Solar heat collectors: Efficiency
Brilliant in the sun, idle at night and in cloud
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Turning sunlight into warm water is far easier than turning it into electricity: a standard glazed collector captures about two thirds of the light striking it under test conditions. Over a year the world's collectors deliver only about 9% of their rated output, because of night, cloud and times when no heat is needed.
Chart: Efficiency ranges
Theoretical limit Commercial In serviceEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Optical ceiling of a glazed collector | 75.6 to 85.5%1 | Theoretical limit | Cover-glass transmittance (0.84 standard to 0.95 anti-reflective) times absorber absorptance of 0.9 (calculated); sunlight reaching the absorber before any heat loss |
| Peak efficiency of reference collectors (no heat loss) | 76 to 90%6 | Commercial | Zero-loss efficiency: evacuated tube 0.76, glazed flat plate 0.78, unglazed 0.90 (averages of EN-tested collectors) |
| Efficiency at rating conditions | 67.1 to 71.7%6 | Commercial | 1,000 W/m2 sunlight, 20 C air; glazed flat plate 67.1% and evacuated tube 71.7% at 50 C fluid, unglazed 70% at 30 C fluid (basis of the 0.7 kWth per m2 convention) |
| Efficiency with fluid 60 C above air temperature | 53 to 66%6 | Commercial | Same reference collectors at 1,000 W/m2: flat plate about 53%, evacuated tube about 66% (calculated from the published loss coefficients) |
| Trough and Fresnel process-heat collectors | 50 to 56%2 | Commercial | Rated output 0.50-0.56 kW per m2 in Europe against 1,000 W/m2 (calculated); heat up to 400 C |
| Average capacity factor of the world fleet | 9.3%3 | In service | 443 TWh delivered in 2024 by 544 GWth of collectors, divided by full output all year (calculated) |
| Capacity factor of industrial solar heat plants | 4 to 29%2 | In service | About 4% (Japan), 16-20% (UAE, India), 29% (Mexico) |
Where the energy is lost
| Step | Loss |
|---|---|
| Reflection and absorption in the cover glass | 5-16% of incoming sunlight (calculated from transmittance of 0.84-0.95)1 |
| Light reflected by the absorber | under 10% (absorptance above 0.9)1 |
| Heat lost from the hot absorber | rises with temperature: about 3.2 W per m2 per degree for flat plates, 1.2 for evacuated tubes, 20 for unglazed mats6 |
A collector's efficiency falls as its working temperature rises above the outside air, because a hotter absorber loses more heat. That is why unglazed mats suit swimming pools, flat plates suit domestic hot water, and vacuum tubes or mirrors are needed for steam. The fleet capacity factor is low partly because systems are sized for summer and sit idle when tanks are already hot.
Sources
- Solar Collectors (workshop lecture, W. Weiss, AEE INTEC) Centre for Renewable and Sustainable Energy Studies, Stellenbosch University, 2013
- Solar Heat for Industrial Processes: Technology Brief E21 IRENA and IEA-ETSAP, 2015
- Solar market trends in a nutshell (Solar Heat Worldwide 2025 webinar, M. Spork-Dur, 24 June 2025) IEA Solar Heating and Cooling Programme / AEE INTEC, 2025
- Technical Note: New Solar Thermal Statistics Conversion (reference collector parameters, 0.7 kWth per m2) IEA Solar Heating and Cooling Programme, 2004