Total equivalent energy efficiency metric for building-integrated photovoltaic windows

We report a metric for evaluating the total efficiency of incident sunlight conversion by solar windows into useful energy in the form of electric power and luminous flux. The transmitted luminous flux is converted into an equivalent energy flux using the luminous efficacy. If the light meets a white light criterion, the luminous efficacy is for ideal white light. Otherwise, it is for light at photopic maximum. The total equivalent energy efficiency metric is formed by adding the equivalent luminous power to the electrical power and dividing the sum by input power. The metric can be used for both global and diffuse incident sunlight. There are two "sweet spots" for efficiency limits as a function of band gap: 1.3 and 2.5 eV. Evaluation of demonstrated semi-transparent perovskite solar cells as an example against the metric follows these predicted trends. The value of spectrum shifting for enhancing total equivalent energy efficiency, especially under diffuse sunlight, can also be quantified. The use of the new efficiency metric will encourage progress in building-integrated photovoltaics by enabling product comparison in terms of their efficiency of energy usage.