Reduction of power consumption is one of an important issue in wireless communications because most mobile devices are battery-driven. There are many power saving techniques for use in a wireless LAN (WLAN) environment at the hardware level and at the MAC protocol level. Their common strategy for power saving is to stay in sleep mode, which consumes very little energy, for as long as possible when no data is being transmitted or received. For effective power saving, therefore, it is important to understand the behaviors of the transport layer protocols used by upper-layer applications, since packet transmission and reception timing are mainly determined by those behaviors. In this paper, we propose a power consumption model based on a treatment of detailed TCP behaviors within a WLAN environment. Our model considers both the TCP-level and MAC-level behaviors of a wireless client. Comparing the model with and without ideal sleeping, we analyze the power consumption of a single wireless client as it sends data to a wired host by TCP. From the numerical results of our analysis, we show the lower-bound for power consumption in upstream TCP data transfer with ideal sleeping. We also discuss the tradeoff between power saving and network performance in TCP data transfer.