strategy, orientation and tilt, shading, flow rate, and the load profile. A collector comparison tells you which panel to buy. Translating modelled yield into delivered hot water is a separate exercise, and the logic behind sizing a solar water heating system sits closer to the load profile than to the datasheet.

7. A Six-Step Method for Comparing Any Two Solar Keymark Collectors

1. 1Verify the exact certified model. Match quotation, certificate and Solar Keymark performance datasheet to the same model designation and size.

2. 2Check the reference area. Confirm both sets of coefficients sit on the same area basis, normally aperture area.

3. 3Identify the test method. Note steady state or quasi-dynamic before drawing any conclusion from differences in a1 and a2.

4. 4Read η0, a1 and a2 together. Think in terms of one efficiency curve at the project's ΔT, not three independent rankings.

5. 5Compare annual yield under identical conditions. Same location, same climate dataset, same operating temperature, same area basis. ScenoCalc is built for exactly this kind of normalised comparison.

6. 6Return to the project constraints. Working pressure, stagnation behaviour, dimensions and mounting, hydraulic integration, and destination-market documentation requirements.

The winner of that process is often not the collector with the highest single coefficient. It is the one that delivers competitive modelled output at the project's real operating point while fitting the hydraulic, safety and certification requirements without redesign.

A supplier worth shortlisting should be able to walk through all six steps with you using its own documentation. If a manufacturer can produce a glossy efficiency figure but not the matching certified datasheet, the reference area, or the test method behind it, that gap is the answer. The wider commercial checks — factory capability, documentation depth, OEM handling — are set out in this guide on how to choose a solar thermal supplier.

For distributors and EPC contractors comparing several certified options, Soletks can supply the relevant flat plate solar collector datasheets and support a like-for-like technical evaluation based on project location, required operating temperature and available collector area. Available model configurations, documentation packages and delivery terms depend on the destination market and confirmed specifications.

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