What is the useful energy of a photon?

The fundamental upper bound on the efficiency of photovoltaic conversion continues to attract interest of the research community. By considering the conversion efficiency of a monochromatic photon gas at constant pressure, we show that this limit is equal to the availability (or exergy), as defined in textbooks on classical thermodynamics. The application of this result to the full spectrum of black-body radiation yields the Petela-Press-Landsberg efficiency. Generalization to include entropy generation on account of the kinetic nature of the conversion process, by drawing a parallel with the efficiency of an infinite tandem converter, yields a theoretical efficiency limit of 85.2%.