Current views of photovoltaic efficiency adopt a ray perspective of light and therefore cannot be applied to devices with sophisticated light coupling mechanisms, where phenomena from the realm of near field optics prevail. We use the fluctuation dissipation theorem to derive a thermodynamic estimation of the electromagnetic source strength within a semiconductor. This allows for a photovoltaic efficiency calculation that considers the full vectorial wave nature of light. The approach is demonstrated by analyzing the performance of a model solar cell down to thicknesses of few nanometers. The effect of near field optics on the performance of the device is highlighted.