The energy balance of a photoinduced electron transfer reaction is given by the Rehm-Weller equation which combines the oxidation potential Eox(D) of the electron donor, the reduction potential Ered(A) of the electron acceptor, an electrostatic correction term C and the excited state energy of the light-absorbing species: It is shown that if light carries a thermodynamic entropy the excitation energy term must be given by ηE*, η being the efficiency of the conversion of the energy of light into chemical free energy. Measurements of fluorescence quenching through electron transfer at very low light intensities show that the Rehm-Weller equation remains valid in spite of its implied assumption that η = 1; it is concluded that contrary to much current thinking light is a form of high grade energy which can be converted in principle entirely into chemical free energy and electrical energy.