Rapid installation of variable renewable power amid a worldwide transition to low-carbon energy systems would impact the stability of the power system due to an anticipated reduction in the synchronous inertia. Converting excess electricity into hydrogen and its use for power generation using gas turbines are gaining increasing attention since the process may help ensure the power system stability. By using an energy system optimization model, the role of the energy storage technology as well as large-scale batteries in decarbonizing Japan's energy system is investigated. The developed model incorporates the power sector with hourly time resolution to deal with variability of the variable renewables and considers a system non-synchronous penetration (SNSP) constraint. Simulated results reveal that around 730GWh of batteries and 50GWh of hydrogen storage are deployed to satisfy an increasing electricity demand toward 2050 with reducing the carbon intensity of electricity to almost zero. The installation of hydrogen storage may increase when the availability of imported hydrogen is limited. Raising SNSP limit could alleviate the associated cost of energy system decarbonization, and development of advanced technologies on emulated inertia would be crucial.