The key function of an electricity transmission network is to transfer electrical energy from generators and imports to all customers reliably, at low cost, and sustainably as far as possible. The regional or country-level networks are usually highly interconnected with a large number of nodes and redundant subnetworks. Various types of power plants provide energy for the network. They are mainly nuclear-, fossil- or renewables-based. The power generation technologies have different availabilities, costs, and sustainability indicators. In the present work, the systems analysis of electricity transmission networks is performed, based on the cost, the availability and sustainability indicators. The P-graph framework has been used, where all feasible structural options are enumerated. Naturally, these indicators are inter-dependent; for example, increasing the share of renewables in the overall energy supply for better sustainability may reduce the availability of the system. On the basis of the current work, the sustainability of the transmission network can be maximised without compromising the overall availability. Hungary’s electricity transmission network has been examined to illustrate the proposed procedure.