A simple dimensionless expression is developed for the natural convection heat transfer. Darcy flow characteristics are assumed for the liquid phase. The solid phase is an anisotropic and heterogeneous saturated porous medium with internal heat generation. Solid characteristics depend on the temperature. The first part of the paper shows that, using a dimensionless formulation, the heat transfer and Darcy equations are similar for two-dimensional problems. The second part reports the numerical results obtained on various cases with a comparison to classical natural convection models.