The UKMOD has an objective to improve the efficiency and flexibility associated with the integration of naval electrical systems into both new and existing platforms. A more specific challenge for the MOD is in the de-risking of the integration of future pulse and stochastic loads such as Laser Directed Energy Weapons. To address this the Power Networks Demonstration Centre (PNDC) naval research programme is focused towards understanding and resolving the associated future power system requirements. To address these challenges, the UK MOD and the PNDC have worked collaboratively to develop a 540kVA Power Hardware in the Loop (PHIL) testing facility. For the UK MOD this supports the “UK-US Advanced Electric Power and Propulsion Project Arrangement (AEP3).” This testing facility has been used to explore the capabilities of PHIL testing and evaluate a Flywheel Energy Storage System (FESS) in a notional ship power system environment. This testing provided an opportunity to develop and further validate the capability of the PHIL platform for continued marine power system research. This paper presents on the results from PHIL testing of the FESS at PNDC, which involved both characterisation and interfacing the FESS within a simulated ship power system. The characterisation tests involved evaluating the: response to step changes in current reference; frequency and impedance characteristics; and response during uncontrolled discharge. The ship power system testing involved interfacing the FESS to a simulated real time notional ship power system model and evaluating the response of the FESS and the impact on the ship power system under a range of different operational scenarios. This paper also discuss the links between FESS characterisation testing and the development of the energy management system implemented in the real time model. This control system was developed to schedule operation of the FESS state (charging, discharging and idle) with the other simulated generation sources (the active front end and battery storage) and with the loads within the ship power system model. Finally, this paper highlights how the testing at PNDC has also supported the comparison and validation of previous FESS testing at Florida State University’s Centre Advanced Power Systems (FSU CAPS) facility, and how the combined efforts help to collectively de-risk future load Total Ship Integration and Evolving Intelligent Platforms in both UK and US programmes via the AEP3 PA.