How to Size a Solar Plant for Your Factory in India 2026

TL;DR — How to Size Solar for an Indian Factory

• The bottom line: the right size of a solar plant for your factory is the smaller of (a) your daytime electricity load that yields 90%+ self-consumption, (b) your usable rooftop area, and (c) the state's net metering cap (1 MW or 2 MW per HT consumer).
• The answer to "should I oversize my solar plant": only if your state allows banking + open access, your DISCOM is reliable for evening drawdown, AND your tariff arbitrage (₹6+/kWh) on banked credits exceeds the export-feed-in tariff loss (typically ₹3.20-3.85/kWh).
• The most important sizing variables in order of impact: (1) Day-shift demand profile (kWh per day during 9 AM - 5 PM), (2) usable rooftop area (after deducting AHU, drainage, walkways, fire-segregation), (3) net metering cap, (4) BESS sizing for evening shift coverage, (5) future expansion margin.
• In short, the most cost-efficient sizing is the largest plant that maintains 88%+ self-consumption ratio in Year 1. Bigger doesn't always mean better — beyond 90% self-consumption, additional capacity exports at low feed-in tariffs that erode IRR.
• Sun Wave Technologies, a leading solar EPC company in India, structures site-specific sizing for Indian factories using PVsyst yield modelling, DISCOM tariff analysis, and shift-profile load mapping.

Step 1: Build Your Factory Load Profile

The first and most important step is mapping kWh consumption by hour-of-day, day-of-week, and month-of-year. Sources:

• DISCOM bills (last 12 months) — gives monthly kWh totals.
• Sub-meter logs if available — gives hourly resolution.
• SCADA / energy management system if installed — gives real-time resolution.
• Plant-survey load study by the EPC firm at proposal stage — fills in the gaps.

For a typical Indian factory, the load profile looks like:

Solar generates roughly: 5% at 8 AM, peak ~85% at noon, 50% at 4 PM, zero by 7 PM. The most important observation: solar generation profile and factory day-shift load profile are highly correlated — most kWh of solar generation directly displaces a kWh of grid imports.

Step 2: Calculate Your Daytime Self-Consumption Capacity

For a solar plant to deliver maximum IRR, target 88-93% self-consumption ratio in Year 1. This means at least 88% of solar generation must be consumed on-site (not exported to the grid).

Self-Consumption Math

For a 1 MW solar plant in your state:

• Annual generation = 1,400-1,650 MWh (state-dependent)
• Daily generation = 4-5 MWh on a sunny day, 2-3 MWh on cloudy day
• Peak hourly generation = 750-850 kWh during noon hours

For 88%+ self-consumption, your factory's noon-hour demand must be ≥ 750 kWh — i.e., your factory must be operating at least 60-75% of peak load during 9 AM to 4 PM.

Quick-Check Formula

Solar capacity (kWh) ≤ 0.88 × (factory's average noon-hour demand × 8 daylight hours) × 365 days

For a factory with 1.2 MW connected load running at 80% during day shift:

• Solar generation ≤ 0.88 × (1.2 × 0.80 × 8) × 365 = 2,469 MWh/year
• Solar capacity = 2,469 / 1,500 (yield/kWp) = 1.65 MW DC

In states with 1 MW net metering cap, install 1 MW. In states with 2 MW net metering cap (UP, MP), install up to 1.65 MW under net metering. Above 1 MW or 2 MW (state-specific), use net billing or open access.

Step 3: Verify Available Roof Area

Standard rooftop solar density for industrial roofs: 65-75 W per sqft (using 540-580 W mono PERC or 600-660 W TOPCon modules).

Subtractions from Gross Roof Area

Before calculating usable area, subtract:

• AHU plant rooms, cooling towers (typically 8-15% of footprint)
• Skylights and translucent panels (5-12%)
• Drainage paths and ponding zones (3-5%)
• Walkways and maintenance access (3-7%)
• Fire-segregation distance from edges (5-8%)
• Helipads, antenna structures, signage (variable)

Net usable roof is typically 55-75% of gross roof area for industrial buildings, 70-85% for warehouses, and 35-50% for hospitals and shopping malls (which have more equipment on roof).

