Flexibility is a key enabler for the smart grid, required to facilitate Demand Side Management (DSM) programs, managing electrical consumption to reduce peaks, balance renewable generation and provide ancillary services to the grid. Flexibility analysis is required to identify and quantify the available electrical load of a site or building which can be shed or increased in response to a DSM signal. A methodology for assessing flexibility is developed, based on flexibility formulations and optimization requirements. The methodology characterizes the loads, storage and on-site generation, incorporates site assessment using the ISO 50002:2014 energy audit standard and benchmarks performance against documented studies. An example application of the methodology is detailed using a pilot site demonstrator.