Originally published May 27, 2015. Updated September 28, 2026.
The duck curve is the real limit on wind and solar, and CAISO projected in 2016 that covering its evening ramp could need up to 11,000 additional megawatts within about three hours by 2020, with spring days needing up to 13,000 megawatts in that same window. That’s an economic limit, not a shortage of sun or wind. Solar and storage together made up 91% of new US generating capacity in the first quarter of 2026, so the grid is already being rebuilt around it.
How the duck curve squeezes a grid
The duck curve is what that economic limit looks like on a chart. CAISO defines net load as the gap between forecast demand and what wind and solar alone will supply, and its 2016 fast facts sheet shows that gap sagging at midday, then climbing steeply at dusk as solar rises and falls each day.
Picture a duck’s outline sitting on top of that chart, with a flat back through the morning and a belly that sags as solar output rises. A neck rises fast once the sun drops and panels stop producing. CAISO built that shape from actual grid data covering every day from 2012 through 2020, watching the belly deepen and the neck steepen as more solar came online each year.
That neck is the same evening window CAISO said could need up to 13,000 more megawatts in about three hours, right when your own dinner is on the stove and the lights come on. Batteries are the fix grid operators reach for. They store the midday surplus and release it during the evening climb instead of curtailing panels or starting up a gas plant.
Batteries are pushing the ceiling higher
Wood Mackenzie counted 7.8 gigawatts of new US solar capacity added in the first quarter of 2026 alone, and a record 45% of home solar installations were paired with a battery in that same quarter. If you’re pricing out a home system, that pairing is close to a coin flip now, and it’s what turns the duck curve’s evening neck from a problem into a non-issue, banking cheap midday power to use after dark.
Solar alone added about 43 gigawatts of new US generating capacity in 2025, 54% of everything new that went on the grid that year, and the fifth year running that solar led the nation’s top new generation source. Pairing that much solar with battery storage is the fix for the timing problem the duck curve describes. It lets a grid sell power in the evening hours instead of only at midday.
How far the ceiling has already moved
Renewables overtook coal in the world’s electricity mix for the first time in 2025, supplying 33.8% of global electricity against coal’s 33.0%, according to Ember. Wind and solar together met 99% of the growth in global electricity demand in 2025, with solar covering about three-quarters of that increase. You won’t see that on your own electric bill yet, since it’s a global figure and the US still trails it, but it’s the same shift now playing out at home, earlier and faster overseas.
What the numbers say about the US grid
The US still lags the rest of the world on total renewable share. Renewables supplied 21.4% of US electricity generation in 2023, with wind contributing 10.2% and solar 3.9%, according to the US Energy Information Administration. Solar’s share has grown fast since then, reaching 8.2% of US generation in the twelve months through August 2025, up from 6.9% in 2024, according to the Department of Energy, which means roughly one in twelve kilowatt-hours reaching your outlets now started as sunlight.
Panels themselves have gotten far cheaper over the same years. A solar module cost $2.51 per watt in 2010 and $0.265 in 2024, both figures in 2025 dollars, a drop of about 89%, according to Our World in Data.
Jesse Jenkins put a number on this ceiling in 2015
Hitting a ceiling before running out of room isn’t a new worry for wind and solar. In a piece published on Energy Collective on May 27, 2015, then-MIT researcher Jesse Jenkins argued that variable renewables hit an economic limit set by a source’s capacity factor, the share of hours it can run at full output. Wind and solar cost almost nothing to run once built, so grid operators dispatch them first, and Jenkins argued that pushes wholesale prices down hardest in the exact hours wind and solar generate the most.
Jenkins put the ceiling for wind at roughly 25% to 35% of electricity supply. Solar’s ceiling, he argued, was lower, at 10% to 20% in most regions. Combined, he estimated variable renewables topping out around one-third to one-half of total electricity.
He pointed to Ireland, where the grid operator was already capping instantaneous wind and solar output at about 50% of demand in 2015. If a US grid capped your local wind and solar output the way Ireland’s did then, you would see it as curtailment, a paid-for panel or turbine throttled back for no reason visible on your bill. Wind supplied 10.2% of US electricity in 2023 and solar added another 3.9%, still far short of the ceiling Jenkins estimated in 2015.
What ERCOT’s summer records mean for a Texas bill
ERCOT’s hourly load averaged a record 74.5 gigawatts for the week ending August 22, 2026, 10% above the highest weekly average from summer 2025. That also beat the prior record of 70.0 gigawatts set in 2023 by 6%. The following week held nearly as high, averaging 73.7 gigawatts for the week ending August 29, 2026, and the six weeks before that one all beat the 2023 record too, according to the US Energy Information Administration.
Wind, solar, storage and gas together kept those hours covered, all leaned on at once by ERCOT through the hottest weeks of summer 2026, so the lights stayed on if you’re on the Texas grid. The grid keeps handling more than it was built for, one added gigawatt of batteries and panels at a time.
