Protein photosensors are versatile tools for studying ligand-regulated allostery and signaling. Fundamental to these processes is the amount of energy that can be provided by a photosensor to control downstream signaling events. Such regulation is exemplified by the phototropins--plant serine/threonine kinases that are activated by blue light via conserved LOV (light, oxygen and voltage) domains. The core photosensor of oat phototropin 1 is a LOV domain that interacts in a light-dependent fashion with an adjacent alpha-helix (J alpha) to control kinase activity. We used solution NMR measurements to quantify the free energy of the LOV domain-J alpha-helix binding equilibrium in the dark and lit states. These data indicate that light shifts this equilibrium by approximately 3.8 kcal mol(-1), thus quantifying the energy available through LOV-J alpha for light-driven allosteric regulation. This study provides insight into the energetics of light sensing by phototropins and benchmark values for engineering photoswitchable systems based on the LOV-J alpha interaction.