CuNbO3 is a promising semiconductor for photovoltaic and photo-electro-chemical conversion. To evaluate its potential, it is necessary to analyze the optical properties and quantify the main features when this material is used as an energy converter. To achieve this aim, first the optical properties are obtained using first-principles. The results, based on the split into contributions, show that the light absorption comes mainly from the p(O)-d (Nb) transitions of the NbO6 octahedra. later the absorption coefficients are used as criteria to evaluate photo-voltages, photocurrents and efficiencies of the solar conversion, and the effect of using total and partial solar spectrum ranges. Furthermore, we show that the efficiencies and photocurrents could be increased significantly using a multi-gap approach. Furthermore some voltage loss effects are analyzed.