We study the influence of the structural modulation on the low energy physics of the Sr14-xCaxCu24O41 oxides, using ab initio determination of the on-site and nearest-neighbor effective parameters. The structural modulations appear to be the key degree of freedom responsible for the low energy properties, such as the electron localization, the formation of dimers in the x=0 compound, or the antiferromagnetic order in the x=13.6 compound.