Wireless mobile ad hoc networks depend on batteries or other fatiguing means for their energy. Power aware design and estimation of transmit power requires familiarity with the energy consumption behavior in wireless systems. Moreover, selection of gateways has not been introduced thoroughly. This paper introduces a mathematical analysis for power adjustment and then we use these equations in selecting gateways in the simulation experiments according to two different criteria: the gateway with the highest energy level and the gateway with the least number of neighbors.