The efficiency of photosynthesis is discussed in analogy to the solar cell. The total efficiency eta can be written as a product of factors eta i concerning different loss processes. The fraction of photon energy hv which is available for photosynthesis in the membrane, usually called the thermodynamic efficiency eta th, is calculated. An upper limit of eta th is found by means of the second law of thermodynamics. Other factors take into account losses by reflection, absorption and by various irreversible processes of the photochemical pathways.