Southwest Power Pool (SPP) used to be the grid operator you could draw inside the Great Plains. Since April 1, 2026 its RTO footprint reaches into the Mountain West and spans both of the big US interconnections. This post sets out what the SPP footprint covers today, what changed, and which western market plans are still to come. Every figure links to SPP, FERC or a participating utility.
Key takeaways
- SPP serves members in all or part of 17 states across roughly 732,086 square miles and has 158 members, per its fast facts updated September 2026 (SPP Fast Facts).
- The western RTO expansion went live at midnight Central on April 1, 2026. Nine load-serving utilities in seven western states led it (SPP, Apr 2 2026).
- FERC accepted the expansion tariff on March 20, 2025 (FERC order, ER24-2184).
- Markets+, SPP's separate western day-ahead market, is scheduled to go live in October 2027 (SPP Markets+).
The SPP footprint by the numbers
All figures below come from SPP Fast Facts, updated September 2026, unless noted.
| Measure | Value |
|---|---|
| States with members (all or part) | 17 |
| States and provinces receiving other energy services | 23 |
| Service territory | ~732,086 sq mi |
| Members | 158 |
| Accredited generating capacity (as of June 2025) | 65,639 MW |
| Transmission miles, eastern interconnection | 72,979 |
| Transmission miles, western interconnection | 12,552 |
| Summer peak load | 56,184 MW (Aug 21 2023) |
| Winter peak load | 48,142 MW (Feb 20 2025) |
| Energy served, 2025 | 300,382 GWh, 36.7% wind |
The capacity figure is dated June 2025, before the western expansion, so it does not include the new western members' generation. SPP's April 2026 release says the expanded western region "is home to 20 million people" (SPP, Apr 2 2026).
The April 2026 western expansion
SPP filed the tariff changes on June 4, 2024 and amended the filing on November 4, 2024. FERC accepted them, subject to condition, on March 20, 2025 (FERC order; SPP release). The FERC order names nine expansion members:
- Basin Electric Power Cooperative
- Colorado Springs Utilities
- Deseret Generation and Transmission Cooperative
- Municipal Energy Agency of Nebraska (MEAN)
- Platte River Power Authority
- Tri-State Generation and Transmission Association
- WAPA Colorado River Storage Project
- WAPA Rocky Mountain Region
- WAPA Upper Great Plains Region (called Upper Great Plains-West in SPP's later release)
SPP and the western utilities confirmed on March 12, 2026 that they would proceed as planned (SPP, Mar 12 2026). Operations began on April 1. SPP described itself as "the first regional transmission organization (RTO) with services spanning two interconnections." The expansion states are Arizona, Colorado, Montana, Nebraska, New Mexico, Utah and Wyoming (SPP, Apr 2 2026).
What happened to WEIS
Before joining the RTO, many of these utilities traded through SPP's Western Energy Imbalance Service. WEIS launched on February 1, 2021 (Tri-State release). SPP discontinued it as part of the RTO expansion, effective April 1, 2026 (SPP WEIS page).
Markets+: the next change in the West
Markets+ is separate from the RTO. It is a day-ahead and real-time market for western utilities that do not join SPP's RTO.
- FERC approved the Markets+ tariff on January 16, 2025 (SPP, Feb 27 2026).
- The program is in Phase Two, implementation, targeting an October 2027 go-live (SPP Markets+).
- Arizona Public Service, Powerex, Public Service Company of Colorado, Salt River Project and Tucson Electric have signed agreements to participate at launch.
- Grant County PUD and Tacoma Power have committed to join on October 1, 2028, alongside Puget Sound Energy and Chelan County PUD (SPP, Feb 27 2026).
- The Bonneville Power Administration set a policy in May 2025 to pursue Markets+ (BPA). In March 2026 it proposed joining with a target go-live of October 1, 2028 (BPA proposed decision). We have not found a final BPA decision, so treat that as a proposal.
Load growth inside the footprint
In August 2026 SPP said it "expects electricity demand in its 17-state region to almost double over the next ten years" (SPP, Aug 11 2026). In January 2026 FERC approved SPP's High Impact Large Load (HILL) process. HILL sets up a 90-day study-and-approval path for large loads paired with new generation (SPP, Jan 15 2026). For how operators turn expectations like these into numbers, see our explainer on how a grid forecast is built.
Following SPP day to day
The SPP live grid status page summarises current load, fuel mix and real-time prices from SPP's public data, with an as-of timestamp and source links. The data comes from SPP's own public feeds; spp.org remains the authoritative source. For prices across the footprint, see the nodal price map. To see where SPP sits among its neighbours, see the balancing authority map.
FAQ
What is the SPP footprint today?
Per SPP's September 2026 fast facts, the SPP footprint covers members in all or part of 17 states and about 732,086 square miles, and it now reaches into both the eastern and western interconnections.
When did the SPP footprint expand west?
The western RTO expansion went live at midnight Central time on April 1, 2026, after FERC accepted the tariff in March 2025.
Which states did the expansion add?
SPP's April 2026 release lists Arizona, Colorado, Montana, Nebraska, New Mexico, Utah and Wyoming as the states of the nine utilities that led the expansion. Nebraska itself was not new to SPP: FERC's expansion order notes that in 2008 it accepted tariff revisions integrating Nebraska Public Power District, Omaha Public Power District and Lincoln Electric System into SPP (FERC order, footnote 151).
Is Markets+ part of the SPP footprint?
Not in the RTO sense. Markets+ is a separate western market that SPP administers, scheduled to go live in October 2027. Its participants do not become SPP RTO members.
Is SPP the same as ERCOT?
No. ERCOT runs most of Texas on its own interconnection. SPP covers parts of the Texas Panhandle and a multi-state region to the north and west. See the ERCOT live page for Texas.
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