Various embodiments include a hybrid energy system that includes a gas-turbine generator configured to provide a up to a full-load power output and an energy storage device configured to store energy, and methods of operating such hybrid energy systems. A hybrid energy system includes a gas turbine generator or combined cycle gas turbine generator system configured to provide up to a full-load power output and a storage device for storing electric energy. The hybrid energy system includes a generator step-up transformer, in which the turbine generator and the storage device are electrically co-located on a low side of the step-up transformer. Methods of operation include controlling power output from the storage device and/or the gas-turbine generator to function as a spinning reserve during scheduled and unscheduled grid power demands to achieve economic and environmental performance advantages, and maintaining a charge state in the storage device.