Now we turn our attention to the key problem: variability. The sun does not always shine, and the wind does not always blow. But we need to go further than making such generic statements. We want to understand and apply the metrics used to quantify this variability: time series, capacity factors, and (in the exercises) load duration curves.

Key takeaways:

• Renewable electricity generation varies at different time scales, from short-term minute-to-minute weather changes, to seasonal fluctuations, and even longer-term shifts in weather patterns (e.g. due to climate change)
• The capacity factor (CF) is a single number summarising the variability of an electricity generation technology for a specific time period
• Capacity factors are a straightforward way to compare the variability of different technologies across different times or places