The Standard Transfer Specification is the global standard for the transfer of electricity and other utility prepayment tokens, ensuring inter-operability between system components from different manufacturers. The Standard Transfer Specification (STS) describes a secure message system for carrying information between a point-of-sale and a meter, and is currently finding wide application in electricity metering and payment systems. The Standard Transfer Specification (STS) describes a secure message system for carrying information between a point-of-sale and a meter, and is currently finding wide application in electricity metering and payment systems. The simplicity and functionality of STS allows each user to seamlessly scale their prepayment infrastructure from the smallest rural settlement, right up to the largest full-scale AMI (Advanced Metering Infrastructure),without requiring multiple systems to achieve this.The interoperability offered by the STS is just one of the many tools offered towards building the Smart Cities of the future.