Sunlight: Fluctuation in time
A steady star, a spinning planet and passing clouds
What this tab covers How availability changes over seconds, days, seasons, years and longer, how predictable it is, and what that means for the plants that use it.Sunlight follows an exact daily and seasonal cycle set by the Earth's rotation and orbit, overlaid by clouds. The Sun itself is steady: its output changes by only about 0.1% over an 11-year cycle.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Daily | Zero at night, rising to a peak around solar noon.3 | From 0 to about 1,000 W/m2 on a clear day. |
| Seasonal (high latitude) | Long summer days, short low-sun winter days.10 | Germany: best month gives about 4.4 times the worst month's PV output. |
| Seasonal (low latitude) | Day length and sun angle change little; monsoon clouds can matter.10 | South Africa ratio about 1.2; India above 1.6 because of the monsoon. |
| Solar cycle (11 years) | Sunspot cycle slightly changes the Sun's output.1 | About 0.1% change in total solar irradiance. |
How predictable it is
The position of the Sun is known exactly for any place and time, so the clear-sky daily and seasonal pattern is fully predictable. Clouds add weather-driven uncertainty that forecasters handle from minutes to days ahead.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| Solar PV, new plants worldwide, 2025 | 16%; 15% in 2010 (2024, IRENA)817 |
| Concentrated solar power, 2025 | 22%; 30% in 2010 (2024, IRENA); higher than PV because of thermal storage817 |
A solar panel runs at its rating only for a few hours around noon on clear days, so over a year it produces about 16% of what it would at full output all the time. Concentrated solar plants with heat storage can run into the evening; new plants built in 2025 averaged about 22%.
The seasonal swing grows with latitude. The World Bank defines a seasonality index as best month over worst month: about 1.2 in South Africa but about 4.4 in Germany, where the darkest winter month yields less than a quarter of the best summer month. In 150 countries with 86% of the world's population, the seasonal gap is below a factor of two. This is why high-latitude systems need seasonal backup, while sunny low-latitude systems mainly need overnight storage.
Sources
- Solar Irradiance: Science NASA Goddard Space Flight Center, 2024
- Solar energy to the Earth Energy Education, University of Calgary, 2024
- Renewable Power Generation Costs in 2025 (Executive summary, Figure S1) IRENA, 2026
- Global Photovoltaic Power Potential by Country (full report) World Bank / ESMAP / Solargis, 2020
- Renewable Power Generation Costs in 2024: Summary (Table S1) IRENA, 2025