With distributed generation and new electrical loads, current electrical grids in buildings are becoming obsolete. DC nanogrid can then improve the overall efficiency of the buildings network. Moreover, as no standards exist yet for such grids, new optimisation criteria can be considered to better address current stakes. Thus, in this paper, a first approach to optimize a DC nanogrid is proposed, integrating power losses in the grid with different voltage values to design the best topology of the system. Then, data associated to CO2 equivalent emissions are integrated to the optimization to size the cross-section cable of a line through an environmental point of view. Finally, we discuss the results and the next steps of the optimisation for the DC nanogrid.