Billions of new energy devices generating energy from variable energy resources are difficult to manage centrally alone. As such, a decentralized power system is a defining feature of the ongoing energy transition development in the corporation of P2P energy trading scheme. A proper design of a decentralized P2P energy trading scheme enables optimal use of renewable energy, with consideration of the overall economic effectiveness. Nevertheless, proper energy planning regarding decentralized power supply management needs to be primed upfront. As the growing penetration of renewable energies increases power system instability, the impact of contingencies needs to be identified in the decentralized P2P energy trading scheme to enhance its reliability. The involved players need to ensure the reliability of the decentralized P2P energy trading scheme and have backup plans to make sure its normal performance can be met. N-1 contingency analysis proposed to synthesize a robust decentralized P2P energy trading scheme. In the illustrated case study, three different entities in P2P energy trading system are analyzed and their back-up plans are identified using N-1 contingency analysis. The results showed that the three entities need to have an excess renewables amount of 98 kWh, 87 kWh and 98 kWh in the case where one of them is having power outages to meet the desired performance. The generated outcomes not only provide a handy lens for the players involved but also for the decision-makers who wish to identify the risks that may occur in P2P energy trading scheme.