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Northwestern Energy's draft IRP highlights trade-offs among gas, storage and nuclear as Colstrip changes loom
Summary
Northwestern Energy presented a 2026 integrated resource plan draft showing the utility is resource-adequate through 2031 but faces capacity shortfalls later; scenarios favor natural gas for near-term lowest-cost capacity, nuclear or storage as coal (Colstrip) retires, and transmission constraints and accreditation values limit solar selection in winter reliability modeling.
Northwestern Energy presented its draft 2026 Integrated Resource Plan (IRP) to the committee, describing modeling approaches, key sensitivities and implications for Montana customers.
John Schaefer, manager of long term energy supply planning at Northwestern, told the committee the IRP is designed to test multiple futures rather than lock the utility into a single path. The modeling couples Western Resource Adequacy Program (WRAP) accreditation rules with production-cost simulations and captures trade-offs among cost, reliability and emissions.
Schaefer said the company is currently capacity-sufficient through about 2031 after acquiring shares of external generating resources, but the system will face significant replacement needs over the next 20 years. In the base case the model selected short-duration storage and natural gas to meet winter capacity needs; if a no-fossil-fuels constraint is imposed after 2035 the model substitutes nuclear plus additional storage and incurs higher net present value costs.
Several stakeholders raised concern that the IRP does not fully account for the ability of added interregional transmission to contribute capacity during winter peaks; environmental stakeholders recommended the IRP include a clearer transmission scenario set to show how interregional lines could reduce in-state capacity needs. Northwestern acknowledged the model27s limits: transmission additions outside a RAP zone can be difficult to value in WRAP accreditation.
Stakeholders also discussed the IRP27s treatment of coal retirements (notably Colstrip): some stakeholders argued that a portfolio that retires Colstrip early appears more expensive in a 20-year net present value calculation but does not always incorporate mechanisms such as securitization that can reduce customer impacts when an asset is retired early.
The IRP draft will be posted, Schaefer said, and Northwestern will use the model and competitive RFPs to select resources when a capacity need is actualized. Committee members asked for clearer, customer-facing scenarios showing monthly bill impacts under alternative portfolios; Schaefer said bill impacts require subsequent rate-case and RFP analyses beyond the IRP.
What happens next: The IRP draft will be publicly posted; Northwestern27s modeling inputs and results will be available for stakeholder review; committee members and agencies encouraged additional sensitivity runs that explicitly include large transmission additions and updated WRAP accreditation values.
