We investigate an industrial state of the art solar c-Si cell including a both sided phosphorous diffusion and aluminum paste on the back side forming the BSF and overcompensates the n-doped region. Front and back side contacts are connected by the n-doped layer around the edge, requiring edge isolation. This is usually done by wet chemical etching or laser processes. Laser edge isolation is usually done on the front side of the solar cell, but it is also possible on back side. Only very little work has been done so far to answer the question on which side and with which kind of technique the isolation should be accomplished to obtain the highest efficiency [1, 2, 3]. Former studies showed higher efficiency for front side edge isolation [3], but a detailed theoretical analysis and experimental evidence using up to date laser isolation technique is still missing. In this paper we give an answer to this question on a broad theoretical and experimental basis.