Cognitive radio (CR) is a novel approach to improving the spectral efficiency of licensed radio frequency bands by opportunistically accessing unused portions of the band without introducing interference to a licensed user. To reliably identify unused portions in a dynamic environment, a collaborative spectrum sensing (CSS) approach is known to be advantageous. In this article we will consider a weighted energy fusion scheme for secondary users with different sensing channel conditions and to analyze the performance we suggest a numerical approach by utilizing a result from probability density function of the weighted sum of noncentral chi-square random variables are used. Simulation experiments are provided to confirm the viability of the proposed numerical approach.