The problem is considered of average power distribution

from the output of an inertial square-law detector that registers the random noise x(t) in the time interval (0,T), the noise possessing the properties of the steady-state normal Markov process of the first order (the Ornstein-Ulenbeck process). An expression is derived, describing the probability distribution density f_X (η) with the arbitrary value η, and generalized for the case of the detector with time-dependent registration efficiency . Analysis is made on the influence of efficiency on the probability distribution properties f_X (η). The asymptotic formula is given for the probability distribution density f_X (η).