As Taiwan has a relatively small area with a dense population, small retail businesses make up the largest proportion of energy consumers in the state. Therefore, the smaller air conditioners such as window-type, split-type, and packaged ones are widely used in the stores. Most store owners often set at lower temperature for the air conditioners to attract customers into their stores. But, such low temperature would cause people to feel uncomfortable either, not to mention the redundant power consumption. According to statistics, air conditioners account for more than 60% electricity usage in retail stores. This paper proposes a control strategy for small air conditioners in tropical areas based on a thermal comfort improvement model. Optimal control is done through the calculation of indoor enthalpy difference using the thermal comfort improvement model. An air-conditioner control system is developed. The main device of the control system communicates with sensors and controllers via wireless 2.4GHz technology. Indoor information is gathered from sensors, and control actions are sent to the air-conditioners after the calculations are done. Human thermal comfort is well considered in the control strategy. The system is used to perform experiments in real stores and the power consumption of air conditioners was reduced by more than 20%.