MISO has overtaken PJM as the largest U.S. power market by installed capacity, reaching 230 GW at the end of Q1 2025 versus PJM’s 227 GW, a shift propelled by 7.45 GW of solar additions across the Midwest over two years. The milestone marks the first time a non-capacity market has claimed the top spot, signaling that renewable buildout in MISO’s footprint – spanning both Democratic and Republican-led states – is accelerating faster than traditional resource adequacy frameworks can absorb.
Midwest Solar Expansion Rewrites Regional Capacity Rankings
MISO’s ascent reflects a concentrated solar deployment wave between April 2024 and March 2026, with Illinois accounting for the largest share followed by Indiana, Arkansas, and Missouri. Unlike PJM, which operates a mandatory capacity auction that procures resources three years forward, MISO runs an energy-only market with a voluntary planning resource auction (PRA) that clears seasonally. The 230 GW figure represents nameplate capacity registered in MISO’s module E tracking system, not accredited firm capacity – a critical distinction because solar’s effective load carrying capability (ELCC) in MISO’s winter-peaking system declines sharply as penetration rises. At roughly 25% annual capacity factor for Midwestern utility-scale solar, the 7.45 GW addition translates to approximately 1.8 GW of firm winter capacity under current accreditation rules, though MISO’s seasonal construct now evaluates summer and winter separately.
The political geography of this buildout is notable: Illinois’ Climate and Equitable Jobs Act (CEJA) mandates 100% clean energy by 2045 and has driven aggressive procurement, while Indiana, Arkansas, and Missouri – all with Republican trifectas – have added gigawatts without comparable mandates, driven instead by corporate power purchase agreements, utility integrated resource plans favoring solar’s levelized cost advantage over new gas, and the Inflation Reduction Act’s production tax credit (PTC) and investment tax credit (ITC) with domestic content bonuses. Florida Power & Light’s parallel achievement – 120 projects totaling nearly 9 GW saving customers an estimated $1.3 billion in avoided fuel costs – underscores that solar’s economic case now transcends partisan lines across the Southeast and Midwest alike.
PJM’s 227 GW capacity total, by contrast, reflects a different dynamic: its capacity market has retained older thermal resources through capacity payments even as energy revenues decline, while its interconnection queue backlog – exceeding 250 GW at last count, predominantly solar and storage – has slowed commercial operation dates. MISO’s queue is also congested (roughly 200 GW pending), but its Long Range Transmission Plan (LRTP) Tranche 1 portfolio, approved in 2022, has unlocked ~53 GW of interconnection rights across 18 Midwestern lines, giving solar projects a clearer path to energization than in PJM’s still-evolving transmission planning process.
Solar Saturation Forces Seasonal Adequacy Reckoning
That points to a deeper structural shift: MISO’s capacity lead is less about absolute megawatts than about how quickly solar saturation is forcing a winter-peaking system to confront seasonal adequacy gaps that PJM’s capacity market was designed to manage. MISO’s 2024 summer PRA cleared at $2.88/MW-day – effectively zero – while the winter 2024-25 PRA cleared at $257/MW-day, a 90-fold spread that reveals the market’s real constraint: not midday summer energy, but winter evening ramp when solar output vanishes and heating load peaks. The 7.45 GW of new solar exacerbates this duck-curve dynamic; each incremental gigawatt reduces summer clearing prices further while doing little for winter reliability unless paired with storage or firm dispatchable resources.
By comparison, PJM’s capacity market, despite its flaws, has maintained a price signal that keeps roughly 150 GW of thermal capacity online – including coal and gas units that might otherwise retire – through the reliability pricing model (RPM). MISO has no equivalent mechanism; its seasonal construct relies on bilateral capacity contracts and utility self-supply obligations. If the current trend holds, MISO will need to accredit roughly 30-40 GW of 4-8 hour storage by 2030 to maintain winter reserve margins above its 17.5% planning reserve margin target, assuming coal retirements continue on schedule (approximately 12 GW announced through 2028) and gas additions lag due to permitting and pipeline constraints. That storage need represents a market opportunity on the order of $15-20 billion in capital deployment, based on current installed cost benchmarks of $1,200-1,500/kW for 4-hour lithium-ion systems.
The IRA’s standalone storage ITC and the PTC’s phase-down timeline (phasing out for projects starting construction after 2032) create a narrow window for developers to monetize both tax credits and capacity value. MISO’s queue currently shows roughly 40 GW of storage projects, but historical completion rates for storage in MISO hover around 15-20% of queued capacity. Closing that gap will require not just economics but transmission upgrades – specifically, the LRTP Tranche 2 and 3 portfolios still under study – to deliver stored energy from solar-rich western MISO zones (Indiana, Illinois, Iowa) to load centers in the east (Michigan, Wisconsin) during winter peaks.
Who This Affects
- Utility planner: Must model solar’s declining winter ELCC trajectory – MISO’s 2024 ELCC study shows solar winter accreditation dropping from 50% at 5% penetration to below 10% above 20% penetration – and factor storage duration requirements into integrated resource plans to avoid capacity shortfalls.
- Storage developer: MISO’s widening summer-winter PRA spread creates a clear arbitrage signal for 4-8 hour storage sited in zones 3-6 (Indiana, Illinois, Michigan) where winter locational marginal prices spike; projects reaching commercial operation by 2027 can capture both IRA tax credits and multiple PRA seasons.
- Policy analyst: The bipartisan solar deployment pattern – blue Illinois leading, red Indiana, Arkansas, Missouri following – undermines narratives that renewable growth depends solely on state mandates; federal tax credits and declining module costs are now sufficient drivers in regulated and competitive markets alike.
- Grid operator: MISO’s seasonal resource adequacy construct faces its first real stress test with a solar-heavy portfolio; the operator must refine its LOLE (loss of load expectation) modeling to account for correlated solar outages during winter storms and evaluate whether the current 17.5% PRM target remains adequate.
What to Watch Next
- MISO’s 2025-2026 planning resource auction results (summer 2025 and winter 2025-26) – specifically whether winter clearing prices sustain above $200/MW-day, signaling persistent adequacy concerns.
- FERC Order 1920 implementation: MISO’s compliance filing for long-range transmission planning due mid-2025, which will determine whether LRTP Tranche 2/3 projects advance to unlock western solar and storage delivery to eastern load pockets.
- Illinois CEJA implementation pace: the state’s 2025-2026 procurement targets (roughly 4.5 GW of new solar/wind) and whether downstate siting opposition slows utility-scale deployment in MISO’s highest-contribution zone.
- PJM capacity market reform outcomes: the June 2025 RPM base residual auction (delayed from 2024) will reveal whether PJM’s proposed reliability pricing model changes stem capacity retirements and alter the MISO-PJM capacity gap trajectory.
Bottom line: The capacity crown swap is a snapshot of a deeper transition – MISO’s solar surge has made it the largest market by nameplate megawatts, but the real test is whether its energy-only framework can translate that nameplate into winter reliability without the capacity market scaffolding PJM spent two decades building.
Read the full report at Energy Central
Note: facts and figures attributed above to reflect that outlet's original reporting. Broader context, cross-sector connections, and forward-looking scenarios reflect independent analysis by our editorial team.
About this article: Drafted by Energy Ai with AI-assisted research and writing based on public reporting, then reviewed under our editorial process before publication.
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