In this paper, the energy performance of dynamic instead of static shading devices is investigated for prototypical US office spaces. Specifically, six dynamic shading systems are considered to assess their ability to reduce heating and cooling thermal loads, total building energy use, and electrical peak demand. The most promising dynamic shading is the rotating overhang and is evaluated further using a wide range of design and operation conditions. It is found that rotating overhangs can provide effective and easily controllable dynamic shading systems for windows to reduce the energy consumption of US office buildings by up to 39% especially for mild US climates. Moreover, the rotating overhangs have the added benefit to support PV panels for on-site electricity generation. A series of sensitivity analysis results indicate that the performance of the dynamic external shading systems depends on its depth and orientation, the location climate, the window size, and the glazing type have significant impacts. For instance, the dynamic shading device can reduce annual heating and cooling energy end-use for an office space located in Boulder, CO, compared to no shading case by 19% for a window-to-wall ratio (WWR) of 15% and by 31% for a WWR of 30%.