A 3D numerical model for flat-plate liquid solar collectors has been developed. This model is envisioned for predicting the efficiency curve of the collector, for which the different heat transfer mechanisms involved are simultaneously taken into account: solar radiation absorption, transmission and reflection; natural convection in the air cavity; heat conduction across the tube-absorber welded junction; mixed convection flow in the risers; and heat losses by convection and radiation to the ambient. To ensure the reliability of the model, the heat transfer results inside the risers and in the air cavity were contrasted with well-known experimental correlations available in the open literature. The thermal efficiency obtained with this numerical model is successfully validated against own experimental data. This heat transfer model is intended for evaluating the impact of different operating conditions and design features on the overall performance of solar collectors, reducing costs in prototype construction and experimentation. (C) 2014 Elsevier Ltd. All rights reserved.