Energy storage systems for aircraft and methods of operating the same are provided. The energy storage system may include a main battery, a propulsion power bus, a non-critical bus, a critical bus, and an engine starter bus. The propulsion power bus may be configured to convey electricity, which is supplied by the main battery, to an electric propulsion machine configured to provide propulsion for the aircraft. The non-critical bus may be configured to convey electricity to a hotel load on the aircraft. The critical bus may be configured to convey electricity, which is supplied by the main battery, to a critical load on the aircraft. The engine starter bus may be configured to convey electricity, which is supplied by the main battery, to an electric starter for a gas turbine engine, where the gas turbine engine is configured to power the electric propulsion machine.