Step 4: Apply State-Specific Net Metering Cap

For projects above the cap, consider net billing (different settlement structure, lower kWh credit value), or open access wheeling, or group captive structures.

Step 5: BESS Sizing for Evening Shift Coverage (or Storage Mandate)

In Maharashtra (mandatory under April 2026 policy) or as voluntary deployment:

BESS Sizing Math

For a 1 MW solar plant with 50% / 2-hour BESS (Maharashtra mandate):

• Battery capacity = 0.5 × 1 MW × 2 hr = 1 MWh ⟹ 1,000 kWh

Wait, the Maharashtra mandate says 50% capacity for 2-hour duration. Let me re-derive:

• Power rating: 50% of solar capacity = 500 kW
• Energy rating: 500 kW × 2 hr = 1,000 kWh / 1 MWh

So a 1 MW solar plant must have a BESS rated 500 kW / 1 MWh in Maharashtra (the post earlier said 500 kWh / 2-hour, which is 250 kW × 2 hr — this corresponds to a stricter interpretation; the Maharashtra rule is 50% capacity × 2-hour duration = 1 MWh battery for 1 MW solar). Confirm via the policy text in our Maharashtra storage mandate post.

Voluntary BESS for Other States

For voluntary BESS in non-Maharashtra states, size to discharge ~25% of daily solar generation for 2-3 hours of evening peak coverage:

• Daily solar generation = 4,500 kWh (for 1 MW system)
• BESS energy = 0.25 × 4,500 = 1,125 kWh
• BESS power = 1,125 / 2.5 hr discharge = 450 kW

A 450 kW / 1,125 kWh LFP BESS for a 1 MW solar plant adds ₹50-65 lakh capex but captures ToD arbitrage of ₹4-7 lakh/year + grid resilience value.

Step 6: Future Expansion Margin

When sizing, leave 10-25% expansion headroom:

• Roof structure: design for 110-125% of Year-1 BoQ to accommodate 4-5 years of business growth.
• Inverter sizing: leave one or two inverter slots empty for future modules.
• Cable and switchgear: size for 110-115% of Year-1 capacity.
• Net metering provisional: re-application is straightforward when expansion happens.

Step 7: PVsyst Yield Modelling

A reputable best solar EPC company in India uses PVsyst (industry-standard solar simulation software) to model:

• Site-specific irradiance based on Meteonorm or NASA POWER data
• Module-specific performance with manufacturer's PAN file
• Shading analysis using site survey 3D model
• Inverter clipping if DC/AC ratio > 1.2
• Degradation modelling over 25 years

Insist on the PVsyst report at proposal stage. The result is a quantified Year-1 generation forecast accurate to within 3-5%, which becomes the basis for your PR guarantee.

Sample Sizing for Indian Factory Scenarios

Scenario A: 1.5 MW connected load, day-shift heavy textile mill in Punjab

• Daytime average load = 1.2 MW
• Usable roof = 16,000 sqft (matches 1.2 MW solar)
• State NM cap = 1 MW (Punjab)
• Recommended size: 1 MW (capped by NM)
• Self-consumption: 92%
• Payback: 4.0-4.6 years

Scenario B: 4 MW connected load, 24×7 cement grinding unit in MP

• Daytime average load = 3.6 MW
• Usable roof = 40,000 sqft (2.5 MW)
• State NM cap = 2 MW (MP)
• Adjacent land = 100 acres available
• Recommended size: 2 MW rooftop net metering + 50 MW captive ground-mount under group captive structure
• See our solar for cement industry post.

Scenario C: 800 kW connected load, single-shift pharma packaging unit in Kerala

• Daytime average load = 700 kW
• Usable roof = 9,500 sqft (700 kW)
• State NM cap = 1 MW (Kerala)
• ANERT 10% solar+BESS grant available
• Recommended size: 700 kW rooftop CAPEX with 350 kWh/2-hour BESS for ToD arbitrage and outage backup
• Payback: 3.5-4.0 years; ANERT grant accelerates BESS payback to ~3 years.

