In this work, the performance degradation in a Wavelength Division Multiplexing (WDM) receiver system is analyzed in terms of power penalty considering the presence of multiple component crosstalk sources. In conventional way, power penalty is calculated considering the presence of all crosstalk sources simultaneously (i.e. worst case) which is not reality in practice and leads to unrealistic estimation. Here a probabilistic approach is adopted which considers all possible probabilistic appearances of the interfering channels and their realistic impact while calculating Bit Error Rate (BER) and penalty. Power penalty considering the presence of signal-crosstalk beat noise is formulated and the effect of the variation of number of interfering channels and crosstalk level are studied for a fixed thermal noise and photocurrent setting of the receiver. In addition the obtained results are compared with the existing worst case analysis model. © 2017 IEEE.