Coinciding with the highest light intensity was also the highest recorded wall-plug-efficiency of 34%. This remarkably high value is not just on par with classical external irradiation systems, but actually exceeds it by a factor of 2.5, suggesting the WLE system is also more energy efficient than external irradiation. This higher efficiency is likely associated with the fact that internal irradiation does not suffer from common loss effects such as off-target irradiation or reflection losses. As a result of both the high light intensity and energy efficiency, the obtained photochemical space time yield (PSTY) of up to 1794 mmol d -1 W -1 is the highest value reported yet.