Solar Panel Efficiency: When It Matters on a Commercial Roof
Updated 23 October 2026 · SEO Dons Editorial
What does solar panel efficiency mean?
Efficiency is the share of the sunlight landing on a module that leaves it as electricity. Datasheets measure it at standard test conditions, which Trina words as "Irradiance 1000W/m2, Cell Temperature 25ºC, Air Mass AM1.5". Divide rated power by the module's area and by 1,000 W per m² and the printed figure appears. Jinko's 670 W Tiger Neo 3.0 measures 2,382 × 1,134 mm, or 2.70 m², so 670 ÷ 2.70 is 248 W per m², which is 24.8%.
Three efficiencies are quoted, and they are not interchangeable. Jinko describes cells that are "27% efficient" inside a module rated at 24.8%; the module figure is measured over the whole module area. Fraunhofer ISE reports laboratory cells up to 27.9% for mono-crystalline silicon, while "commercial silicon modules have improved in efficiency from about 17% to just under 25% over the past ten years"; the National Laboratory of the Rockies (NLR) keeps a chart of record research cells, a benchmark and not a product list. System efficiency adds the rest: PVGIS recommends a 14% system loss, set "considering the system losses and the losses due to ageing".
What efficiencies do current commercial datasheets show?
The table lists one module per row from the manufacturer's own sheet or product page, ordered by technology and not by merit. Models and power classes change, so the exact model in a quote is what counts.
| Technology | Example module | Power class (W) | Top efficiency (%) |
|---|---|---|---|
| PERC | Canadian Solar BiHiKu7 CS7N-MB-AG | 640–670 | 21.6 |
| TOPCon | Trina Vertex N TSM-NEG19RC.20 | 625–650 | 24.1 |
| TOPCon | Jinko Tiger Neo 3.0 66QL6 | 650–670 | 24.8 |
| TOPCon | Canadian Solar TOPBiHiKu6 CS6.3-66TB | 645–670 | 24.8 |
| n-type, architecture not named on the sheet | DMEGC DMxxxG12RT-B66HSW | 640–665 | 24.6 |
| HJT | Canadian Solar HiHero+ CS6.2-66HB | 620–645 | 23.9 |
| Back contact | LONGi Hi-MO X10 LR8-66HVD | 640–670 | 24.8 |
| Back contact | Aiko Comet 3N72 | 655–690 | 25.5 |
Within one model the figure depends on the power class: Jinko's 66QL6 sheet lists 24.06% at 650 W and 24.80% at 670 W on the same module. The spread among the TOPCon rows, 24.1% to 24.8%, is wider than the gap between the lowest of them and the HJT row (24.1% against 23.9%), so the model sheet predicts more than the technology name does. The TOPCon, HJT and PERC page compares the technologies themselves.
When does efficiency decide the system size?
Only when roof area is the limit. For example, take a hypothetical 1,000 m² of module face. At 21.6% (216 W per m²) that is 216 kWp, and at 24.8% (248 W per m²) it is 248 kWp, which is 14.8% more. PVGIS gives these yields per kWp for a fixed array at 10 degrees facing south, with its recommended 14% system loss:
| PVGIS point | kWh per kWp a year | 216 kWp (21.6%) | 248 kWp (24.8%) | Difference (kWh) |
|---|---|---|---|---|
| Southampton | 969 | 209,393 | 240,414 | 31,021 |
| Birmingham | 875 | 189,080 | 217,092 | 28,012 |
| Manchester | 807 | 174,236 | 200,049 | 25,813 |
| Glasgow | 777 | 167,841 | 192,706 | 24,865 |
Cost per kWh then depends on how much more the higher-efficiency system costs for the same roof. As a ratio, with no prices needed, the output ratio is 248 ÷ 216 = 1.148:
| Total cost of the 24.8% system relative to the 21.6% system | Cost per kWh relative to the 21.6% system |
|---|---|
| 1.05 | 0.915 |
| 1.10 | 0.958 |
| 1.148 | 1.000 |
| 1.20 | 1.045 |
Below a 14.8% cost premium the higher-efficiency roof delivers cheaper kWh, and above it does not.
The second case is when something other than the roof sets the size. Suppose an export limit or daytime demand caps the array at 150 kWp. At 216 W per m² that needs 694 m² of module face, and at 248 W per m² it needs 605 m². Efficiency saves about 90 m² of roof but produces the same energy. Export limits are covered on the G100 export limitation page and demand-led sizing on the sizing page; module formats are on the sizes and weights page.
What cuts efficiency on a real UK roof?
