Alexander Hogeveen Rutter Martand Shardul

In March 2026, the Central Electricity Authority released the National Generation Adequacy Plan (NGAP) for 2026-27 to 2035-36. It is one of the most comprehensive power planning exercises, combining least-cost optimization, dispatch modelling and probabilistic reliability analysis to develop a generation mix for 2035-36. It projects a total capacity of over 1,100 GW, with about 70% non-fossil capacity and around 174 GW/888 GWh of storage.

An important output is the addition of 87.2 GW of coal capacity between 2025-26 and 2035-36. Given the scale and its long-term implications, an important question arises: can this planned coal capacity be replaced with additional renewable energy (RE) and storage while maintaining the same adequacy and reliability standards?

The authors undertook a deterministic stress-test modelling exercise to evaluate whether the incremental energy and adequacy contribution expected from the planned 87.2 GW coal addition could be approximated through additional renewable energy and storage under simplified stress conditions.

Indicative modelling suggests that an additional 191 GW of solar, 51 GW of wind, 29 GW of battery energy storage systems, and 9 GW of pumped storage may be required to approximate this contribution under the deterministic assumptions used. This would increase overall RE installations to about 1,006 GW, compared with around 764 GW in the NGAP pathway.

However, unlike NGAP, the exercise does not evaluate full-system interactions across the entire generation fleet and should therefore be interpreted as a targeted stress-test exercise rather than a full-system optimization outcome.

Indicative early results suggest that such an RE pathway could reduce system costs by over ₹1.4 lakh crore per year, even under conservative assumptions on overbuild and storage requirements needed to manage stress periods such as weak wind conditions. These estimates remain sensitive to assumptions regarding fuel prices, storage costs, financing conditions and system integration requirements. If coal fuel price escalation is considered, the divergence becomes larger over the asset lifecycle.

Further, as India imports 25-30% of its coal, fuel price risk over a 20-25 year asset life cannot be ignored. In contrast, RE generation reduces exposure to recurring fuel supply risks and shifts the cost structure toward upfront investment and system integration.

An important aspect of NGAP is its emphasis on adequacy and reliability. Therefore, if RE is deployed as an alternative to coal, the system must reliably meet demand across all time blocks. Therefore, three system constraints become pivotal.

First, storage continuity is essential. Storage must remain available across consecutive demand cycles. If renewable output is weak for several days, storage levels can fall. Once depleted, the system becomes exposed and can lead to demand supply gaps.

Second, charging sufficiency must be ensured. Storage can only discharge what has been charged. With round-trip efficiencies of 80-88%, renewable generation must exceed demand in order to both serve load and recharge. This creates a requirement for surplus capacity and/or overbuild, along with periodic curtailment. Testing system behaviour across consecutive days shows that adequacy depends not only on installed capacity (MW), but also on available stored energy (MWh).

Third, even when annual energy is sufficient, shortfalls can emerge in specific time windows, for example, during the transition from solar to non-solar hours or extended low-wind periods resulting in block level deficits.

NGAP already captures many of these dynamics through sophisticated probabilistic modelling and chronological dispatch analysis. However, the published results do not make it easy to evaluate how the system behaves under explicit coal-replacement stress scenarios, particularly across consecutive periods of low renewable output or storage depletion.

Deterministic stress tests can complement the NGAP framework by making these system behaviours more transparent and interpretable. This would not replace NGAP’s methodology, but could improve transparency around key planning choices. It would also help establish whether the identified coal capacity reflects binding system constraints such as storage duration, transmission readiness or deployment timelines, or whether outcomes are sensitive to assumptions that could evolve over time.

Given that thermal plants typically require 6-8 years to be commissioned, while storage systems can often be deployed faster, evaluating alternatives transparently becomes increasingly important. Episodes of El Nino and La Nina impact power demand and hence reserve capacityplanning to mitigate deficits, where thermal, storage and hydro can play a profound role, must account forsuch climate impacts.

For too long, India’s RE targets have been framed as part of an energy transition story. Instead, RE, storage and thermal targets should be set based on what minimizes costs to consumers while still maintaining adequacy and system reliability under uncertainty.

NGAP’s rigorous methodology has already provided a strong foundation. The next step is to test alternative pathways under stress conditions to identify the route which can best maximize affordability, reliability and energy security.

(Alexander Hogeveen Rutter is an independent energy expert with experience in resource adequacy planning, and an Emergent Ventures grantee focused on energy policy. Martand Shardul is former Policy Director (India) at GWEC and a former Fellow at TERI. All views are personal)