India's C&I energy landscape is changing faster than most EPC firms can keep up with. Time-of-use tariffs, demand charge restructuring, and open access regulations are making battery storage economically attractive for the first time for commercial and industrial clients. But most BESS proposals C&I clients receive are sized incorrectly — leading to systems that underperform against their financial model within three years of commissioning.

This guide covers the four fundamentals of correct BESS sizing for C&I projects.

  1. Start with the load profile, not the solar output The most common sizing error is building the BESS model around peak solar generation and asking how much storage is needed to shift that energy. The correct approach is the reverse: start with the client's half-hourly demand profile across all months, identify the peak demand periods and tariff windows where storage has the highest economic value, and then size the solar and storage system together to serve that demand curve. For most C&I clients in India, the highest-value dispatch window is between 6 PM and 10 PM — after solar generation ends and before demand drops. A BESS system sized to serve this window, charged by solar during the afternoon generation peak, delivers the maximum reduction in evening demand charges.
  2. Understand C-rate before specifying capacity Battery capacity (in MWh) and battery power (in MW) are two separate specifications, connected by the C-rate. A 1 MW / 2 MWh system has a C-rate of 0.5C — meaning it can fully charge or discharge in two hours. A 1 MW / 1 MWh system is a 1C battery — fully charged or discharged in one hour. For most C&I peak-shaving applications, a 0.5C battery (two-hour duration) is appropriate. For frequency regulation or short-duration demand response, a 1C or higher system is needed. Specifying the wrong C-rate results in a battery that either cannot deliver enough power during peak demand events or degrades faster than modelled because it is being cycled at too high a rate.
  3. Overlay the P50 and P90 solar generation scenarios A BESS system sized against the P50 (median) solar generation scenario will underperform in years where actual generation falls toward the P90 level. For a C&I client whose economics depend on the BESS system reliably reducing peak demand charges every evening, a P90-sized solar input is the conservative and correct assumption. This is particularly important in states with high variability between monsoon and non-monsoon generation periods — Karnataka, Maharashtra, and parts of Gujarat — where the gap between P50 and P90 generation can exceed 15% on an annual basis.
  4. Build a state-specific tariff model Time-of-use tariff structures vary significantly across Indian states and distribution companies. The economic value of a BESS system in Karnataka (where ToU tariffs have multiple pricing windows and demand charges are significant) is materially different from its value in Rajasthan or Gujarat. Before finalising BESS sizing, build a detailed tariff model incorporating: the applicable HT tariff schedule for the client's sanctioned load, the ToU pricing windows and rates, demand charge structure (kVA or kW-based), reactive energy charges, and the impact of net metering or banking regulations on any excess solar generation.

A BESS system that is sized correctly from feasibility, against a rigorous tariff model and a P90 solar generation scenario, will consistently outperform its financial model. One that is sized as a bolt-on afterthought will not.