Meeting future electricity demands with increasing shares of variable renewable energy will require energy storage to provide clean and resilient power supplies. A challenge for models used for long-term power system planning is to balance high-level multi-sector dynamics operating at the annual and decadal timescales with sufficient technological and regional detail of the power sector operating at the hourly and daily timescales. Here we incorporate improved sub-annual representations and an energy storage module into the power sector component of a well-established multi-sector model with state-level details in the US (GCAM-USA). Electricity dispatch and supply are modeled through 24 hourly segments in a day for each of the four seasons, and an additional segment, “superpeak” to represent the top ten load demand hours of the year (97 dispatch segments in total). The intent is to represent an average diurnal profile. Using this improved model, we estimate the rates of energy storage capacity additions implied by alternative electric power expansion pathways that vary across assumptions about socioeconomic growth, decarbonization, and technological change. We will explore the interaction of variable energy penetration with energy storage needs to increase clean firm power and power sector resiliency. We expect that energy storage will provide additional firm capacity during net peak hours and contribute to reductions in renewable generation curtailments and increased asset utilization during oversupply periods.