Natural convection in trapezoidal cavities is important in industrial processing applications and the entropy generation technique has been used for the assessment of the energy efficiency. The results are obtained in terms of the isotherms (θ), streamlines (ψ), entropy generation due to the heat transfer (Sθ) and fluid friction Sψ), total entropy generation (Stotal), average Bejan number (Beav) and average Nusselt numbers (Nul- and Nub-). It was found that, Sθ, max occurs at the top corners for the case 1 (uniformly heated bottom wall, adiabatic top wall and linearly heated side walls) whereas, Sθ, max occurs at the bottom right corner for the case 2 (linearly heated left wall and isothermally cold right wall with other walls identical with the case 1). On the other hand, Sψ, max occurs along the side walls for the case 1 whereas, Sψ, max occurs along the bottom or right walls for the case 2. The local maxima of entropy generation terms (Sθ, max, Sψ, max) contribute significantly on the total entropy generation (Stotal). Finally, the optimal strategy for natural convection involving the cases 1 and 2 are decided based on the interplay between Stotal and Nub- for various ranges of modified Darcy numbers (Dam). © 2016 Taiwan Institute of Chemical Engineers.