Sizing a 10,506 MWh Industrial Site: 7.2 MWp Solar + 25 MWh BESS Cuts Bill 81%

This is a real sizing study on a real industrial site, redacted for the client. The site draws 10,506 MWh a year and buys almost all of it from the grid at close to ₹13.7 a unit. Much of that load runs well after sundown, so adding more panels alone would have hit a ceiling fast. The annual power bill: ₹14.38 crore.

Ingro Sims ran 2,961 solar-and-battery configurations against the site's actual year — hour by hour, on its real time-of-day tariff. The winning build: 7.2 MWp of solar with 25 MWh of storage.

The recommended build

• Annual bill: ₹14.38 Cr → ₹2.71 Cr, a saving of ₹11.67 Cr a year (81.2%)
• Self-supply: 82% of the site's power comes from its own solar; the grid covers what's left
• Day-one capital: solar ₹35.28 Cr + battery ₹27.43 Cr = ₹62.72 Cr, before any subsidy or grant
• Break-even: 4.3 years; project IRR: 27.4% (pre-tax)
• Over 15 years: ₹279.54 Cr of bills never paid — about 4.4× the money back over the life, and ₹212.2 Cr ahead by year 15 on a net-cash basis

Over one simulated year the plant generates 10,512 MWh of solar. 2,303 MWh is used directly; 7,535 MWh is stored and 6,295 MWh comes back out of the battery into the costly hours. The grid supplies 1,908 MWh. Two losses are already costed into the bill and payback: 1,240 MWh of battery round-trip losses, and 675 MWh of solar surplus curtailed on bright afternoons when both site and battery are full. The year-in-numbers view puts load served by the plant at 81.8% and battery duty at 279.8 cycles a year (0.77 a day) at 76.6% utilisation.

Why 7.2 MWp — the full search

Every one of the 2,961 configurations was simulated and scored on annual benefit, with the strongest gathering around the winner. For each solar size, the best battery pairing shows the trade-off: as solar grows, grid drawal falls but surplus climbs. The knee of that curve sits at 7.9 MWp; this configuration builds 7.2 MWp.

Month by month, savings range from 20.2% in July (₹99.73 L before, ₹79.59 L after) to 54.7% in May (₹1.23 Cr before, ₹55.79 L after), tracking monsoon and summer solar output against the site's load.

Fifteen years of cash

The study models an owned (CAPEX) plant over 15 years at ₹45.0k/kWp solar and ₹10.0k/kWh battery, GST included in the outlay, with tariffs escalating 4% a year. Demand charge and electricity duty are excluded on both sides. Net cash flow is the saving less running costs, plus the GST credit, less any battery replacement — the same series the IRR and break-even are solved on, pre-tax and unlevered.

• Year 1: bill without the plant ₹15.82 Cr, residual ₹3.04 Cr, saving ₹12.78 Cr; cumulative position −₹48.27 Cr
• Year 4: cumulative −₹4.00 Cr
• Year 5: cumulative +₹10.75 Cr
• Year 10: cumulative ₹97.98 Cr
• Year 15: saving ₹24.51 Cr; cumulative ₹212.15 Cr
• 15-year totals: bills avoided ₹279.54 Cr, EBITDA ₹261.93 Cr, net cash flow ₹274.87 Cr

How robust is the answer

Over 600 sampled cases, project IRR runs from 19.1% (P90) to 25.9% (P50), with payback of 5.4 to 4.3 years, against the 27.4% point estimate. P90 is the conservative end: nine cases in ten come out better. The levers that move IRR most, each across its full range with everything else held:

1. Tariff escalation: 18.5% → 34.0%
2. Grid tariff level: 19.8% → 34.9%
3. Solar capex: 21.9% → 33.4%
4. Battery capex: 23.2% → 31.6%
5. Plant load growth: 22.5% → 29.9%
6. ToD peak/off-peak spread: 25.1% → 32.2%
7. Solar yield vs dispatch: 23.4% → 29.9%
8. Plant availability: 27.1% → 28.6%
9. Annual capacity fade: 27.0% → 28.1%
10. O&M cost vs estimate: 27.0% → 27.7%

The full study shows the whole search, every assumption and the year-by-year cash — so anyone can check where they would have sized it differently.