Regular graphs with maximal energy per vertex

We study the energy per vertex in regular graphs. For every k⩾2k⩾2, we give an upper bound for the energy per vertex of a k  -regular graph, and show that a graph attains the upper bound if and only if it is the disjoint union of incidence graphs of projective planes of order k−1k−1 or, in case k=2k=2, the disjoint union of triangles and hexagons. For every k, we also construct k-regular subgraphs of incidence graphs of projective planes for which the energy per vertex is close to the upper bound. In this way, we show that this upper bound is asymptotically tight.