Capacity Factor

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Capacity factor is a crucial concept for understanding the potential output achievable by a power generator. Imagine a perfectly reliable device that runs 24 hours a day, 365 days a year at its full rated capacity. That device has a capacity factor of 100%. In the real world, the capacity factor is always less than 100, sometimes much less. Reasons might be:

  • Mechanical breakdown or shutdown for maintenance
  • Lack of demand for the electricity generated
  • Intermittent or interrupted energy source
  • Variability of the energy source

Solar Photo-voltaic in the UK has a capacity factor of around 11% while wind power in the UK has a capacity factor in the low 30's % [1]
The naïve solution of building three times as many wind farms - or nine times as many solar PV installations won't work because of the 'Just In Time' nature of our national electricity grid


  1. Wikipedia [1]