Recently, many research activities have targeted at reducing energy consumption of computer systems through software optimizations. By using the functions of dynamic voltage scaling (DVS) and turning off unused system units (TOSU), some software techniques can be used to reduce the wasted energy consumption, and one of the techniques is the compiler-directed method. Many hardware details are complex and fuzzy about DVS and TOSU, and so the paper has established an analytic model for compilation. Based on analyzing the model, the paper gives the idea about localizing the use of system units. If the systems have functions of DVS and TOSU, localizing the use of system units is an effective method of low-power compilation optimization.