Start from the tariff, not the roof
Commercial and HT tariffs in Tamil Nadu are high enough that every solar unit displaces expensive grid power. Model your savings against your actual slab and demand charges rather than a generic per-unit figure.
CAPEX or OPEX
CAPEX means you own the plant and capture the full benefit, including depreciation. OPEX means a developer builds it and you buy power at a fixed discounted rate with no upfront cost. CAPEX wins on total return; OPEX wins when capital is tight.
Accelerated depreciation
Businesses can claim 40 percent accelerated depreciation on solar assets in year one plus additional depreciation, which materially improves post-tax returns and often shortens effective payback below three years.
Don't ignore O&M in the model
Budget for cleaning, monitoring and preventive maintenance from day one. A plant running at 78 percent performance ratio instead of 82 costs more over twenty-five years than the entire maintenance contract.