this paper, we present a systematic analysis on the energy consumption of specific implementation of a WSN for rural areas. The implementation uses extremely low cost commercially available devices and transceivers operating in the 240 to 930 MHz thus covering the main sub-mega Hz ISM bands. The main objective of our analysis is the minimization of the energy consumption for a fairly generic type of WSN similar to the one deployed and tested in a rural area in Peru. The study is based on practical considerations at the design and implementation stage and the use of extremely low cost devices and radio transceivers available in the market. Our study provides an accurate characterization of the Intra cluster total energy consumption in terms of all parameters of interest of our actual WSN implementation. We have determined the optimum values for adjusting the transmission power for realistic scenarios and commercially available low cost devices. In addition, we have also characterized the actual energy consumption of our WSN using commercially available devices and compare them to their optimum values. As a result of our systematic analysis of the energy consumption we improved the antennas, reduced cable losses, improved antenna impedance matching, used elevated fixtures to place the various nodes to reduce path losses, and used appropriate operating frequencies.