The maximal power available to drive an external load is less than the product of the maximal rotational rate and the free energy change, because loading the motor decreases its rotation rate. To get a sense for the power available from Motor S, we used WESTPA to recalculate the THI rate constant in the presence of a constant external torque along torsion angle 43-15-14-29. The external torque opposes forward rotation, with an energy difference (load) of 3.4 kcal mol -1 between the metastable form and the stable form. Adding this load dropped the THI rate constant to 1 × 10 5 s -1 (Figure in the ESI), about an order of magnitude below the unloaded motor . This loaded THI rate constant yields a power output of about 4 × 10 5 kcal mol -1 s -1 , when not bottlenecked by photoisomerization . This potential power output is higher than typical biochemical reactions, because even fast enzymes, such as ATP synthase (1 × 10 3 s -1 ) and catalase (1 × 10 4 s -1 ), cycle much more slowly.