Photoswitches convert light into mechanical energy by exerting forces on their environment during photoisomerization. However, the mechanical efficiency of this conversion is limited because a plethora of internal modes of the photoswitch do not contribute to the desired switching function but are also changed during the photoisomerization. Here we present a computational approach to quantify the efficiency of a photoswitch during the initial motion on the excited-state potential energy surface. We demonstrate the gist of our method by looking at the excited-state relaxation of carbon monoxide. Subsequently, the photoswitching efficiency of p-coumaric acid is analyzed as one representative example of the approach.