Reversible Solid Oxide electrolyser (rSOE) is a promising solution for long-duration energy storage due to its exceptional features such as inherent reversibility, co-electrolysis capability in electrolysis mode, high efficiency, adaptability to multiple fuels and the ability to concentrate CO2 in fuel cell mode (SOFC) during the processing of carbon-based fuels. However, its economic viability in power-to-X-to-power (P2X2P) applications remains underexplored. This study evaluates the levelized cost of storage (LCOS) of a 250 MW rSOE across six energy storage forms, considering different discharging durations (168–1440 h) and varied annual charging–discharging cycles (1–30). H2 stored in salt caverns, CH4 stored in gas grids and CH3OH stored in tanks are identified as the most viable storage options. The impact of stack lifetime and degradation on LCOS is also analyzed, revealing that associated benefits in terms of LCOS from extending the stack lifetime are marginal. Furthermore, Capital expenditure (Capex) thresholds for achieving an LCOS target of 0.2 $/kWh are calculated for rSOE scales ranging from 10 MW to 1000 MW under scenarios with discharging durations exceeding 700 h and 1–3 charging–discharging cycles per year. Results indicate that H2 stored in salt caverns is optimal for smaller scales and shorter discharging durations, while CH4 stored in gas grids becomes more cost-effective at larger scales with longer durations. Also, it is found that there is a minimum deployment scale for rSOE to meet the LCOS target of 0.2 $/kWh. The minimum deployment scale varies with different energy carriers and energy storage scenarios. Moreover, the Capex thresholds to achieve 0.2 $/kWh LCOS for Alkaline Water Electrolyser (AWE) and Proton Exchange Membrane Water Electrolyser (PEMWE) are also uncovered. The results revealed that rSOE outperforms AWE and PEMWE at MW-scale seasonal energy storage due to its reversibility. For carbon-based fuels (CH4, CH3OH), rSOE demonstrates unique advantages, as its reverse SOFC mode inherently enables CO2 concentration, significantly reducing CO2 capture costs.