It matters whether a capacity margin is constituted by nuclear generation capacity, gas- or oil-fired capacity that can be brought on-stream at will or intermittent renewables. Renewable energy that constitutes a growing share of generating capacity in OECD countries might instead not be available when needed. The intermittency of renewable energies (especially wind and solar) which are governed by the weather and not by the needs of electricity consumers creates serious stress for power generation infrastructures and has lead to the distinction between gross capacity margins and “reliably available capacity”. A power system must provide stable amounts of baseload power and cover peak demand at all time.