Power interruptions are frequent in many developing countries, adversely affecting residences and small businesses alike. Design of a Low-Cost Back-to-Back MOSFET-Based Automatic Transfer Switch for Single-Phase Residential and Business Loads is to mitigate these interruptions, but many existing designs are expensive or rely on mechanical switching devices with relatively slow response. This paper addresses lack of affordable ATS for single-phase residential users, and limited solutions optimized for mixed residential-business loads. A low-cost microcontrollercontrolled ATS using back-to-back MOSFETs for automatic switching is proposed. Experimental results indicate transfer times under 50μs, negligible voltage sag, and stable operation up to 90% of rated load.