One key parameter for assessing the CO₂ fixation in aquatic ecosystems but also for the productivity of photobioreactors is the energy conversion efficiency (PE) by the photosynthetic apparatus. PE strictly depends on a range of different fluctuating environmental conditions and is therefore highly variable. PE is the result of complex metabolic control. At the moment PE can only be determined indirectly. Furthermore, the currently available techniques either capture only short time processes, thus reflecting only parts of the photosynthetic engine, or quantify the total process but only with limited time resolution. To close this gap, we suggest for the first time the direct measurement of the fixed energy combined with respirometry, called photocalorespirometry (Photo-CR). The proof of the principle of Photo-CR was established with the microalga Chlamydomonas reinhardtii. The simultaneous measurement of oxygen production and energy fixation provides an calorespirometric ratio of −(437.9 ± 0.7) kJ mol⁻¹ under low light conditions. The elevated calorespirometric ratio under high light conditions provides an indication of photo-protective mechanisms. The Photo-CR delivers the PE in real time, depending on the light intensity. Energetic differences less than 0.14% at radiation densities of up to 800 μE m⁻² s⁻¹ can be quantified. Other photosynthetic growth parameters (e.g. the specific growth rate of 0.071 h⁻¹, the cell specific energy conservation of 30.9 ± 1.3 pW cell⁻¹ at 150 µE m⁻² s⁻¹ and the number of photons (86.8) required to fix one molecule of CO₂) can easily be derived from the Photo-CR data.