Passive solar and natural cooling: Efficiency
Measured by what machines you no longer need
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.Passive design is judged by how much machine heating, cooling and lighting it avoids rather than by a conversion efficiency. Passive cooling measures can cut a building's cooling energy by 25-40%, evaporative coolers use about a quarter of the electricity of air conditioning, and a measured Trombe wall supplied a fifth of a building's heating.
Chart: Efficiency ranges
Commercial In serviceEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Window glass transmittance | 84 to 95%1 | Commercial | Sunlight passing through one pane: standard glass 0.84, low-iron 0.91, anti-reflective 0.95 |
| Cooling energy saved by passive techniques | 25 to 40%2 | In service | Reduction in a building's cooling energy from shading, insulation, ventilation and similar measures |
| Evaporative cooler effectiveness | 70 to 95%3 | Commercial | How far air is cooled towards the wet-bulb temperature: aspen pads about 70%, thick media 75-93%, two-stage indirect/direct units 95% or more |
| Electricity of evaporative coolers relative to central air conditioning | 25%3 | Commercial | About one quarter of the energy for the same cooling job |
| Trombe wall efficiency, Zion National Park Visitor Center | 13%6 | In service | Heat delivered indoors divided by sunlight striking the wall, 2001-2002 heating season; the wall supplied 20% of the building's heating |
| Electricity saved by seawater air conditioning | 83 to 94.5%5 | Commercial | Feasibility estimates for Grenada, Jamaica and the Dominican Republic against chillers |
Where the energy is lost
| Step | Loss |
|---|---|
| Evaporative cooling in humid air | temperature drop falls from 27 F (15 C) at 10% humidity to 11 F (6 C) at 50% humidity (Celsius calculated)3 |
| Window glass reflects and absorbs sunlight | 5-16% per pane (calculated)1 |
Glazing and thermal mass collect and store solar heat, so their benefit depends on climate and use. Cooling measures exploit natural heat sinks: evaporating water (only in dry air), the cold night sky, the stable temperature of the ground and cold deep seawater. They are best compared with the electricity an air conditioner would otherwise use. Measures in other units (earth-tube air warming in degrees, radiative cooling in watts per square metre) are given under Techniques.
Sources
- Solar Collectors (workshop lecture, W. Weiss, AEE INTEC) Centre for Renewable and Sustainable Energy Studies, Stellenbosch University, 2013
- Buildings Decarbonisation Report: Summary, Cooling Energy Transitions Commission, 2025
- Evaporative Cooling Systems (Building America Solution Center) Pacific Northwest National Laboratory / US Department of Energy, 2024
- Sustainable cooling: a deep dive into seawater air-conditioning solutions for the Caribbean World Bank Blogs, 2024
- Trombe Walls in Low-Energy Buildings: Practical Experiences (Torcellini and Pless, NREL/CP-550-36277) National Renewable Energy Laboratory, 2004