– The term 'Firm power' as it applies to the area of energy can be defined as ' Power or power-producing capacity, intended to be available at all times during the period covered by a guaranteed commitment to deliver, even under adverse conditions'.
The firm power output is decisive for the installed capacity in design, and represents the reliability of the power generation when the power plant is put into operation. To improve the firm power, the whole generation process needs to be as stable as possible, while the maximization of power generation requires a rapid rise of the water level at the beginning of the storage period
Firm power can be used as an indicator for the reliability of power generation. Firm power is the mean power output in a certain critical period. The critical periods are distinct for different kinds of hydropower plants, for instance, run-of-river or daily regulated plants take a day as the critical period, and annually regulated plants take the dry season as the critical period.