Firm capacity is defined as the product of plant efficiency and gross plant capacity.
In case of power utilities and in field of electrical power and energy engineering, the firm capacity is specified by a supplier and it is the power that is intended to remain available 24/7 during the period covered by commitment.
The firm power and the primary indicator of reliability of a power plant. The firm power of a power plant is an indicator of stability and reliability and it is the mean power output during a critical period of time.
Example: At a 50 m high head, 84% efficient hydropower plant has a minimum run-off of 85 m³/sec. Calculate the firm capacity of the power plant.
Water weight available (W) = 85 * 1000 = 85000 kg/sec
Available water head = 50 m
Gross capacity (Work done/sec) = W * H = 85000 * 9.81 * 50 watts = 41692500 Watts = 41693 X 10³ W = 41693 kW
Firm capacity = Plant efficiency * Gross Plant Capacity = 0.84 X 41693=35022 kW