Scenario D: 25 MW connected load, 24×7 IT data centre in Hyderabad

• Daytime average load = 22 MW
• Usable roof = 18,000 sqft (1.2 MW)
• Floating pond = 3 acres (~1 MW)
• State NM cap = 1 MW (Telangana)
• Recommended size: 1.2 MW rooftop CAPEX + 1 MW floating + 60 MW group captive open access + 25 MWh BESS
• See our solar for data centers post.

Frequently Asked Questions

What's the right size for solar at my factory?

The right size is the smaller of (a) your daytime load that yields 88%+ self-consumption, (b) your usable rooftop area, and (c) the state's net metering cap (1 MW for most states, 2 MW for UP and MP). For projects above the NM cap, additional capacity should be structured as net billing, open access wheeling, or group captive — not as net-metered rooftop.

Should I oversize my solar plant for future expansion?

Leave 10-25% expansion headroom in roof structure, inverter slots, and cable/switchgear sizing. Oversizing the actual module installation beyond your daytime self-consumption capacity is uneconomical because exported energy receives only the DISCOM feed-in tariff (₹3.20-3.85/kWh) versus your grid alternative tariff of ₹7-9/kWh — the export delta erodes IRR rapidly.

How does net metering cap affect sizing?

States cap net metering at 0.5-2 MW per HT consumer (1 MW for most major industrial states; 2 MW for UP and MP). Projects above the cap can still install solar but must use net billing (lower kWh credit value), open access wheeling, or group captive structures. Multi-MW factory rooftops are typically structured as 1-2 MW net metering + balance net billing or open access.

What's the math for self-consumption ratio?

Self-consumption ratio = (kWh consumed on-site from solar) / (total kWh of solar generation). For maximum project IRR, target 88-93%. Below 80%, the lost export revenue erodes IRR significantly. Above 93%, you've left potential capacity on the table that could have been profitably installed. Use PVsyst with hourly load profile data to validate your target ratio at proposal stage.

How big should the BESS be for my factory solar?

For Maharashtra (April 2026 storage mandate), BESS must equal at least 50% of solar capacity for 2-hour duration. For a 1 MW solar plant, that's 500 kW × 2 hr = 1 MWh battery. For voluntary BESS in other states, size to discharge ~25% of daily solar generation for 2-3 hours of evening peak coverage — typically 400-500 kWh for a 1 MW system. See our Maharashtra storage mandate post.

Should I include accelerated depreciation in sizing decisions?

Yes. The 40% Year-1 accelerated depreciation under Section 32(1)(iia) effectively reduces your Year-1 net cost by ~10% for a tax-paying corporate. For sizing decisions, this is a one-time benefit that doesn't change the right sizing ratio (88-93% self-consumption) but improves the IRR of the chosen size. See our solar accelerated depreciation guide.

How do I calculate my factory's roof solar capacity?

1. Measure gross roof area (sqft).
2. Subtract AHU plant, skylights, drainage, walkways, fire-segregation = net usable area.
3. For industrial roofs, net usable is ~60-70% of gross.
4. Multiply net usable (sqft) × 70 W/sqft = solar capacity in watts.
5. Compare with state NM cap and your day-shift load.

For a 30,000 sqft industrial roof, net usable is ~20,000 sqft, supporting ~1.4 MW DC of solar. Above 1 MW (most state NM cap), structure as 1 MW net metering + balance via net billing or open access.

Does future tariff escalation change the right sizing?

Yes, marginally toward larger plants. If grid tariff is expected to rise 4-7% YoY (typical Indian C&I trajectory), oversizing slightly toward 86-87% self-consumption (vs 90%+) is justified because the future-year arbitrage on exported kWh improves. However, the difference is modest — sizing is fundamentally about today's self-consumption math, with future tariff escalation providing 1-2 percentage points of IRR upside on either sizing choice.

Sources

• PVsyst v8 (industry-standard solar yield simulation)
• MNRE Net Metering Guidelines (state-specific)
• India installs record 45 GW solar capacity in FY2026 — pv magazine India