NEW DELHI, Sept 3 (Reuters) - India will test the installation of battery storage systems at some coal power plants, as the country grapples with integrating massive solar capacity while maintaining reliable electricity supply, an advisor to the country's power ministry said.

The concept addresses a critical challenge facing India's power grid, where thermal plants must ramp down during peak solar hours but maintain capacity for evening demand when solar generation drops.

The Central Electricity Authority (CEA) has been working on guidelines for coal-based power plants and technical minimum load requirements as the country rapidly expands renewable energy capacity.

India is aiming to expand its non-fossil fuel capacity to 500 GW by 2030, but coal remains central to its energy security. The government plans to increase coal-based capacity by 97 GW by 2035, taking the total to around 307 GW to ensure round-the-clock power.

"At times there are only two choices. Either you shut down the coal plant (during excess solar generation) or lose the thermal capacity in the evening, which we don't want," CEA chairman Ghanshyam Prasad told Reuters on the sidelines of PowerGen India 2025 event in New Delhi.

"We are just trying this as an experiment," he said, adding that the country's top coal power generator NTPC (NTPC.NS), had been tasked with testing this at some plants and given funding support.

The batteries would allow the coal plants to capture excess energy and dispatch it to the grid at a later point when needed, allowing the plants to operate at a stable rate, saving costs and extending their lives, CEA's Prasad said.

Recently, NTPC floated a tender for setting up of 1.7 GW of battery storage across 11 coal plants.