Complex sentence: The existing grids were built in accordance with the centralized energy system of the four main energy companies in Germany, namely, as of 2016, E.ON, EnBW, RWE, and Vattenfall.
Split into simpler sentences: The existing grids were built in accordance with the centralized energy system of the four main energy companies in Germany. The companies comprise, as of 2016, E.ON, EnBW, RWE, and Vattenfall.