It has been shown by previous work [M. Takai er nl., J. Vac. Sci. Technol. B 13, 441 (1995)] that field emission of silicon emitters can be enhanced by anodization forming a layer of porous silicon. Typically, an increase of the emission current of a factor of 10 could be measured. The effect of enhanced emission is usually attributed to nanometer sized silicon rods on the surface, leading to local peaks of electrical field strength [E. C. Boswell et al., J. Vac. Sci. Technol. B 14, 1895 (1996)]. The possibility of a sharpening of the tip has also been discussed. The theoretical part of the above-mentioned investigations has been qualitative in nature. In this work, a quantitative discussion of these effects is performed. For this purpose, a finite element (FEM) model has been constructed on the basis of the test structures. Since the performance of FEM degrades in the vicinity of singularities, such as sharp tips analytical solutions have been employed in order to check the accuracy of the numerical solution and to discuss the behavior of the electrical field close to ''rough'' surfaces.