Recently, user cooperation aided traffic forwarding is widely considered as one of promising techniques to improve energy efficiency in battery-operated mobile terminal (MT). This study tries to theoretically analyze the power consumption of such kind of technique in cellular networks over frequency selective fading channels. We initially derive some expressions to calculate transmission power for non- and cooperative scenarios by a generalized model, in which spatially correlated shadowing, an easily overlooked issue, is considered. Then we mathematically analyze consumed power in aforementioned scenarios with the help of a recent and experiment based Long Term Evolution smartphone power model. Numerical results indicate that the benefits of cooperative forwarding over frequency selective fading channel are substantial, however, due to several physical constraints on MT, such as maximal transmission power, the benefits cannot be fully achieved in realistic environment. Several interesting insights about cooperative forwarding are also obtained in our study.