India's rapid expansion of solar power is beginning to outpace the flexibility of its coal-fired fleet, leading to rising curtailment of renewable energy and highlighting an urgent need for battery storage, Ember strategists said in a report Wednesday.
"Solar and wind curtailment is becoming a visible part of India's real-time grid balancing. The volumes are already noticeable and rising," said Neshwin Rodrigues, senior energy analyst at Ember.
Solar and wind account for about 17% of India's annual electricity generation. However, Ember said the share of solar and wind in India's energy mix can rise to as much as 41% during peak midday hours, forcing coal plants to operate close to or below their technical minimum output levels.
Coal, which remains the main source of balancing reserves for the grid, is increasingly being cycled from near full output overnight to its lowest levels during the day as solar generation surges.
Rodrigues said the system required coal to generate below 55% of its rated capacity in more than half of the midday intervals in April 2026.
Ember said once coal plants reach their minimum operating levels, they are unable to reduce output further, leaving grid operators with little choice but to curtail renewable generation to maintain system stability.
The energy think tank projected that maintaining coal plants above their technical minimum resulted in about 2.1 terawatt-hours of renewable energy curtailment in 2025/26, equivalent to about 6.29 billion Indian rupees ($73.1 million) in lost electricity.
The report indicated that during peak periods, as much as 6% of available solar and wind generation was curtailed, not because of transmission congestion or a lack of demand, but because coal units could not reduce output further.
Renewable curtailment accelerated over the past year, Ember said, with tertiary reserve ancillary service instructions causing more than 3.6 TWh of curtailed solar and wind generation by early June 2026, compared with negligible levels in early 2025.
India added about 24 gigawatts of solar capacity between October 2025 and April 2026, lifting installed solar capacity to about 154 GW.
Ember said without additional flexibility, curtailment could worsen during the post-monsoon period when high solar and wind output coincide.
The energy think tank forecast that about 10 gigawatt-hours of battery storage charging during midday hours would have been sufficient to absorb surplus renewable energy and avoid most curtailment during the most constrained months of fiscal year 2025/26.
Battery energy storage systems can also provide downward reserves once coal plants reach their minimum operating levels, reducing the need to spill clean energy.
However, Ember said regulatory and grid-connectivity rules are limiting battery projects' ability to operate as system-wide flexibility assets.
The think tank said current rules often require battery projects to install matching renewable generation capacity before gaining long-term permission to draw power from the grid, slowing deployment and limiting batteries' ability to absorb excess power during solar-rich hours.
The flexibility problem is already here, Rodrigues said, adding that battery storage is becoming essential for the next phase of renewable energy growth.