Heat comes first. PVGIS says: "Generally, the efficiency decreases with increasing temperature, and the strength of this effect depends on the PV technology." It adds that module temperature rises above the local air temperature when the sun shines. The coefficient on each datasheet is the percentage of power lost per degree above 25 °C. With a cell at 60 °C, 35 degrees above the test condition, the power lost is:
| Technology and examples | Pmax temperature coefficient (% per °C) | Power lost with the cell at 60 °C (%) |
|---|---|---|
| PERC (Canadian Solar CS7N-MB-AG) | -0.34 | 11.9 |
| TOPCon or n-type (Trina, DMEGC) | -0.29 | 10.2 |
| TOPCon (Jinko, Canadian Solar) and back contact (LONGi, Aiko) | -0.26 | 9.1 |
| HJT (Canadian Solar HiHero+) | -0.24 | 8.4 |
On a hot afternoon the PERC-to-HJT gap is 3.5 points. Most UK generation happens at much cooler cell temperatures, so the annual gap is far smaller; the TOPCon, HJT and PERC page works it through with PVGIS hourly data.
Low light is the second factor. PVGIS says efficiency tends to fall below about 400 W per m², and for a generic crystalline-silicon module it reports a combined temperature-and-low-irradiance loss of 4.4% to 4.7% a year at the four sites above. On the estimate worked through on the technology page, heat accounts for between 0.3 and 2 points of that at Birmingham and Southampton, depending on the module's coefficient and NMOT, so most of it is low-irradiance behaviour. Makers publish their own low-light figures: Aiko states a relative conversion efficiency of at least 97% at 200 W per m² for its TOPCon cells, which is a statement about its cells and not an independent test. Soiling and shading are separate losses, covered on the cleaning page and the shading page.
How should the efficiency line in a quote be read?
- Ask for the datasheet and the power class named, because efficiency changes between classes.
- Multiply efficiency by module area to confirm power per module, and check that module count times power equals the quoted kWp.
- Establish what limits the size (roof, demand or grid connection) before paying for efficiency.
- If the roof is the limit, compare installed cost per kWh on the same roof using the ratio above.
- Treat the temperature coefficient as a tie-breaker, because on UK sites it moves annual output by tenths of a percent.
The commercial PV panels page lists the other datasheet lines to check. An enquiry about a specific quote or roof can be sent through this site's form.
Sources
- Fraunhofer ISE: Photovoltaics Report, updated 14 July 2026, checked 22 October 2026.
- NLR: Research-cell efficiency chart, last updated 14 August 2026, checked 22 October 2026.
- European Commission Joint Research Centre: PVGIS data sources and calculation methods; PVGIS PVcalc output, Southampton; PVGIS PVcalc output, Birmingham; PVGIS PVcalc output, Manchester; PVGIS PVcalc output, Glasgow (PVGIS-SARAH3 data, fixed array at 10 degrees facing south, 14% system loss), checked 22 October 2026.
- JinkoSolar: Tiger Neo 3.0 page; JKM650-670N-66QL6-BDV datasheet, © 2026, checked 22 October 2026.
- Trina Solar: TSM-NEG19RC.20 datasheet (TSM_EN_2025_D), checked 22 October 2026.
- Canadian Solar: CS6.3-66TB datasheet v1.11; CS7N-MB-AG datasheet v2.8; CS6.2-66HB datasheet v1.5, checked 22 October 2026.
- DMEGC: DMxxxG12RT-B66HSW datasheet, document ID dated 15 September 2026, checked 22 October 2026.
- LONGi: LR8-66HVD datasheet page, checked 22 October 2026.
- Aiko: Comet 3N72 product page; Comet 3N72 datasheet, 202608 V1.1; Bifacial mono TOPCon cell page, checked 22 October 2026.
Common questions
Is a higher efficiency panel always worth paying for?
No. When roof area is the limit, higher efficiency fits more kWp. When demand, an export limit or grid capacity is the limit, it only reduces the roof used. Compare installed cost per kWh on the same roof, using the cost ratio in the worked example.
Why does efficiency differ between power classes of one model?
Every power class of a model has the same area, so efficiency rises with power. Jinko's 66QL6 sheet lists 24.06% at 650 W and 24.80% at 670 W on one 2,382 × 1,134 mm module. The power class named in the quote therefore decides which efficiency applies.
How do laboratory cell efficiencies compare with roof modules?
Fraunhofer ISE reports laboratory cells up to 27.9% for mono-crystalline silicon, against commercial silicon modules that have improved to just under 25%. A module figure is lower partly because it is measured over the whole module area, and lab cells are not production products.
Do panels lose efficiency on cloudy days?
A little. PVGIS says efficiency is nearly constant down to about 400 W per m² and tends to decrease below that. For a generic crystalline-silicon module it reports a combined temperature and low-irradiance loss of about 4.4% to 4.7% a year at four UK sites. Maker low-light claims are statements about their own cells.