Energy systems are rapidly becoming decentralized due to advances in distributed renewable generation, storage technologies, and electric vehicles (EVs). One consequence of decentralization is the loss of a central trusted party. There is, therefore, a need for a solution that enables interactions between mutually untrusting agents. In this paper, we present a general methodology for blockchain-oriented system design and demonstrate its use to design a system for EV charging in a decentralized network of charging stations. We also show how a blockchain can be integrated with minimal changes to a legacy back end. Our solution is scheduled for roll-out by the EV charging service provider SWTCH in the near future.