2025 saw significant solar curtailment to preserve system stability

A combination of forecast error, low daytime demand and rising solar generation led to periods of daytime oversupply in 2025. The inability to flex the coal fleet beyond technical limits restricted the system’s ability to provide sufficient headroom, making solar curtailment necessary to maintain grid security.

Rapid growth in solar capacity is enabling solar to serve an increasing share of daytime demand, altering net load patterns that conventional generators have historically supplied.

In 2025, weaker-than-forecast daytime demand coincided with continued solar expansion, resulting in periods where cumulative supply was higher than demand. This was because enough coal capacity had to remain online to meet the evening demand, with the mismatch leading to grid security concerns. As a result, we estimate that the system operator had to curtail 2.3 TWh of solar energy via emergency TRAS between late May, when reporting started and December 2025, with ~0.9 TWh in October alone.

At current volumes, this curtailment is smaller than that stemming from transmission constraints, and its causes are complex. Operational issues are the main proximate causes, with forecast error often high during high curtailment periods. But more structural issues like solar growth create the environment in which operational problems occur. Regardless of precise causes, this curtailment typifies the kind of integration challenges that could become more common in future if system flexibility does not keep pace with solar growth.

Solar power ramps up steeply in the morning to a peak near midday, before declining to zero at sunset. Nationally, demand has a much more stable profile through the day, generally peaking in the evening after sunset. Introducing solar power, therefore, requires increasing flexibility in other sources, which must ramp down sharply in the morning before firing up quickly to meet the evening demand peak. Owing to solar power’s “must-run” status, national dispatch generally requires the system to operate around it.

In India, this ramping up and down is primarily supplied by hydropower, which is technically flexible, and coal, for which this profile can be challenging. In particular, most coal plants can only operate with power output above a minimum thermal load (MTL), below which they must shut down and remain offline for several hours. MTL is mandated at 55% for all central sector plants, and states are directed to align as well. But the compliance across all coal power plants is not yet 100%. Additionally, it is likely that more than 40% of coal capacity is not under the purview of the NLDC for ancillary services, so may not be operating as flexibly as it could.

Between coal and other generation sources, the system is normally able to flex enough to absorb scheduled solar generation. In some circumstances, however, and particularly when demand is lower than forecast, the NLDC is not able to turn down non-solar generation sufficiently to fully accommodate midday solar while still keeping enough coal online to meet evening demand. Despite the continued improvement in the coal fleet’s flexibility, this situation occurred frequently in H2-2025, most notably in October. In response, the NLDC regularly resorted to curtailing solar using emergency TRAS down.

The chart above illustrates this situation in the first half of October 2025. At midday, available solar capacity was around 50-65 GW, while the coal fleet was maintained above approximately 100 GW to ensure availability for its expected evening output of 140-160 GW. Consequently, solar generation was curtailed during midday hours in proportion to coal’s limited ability to ramp down further, as reflected by the gap between actual and potential solar capacity. Over this period the coal fleet may have been operating close to the mandated MTL of 55%; on the 12th of October, assuming a 90% load factor in the evening peak, there would have been approximately 167 GW of total coal capacity online. The midday coal fleet load factor would therefore have been approximately 60%.

It is important to note that even on the most extreme days, solar availability was never more than 40% of power demand. Solar curtailment in 2025, therefore, did not reflect a lack of demand for clean power. Rather, it reflected the flexibility required from the rest of the system to integrate this clean power.

India’s electricity demand has generally followed an upward trajectory, increasing between 110-120 TWh every year between 2021-2024. Following this, the demand in 2025 was also anticipated to grow over 2024 levels; even under the 20th Electric Power Survey, a 6% increase was projected for the fiscal year (FY) 2025–26 over FY 2024-25.

Time-to-time, based on anticipated demand growth, India’s Ministry of Power takes precautionary measures, such as directing all coal-based power plants to make full capacity available for generation. Even the capacity addition targets under the National Electricity Plan 2031-32 are based on these demand projections.

However, the actual demand growth in 2025 was less than 20 TWh, far weaker than anticipated. Further, demand fell year-on-year in May, June and October. A key reason for this stagnation was the exceptionally mild temperatures seen throughout the year, which reduced cooling needs compared to 2024.

As cooling demand in most Indian states trended downwards, associated power demand fell, with the effect more pronounced during the daytime compared to evening. Another contributing factor was the growth of behind-the-meter solar generation, which appears to the system operator as a fall in demand from the network.

The cumulative result was that in October, average demand at 1 p.m. fell by 14 GW (7.2%), twice as much as the fall at 6 p.m. of 7 GW (3.5%). This reduced daytime demand, coincident with rising solar output, exacerbated the midday-evening net load spread (overall demand minus total variable renewable energy generation) and the flexibility required from the rest of the system.

The unusual demand patterns likely contributed to curtailment both structurally and operationally. Early October 2025, when midday demand was particularly low amidst unusually heavy rainfall, saw relatively high day ahead forecast error. This error was likely the proximate cause of the high curtailment in this period, with hydro generation, expecting higher demand, not scheduled to flex as much as it can.

Nonetheless, curtailment is ultimately downstream of both operational factors like forecasting error, and the broader structural issues described earlier.

In chapter 3, we will discuss that solar is projected to grow faster than demand, almost irrespective of how demand growth evolves in the coming years. The constraints exposed in the coal fleet’s operations under these circumstances underscore the need to improve flexibility, emphasising that a reasonably flexible system can comfortably accommodate the solar output.