The efficiency of a photovoltaic converter (solar cell) illuminated by a thermal source (sun) is commonly determined with Shockley and Queisser’s approach. The strength of this approach lies in its simplicity: All one needs to know is the solar cell’s bandgap, and the efficiency emerges from a detailed balance equation of the electron-hole pair generation and depletion rates at a given temperature. This article studies a single junction cell in outer space to show that a detailed balance approach is not always thermodynamically compatible. We then show that this inconstancy resolves once the cell’s sub-bandgap emission and absorption are included in its energy balance. Generalizing this result, we propose a unified formulation for a photovoltaic process that maintains its detailed balance constraints while not giving away thermodynamics’ first and second laws at all times and under any circumstances. Most importantly, our unified model allows heat conduction consideration to enter the photovoltaic analysis. Therefore, the proposed approach is critical for a single-junction cell and every photovoltaic process with an ample radiative power supply or limited conduction of heat such as concentrated space solar, thermo-photovoltaics thermoradiative, and thermophotonics power schemes.