To enhance the transmission efficiency of wireless communications systems, there is extensive research going on in the field of Cognitive Radio network along with the improvement in antenna technology. Use of multiple RF devices for various applications increases the need of multiple antennas at the transceiver which results in more computational complexity and large power consumption at high data rates. Incorporating the spectrum sensing abilities of Cognitive Radio for higher spectrum efficiency along with MIMO technology increases the need to determine the optimum power levels for the antennas at which data transmission takes place. To overcome the challenges for optimizing energy efficiency, we propose an efficient algorithm to compute the transmit-power that is to be allocated using water filling method and antenna subset selection schemes. For this, an antenna selection algorithm is proposed which has lesser computations to be performed and the results of the channel capacity are compared with that of exhaustive search for various antenna configurations. © 2016 IEEE.