​ISO-NE energy prices rose year on year in May 2026, with the gains concentrated in the evening peak. Internal Hub real-time prices averaged $49/MWh, up from $33 in May 2025. Day-ahead prices averaged $46/MWh, up from $35.

Battery arbitrage value rose alongside prices. Four-hour top-bottom (TB) spreads at the Internal Hub averaged $214/MW-day in real time, 85% above the prior May. The story of the month is a tighter evening market and a wider gap between midday and evening prices.

Load stayed mild most of the month, averaging 10.8 GW, but a warm spell over May 18 to 20 pushed demand to a monthly peak of 18.9 GW. That surge briefly tightened the system and drove the month's highest prices.

Key takeaways

• Real-time prices at the Internal Hub averaged $49/MWh, up from $33 a year earlier. Day-ahead prices averaged $46/MWh.
• Gains concentrated in the evening, not across the board. Real-time prices averaged $40/MWh at midday and $67 by 7 PM, widening the intraday range batteries capture.
• Four-hour real-time TB spreads averaged $214/MW-day, up 85% year on year. May spreads are more in line with April's $177 but below January's weather-driven $491 winter peak.
• Maine had the widest four-hour real-time spreads at $263/MW-day, and Connecticut had the narrowest at $204. Maine also had the lowest average energy prices.
• Nuclear generation returned from refueling and wind generation rose 70% compared to 2025 as offshore capacity came online, with wind at a 103% capture rate.

Real-time prices averaged $49/MWh, with the gains in the evening peak

Internal Hub real-time prices averaged $49/MWh in May 2026, up from $33 in May 2025. Day-ahead prices averaged $46/MWh, up from $35.

The rise was not a broad, all-hours lift. Hourly real-time prices held at $40/MWh at midday, then peaked at $67/MWh at 7 PM, as solar fell off and demand built into the evening. Average peak load rose only 4% to 13.6 GW. The gains reflect evening tightness rather than load growth, and the widening intraday range is what fed the bigger spreads.

Prices also varied by zone. Maine cleared lowest, at $38/MWh day-ahead. Southeast Massachusetts and the Boston zone cleared highest, at $46.5/MWh.

Gas supplied 46% of generation as nuclear returned from refueling

Natural gas provided 4.4 GW on average, or 46% of in-region generation. That was down from 5.2 GW in May 2025.

Nuclear output recovered through the month. Seabrook and Millstone returned from spring refueling outages, lifting nuclear from about 1.2 GW on May 1 to about 3.0 GW by May 31. The month averaged 2.6 GW.

Wind generation rose 70% to 503 GWh and captured 103% of the average price

Wind output totaled 503 GWh in May 2026, up from 297 GWh a year earlier. The increase tracks new offshore capacity, as Vineyard Wind installed its final turbine blades in March 2026.

Wind's capture rate reached 103%, up from 97% in May 2025, despite a large increase in generation. A generation surge may cannibalize its own hours and pull capture rates below 100%. ISO-NE wind avoided that because its shape runs opposite to solar. At midday when solar floods the market and prices bottom out, wind averaged 554 MW. From late afternoon into the overnight hours when prices are elevated, wind generation averaged 754 MW. That profile, not the sheer volume, kept capture above 100%. At 6% of supply, wind is too small to move the evening peak, so an above-average capture rate and rising system prices hold at once.

Real-time four-hour spreads averaged $214/MW-day, widest in Maine

Four-hour real-time TB spreads at the Internal Hub averaged $214/MW-day, up 85% from May 2025. Day-ahead four-hour spreads averaged $117/MW-day, up 31%.

That figure reads strong year on year, but reflects New England’s lesser spring opportunities relative to winter. It is above April’s $177/MW-day, yet well below the winter peak TBs, with January 2026 winter peak of $491 and February’s $404/MW-day.

Spreads differed across the region, with a clear north-to-south gradient. Maine held the widest four-hour real-time spreads at $263/MW-day. Connecticut held the narrowest at $204. The day-ahead market followed the same order: Maine at $138/MW-day, Connecticut at $110. The north-south transmission constraint backs up surplus supply in Maine, holding its prices below the rest of ISO-NE. NECEC's 1,200 MW of Hydro-Québec imports, energized in January 2026, add to that surplus, deepening Maine's price troughs and widening its top-bottom spread even as average prices stay low.

Ancillary services stay a smaller, battery-heavy stream

Arbitrage drove the value, and ancillary services added only at the margin for storage. In May, day-ahead reserves cleared at $11/MWh for ten-minute spinning reserve and $7 for the slower products. Regulation capacity cleared at $7.5/MWh.

Those monthly averages mask one sharp spike. On May 19, a heat-driven demand surge pushed day-ahead reserves to $40/MWh and regulation capacity to $23. Peak load reached 18.8 GW on May 19 and 18.9 GW on May 20, well above the month's typical evening peak of 13.6 GW. Day-ahead energy hit $88/MWh the same day, its monthly high. Energy and reserve prices rose together, so the spike reflects system-wide scarcity rather than anything specific to the reserve market.

Batteries are concentrated in this market, holding 84% of ISO-NE's cleared regulation capacity. Regulation is a small market, however, and batteries are less dominant in the larger reserve products.

What May signals for ISO-NE batteries

May is a shoulder month for New England. Spreads widened year on year but stayed far below January's $491 peak, and the clearest value came from one hot stretch, May 18 to 20. ISO-NE pays batteries best when winter heating and summer cooling stretch the system.

Nuclear is back at full output ahead of summer cooling demand, which ISO-NE's 2046 load forecast expects to keep climbing. Higher summer peaks and increased renewable output should drive intraday volatility that lifts battery returns.