Adequacy is related to the long run characteristics of an electrical system. In the last decade the long-term adequacy of the electrical system has been becoming an issue in many OECD countries due to a progressive slow-down in the investments in new dispatchable capacity, a growth in peak demand and increasing doubts about the ability of variable renewables to contribute to long-term system adequacy. The reliably available capacity takes into account the average availability of the power plant due to outages and maintenance shutdowns, the limitations to power output due to natural conditions (such as wind speed and direction, water temperature, precipitation and the solar irradiation factor) as well as the reserve capacity used for short-term balancing.