China lost a quarter of the wind and solar power it generated in the first half of this year because its grid had no room for it, wasting enough electricity to run the United Kingdom or Mexico for more than a year.
The country turned away 360 terawatt hours (TWh) between January and June, 49 per cent more than the same period last year, according to a report by the Centre for Research on Energy and Clean Air and Global Energy Monitor.
Had that power been used, it would have covered the entire growth in Chinese electricity demand over the six months and allowed coal generation to fall. Instead, China's coal generation rose 3.4 per cent year on year, reversing a decline that was the first in a decade.
The practice of rejecting electricity is known as curtailment, and it means switching off wind turbines and solar farms, or refusing to accept what they generate, because the network has no room for the electricity at that moment.
Electricity cannot be held on a wire, so power that is not used as it is produced is lost, and the fossil fuel plants that could have been displaced keep running.
At the root of it is a coal building programme that has continued despite orders to rein it in, researchers said.
China brought 30 gigawatts (GW) of new coal-fired capacity into operation in the first half of the year, the most in any first half for a decade and 43 per cent more than last year, while retiring only 2.7GW. Another 25.4GW began construction, and 274GW are in the pipeline, equivalent to 22 per cent of the coal fleet already running.
This has resulted in a system with more generating capacity than it can use. Coal plants are running fewer hours, yet long-term contracts still guarantee them a share of demand, with coal generators expected to sign annual contracts this year covering 60 to 70 per cent of the electricity they delivered the year before. That reserves space on the grid regardless of how much cheaper clean power is available.
"China's coal buildout is a warning about overcapacity, not a blueprint for energy security," said Qi Qin, China analyst at the research centre. "Coal capacity is still growing because of legacy approvals and policy protections, even as plants run fewer hours and large volumes of clean electricity go unused."
Transmission is the other constraint. Lines have not been built fast enough to move electricity from the deserts and grasslands of the north and west, where China has built its largest solar and wind farms, to the industrial cities of the east that consume it.
"Curtailment in China is structural, not a temporary bottleneck. We expect curtailment pressure to continue through the rest of this decade," Yuan Ren, an analyst at the consultancy Wood Mackenzie, told Reuters.
The central government called in April for tighter control of both coal capacity and generation, and permits have slowed sharply, with only 8.6GW approved in the first half of the year. But developers proposed 70GW of new coal projects and revived a further 20GW over the same period, a quarter more than last year.
"China's power sector is receiving mixed signals," said Christine Shearer, a research analyst at Global Energy Monitor.
"Policymakers are calling for tighter control of coal, yet record numbers of new coal plants continue to enter operation, supported by market mechanisms that guarantee coal both capacity payments and significant electricity sales."
The waste is far above the government's own account. China's National Energy Administration says 8.6 per cent of solar output and 9.1 per cent of wind output were curtailed in the first half, against the 26.1 per cent the report calculates using weather-adjusted data to capture output it says goes unreported. The agency stopped publishing monthly province-by-province figures in March and did not respond to a request for comment from Reuters.
New solar installations in China have fallen 66 per cent this year, a drop driven by curtailment and by a policy change that removed the guaranteed fixed price renewable generators used to receive. Investors are shifting towards projects that pair panels with batteries, Mr Ren said, so that power can be stored rather than wasted.
The problem is not confined to China alone.
Australia's National Electricity Market curtailed 2.93TWh in the first half of the year, up 37 per cent and equal to 7 per cent of its wind and solar output, while Japan's grid rejected 2.35TWh, a rise of 34 per cent. India, the world's third largest solar generator, curtailed 8.13TWh in the quarter to June, about 14 per cent of its solar output over those three months.
Behind all of it is a build-out that has outpaced the wires.
The world added a record 692GW of renewable capacity last year, three quarters of it solar, taking total renewable capacity to 5,149GW, or about half of all installed generating capacity worldwide, according to the International Renewable Energy Agency.
China alone holds about half the world's operating solar.
Batteries are the answer both analysts and researchers keep returning to, since storing surplus midday output and releasing it in the evening removes the moment of oversupply that forces curtailment.
"Chile added 4GWh of batteries in 2025, more than doubling its installed capacity. Most of this new storage was co-located with solar plants, helping reduce curtailment," said Kostantsa Rangelova, an analyst at the energy think tank Ember, who pointed to Chile and Bulgaria as models others could follow.