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Seattle City Light outlines 20-year resource plan, models need for thousands of megawatts and new transmission
Summary
City Light presented its 2026 Integrated Resource Plan to the committee, saying modeling shows a need for roughly 3,738 MW of new resources over 20 years and about 505 MW of transmission capacity to reliably meet projected load growth; the IRP was presented for committee discussion but not a vote.
Seattle City Light staff presented the department's 2026 Integrated Resource Plan (IRP) to the Parks and City Light Committee on June 3, describing the IRP as a 20-year planning document required under state law and due to the Department of Commerce by Sept. 1, 2026. City Light said the IRP is a long-range outlook, not an acquisition plan, and that modeling used 900 weather variations to evaluate resource adequacy across plausible futures.
Siobhan Doherty, City Light supply officer, and resource planning staff outlined results showing sustained load growth and increasing winter peaks driven by electrification and economic growth. Under the IRP's baseline modeling — which prioritized wind, solar and short-duration batteries and layered "generic firm energy" to ensure reliability — the staff presented supply-side needs totaling about 1,711 MW in the first ten years and a grand total modeled need of roughly 3,738 MW over 20 years. The plan also modeled transmission needs: about 302 MW of additional transmission in the first ten years and about 505 MW over 20 years to bring new generation to the city.
City Light staff stressed that the portfolio needs a mix of intermittent renewable resources and 'firm' clean resources such as enhanced geothermal, small modular reactors or long-duration storage to cover sustained winter peaks and extreme weather events. Presenters also noted that much of the region's transmission is controlled by the Bonneville Power Administration and that building new transmission lines is a lengthy, complex process that can take 15–20 years.
Committee members praised the department's outreach and asked technical clarifying questions about load drivers, customer classes and the agency's contracts (including long-term purchases through BPA). Staff said follow-up written questions would be addressed and that the IRP will return for further committee review in mid-June and later to full council; no committee vote was taken on Res. 32206 during the session.
The presentation included specific model outputs and planning assumptions: the IRP used end-of-2024 assumptions for costs and market conditions and noted the modeling did not reflect some recent federal policy and market changes; City Light said it will refresh analyses in future IRPs and do deeper dives on firm resources and long-duration solutions in the 2028 update.
The department asked the committee to acknowledge the IRP and the plan's inclusion as attachment to the proposed resolution; staff will continue stakeholder outreach and return with additional detail.
