The future Nordic power system is expected to have reduced system rotational energy and challenging frequency stability because of an increased share of inverter-based renewable generation. This paper develops a rotational energy estimation model by estimating synchronised generation capacity from generation output using technology-specific loading factors and validates against 2022 measurements. The results show that the model has a mean relative error of −1.3% and a standard deviation of 7.2%. The model is then applied to a cost-optimal decarbonised 2050 Nordic scenario to investigate future system rotational energy. The results indicate that the future rotational energy ranges from 19.8 to 293.4 GWs, with values below 120 GWs for 22.8% of all investigated operating conditions. The analysis further shows that synchronised hydropower upward reserves are below the frequency containment reserve for disturbances (FCR-D) up requirement (1450 MW) for 73.7% of the time. Dynamic frequency analysis of a selected critical operating point shows that the system frequency nadir may reach as low as 47.5 Hz when hydropower is the only FCR-D provider. This will trigger under-frequency load shedding, highlighting the importance of considering frequency stability criteria when developing cost-optimal capacity expansion energy system models.