As passive energy consumers increasingly become active prosumers, quantifying the benefits of engaging in the energy market has been crucial. An example is by estimating the level of satisfaction a prosumer derives from selling its excess energy generation units. This economic theory is usually referred to as utility and could be an incentive to motivate prosumers in distributed energy generation. This paper studies the maximising power-network utility for small-scale energy prosumers engaging in peer-to-peer (P2P) energy trading with delivery contracts. We formulate these as an optimisation problem and solve the same using dual decomposition, which involves a distributed algorithm to decouple the trading variable. To ensure the feasibility of the proposed model, we consider small sets of P2P energy traders formed into virtual clusters (referred to as virtual microgrids) using communication infrastructure for easier management. Results show that the utility derived by prosumers depends on the certain weights assigned to the formulated utility model. We attribute the "psychological" interpretation of these weights to the prosumer willingness to sell the excess energy generated. Index Terms-delivery contract, distributed energy trading, network utility maximisation, peer-to-peer energy trading, smart grid, virtual microgrids (VMG).