Commercial Solar System Sizing in India

Commercial solar projects in India, factories, warehouses, schools, hospitals, malls, are the highest-revenue segment for most EPC owners. They are also the segment where a sizing mistake costs the most: an undersized system leaves savings on the table and a disappointed client; an oversized system creates net metering cap problems and stranded capital. Getting commercial sizing right is a core EPC competency, and it starts with a structured process, not a back-of-envelope guess.

This guide walks through the complete commercial solar sizing methodology used by professional EPC companies in India: from reading the electricity bill, through load analysis, roof area calculation, inverter ratio, string design, and the battery-vs-grid-tie decision, all the way to a worked example with a 100 kW factory.

Why commercial sizing is different from residential

Residential solar sizing in India is relatively straightforward: 3 kW, 5 kW, or 10 kW systems on RCC terraces with standard roof configurations. The PM Surya Ghar framework even provides a rough reference, subsidy slabs start at 1 kW and the most common size is 3 kW for a 2–4 BHK home.

Commercial sizing has none of these simplifications. A 10,000 sq ft warehouse may have 200 kW of connected load but actually consume only 30,000 kWh/month because machinery runs in two shifts. A hospital may have 50 kW of connected load but a near-100% load factor due to 24/7 critical equipment. A school may have high connected load but generate most consumption during daytime hours, which aligns perfectly with solar generation.

Additionally, commercial connections in India face regulatory complexity: net metering caps at 25 kW for LT connections (in most states), captive vs. third-party ownership structures, accelerated depreciation eligibility, and different GST treatment for HT commercial connections versus residential. According to CEEW (Council on Energy, Environment and Water), India's commercial and industrial (C&I) segment accounts for over 40% of total electricity consumption, making it the single largest opportunity for rooftop solar deployment. Missing any of the regulatory factors in your sizing exercise leads to a proposal that cannot be executed or one that fails to deliver promised returns.

Step 1, Read the electricity bill correctly

The electricity bill is your primary data source for commercial sizing. You need three numbers:

Monthly consumption in kWh, found in the "Units consumed" or "Energy charges" section. For commercial customers, use the average over the last 12 months, seasonal variation is significant (summer air-conditioning loads may be 2–3× winter loads). If the customer only has the last 3 months' bills, ask for the previous year's bills or use the billing software if DISCOM provides online history.

Sanctioned load in kW, the contracted demand with the DISCOM. This is the upper cap for solar system size in many states, often expressed as: system size ≤ sanctioned load. If the sanctioned load is 50 kW and the client wants 100 kW of solar, they may need to upgrade their sanctioned load, which involves a separate DISCOM application process.

Tariff rate per unit (₹/kWh), the commercial tariff determines the financial case for solar. Industrial LT consumers in most Indian states pay ₹6–10/kWh; HT consumers pay lower per-unit rates but have demand charges that change the ROI calculation significantly. Central Electricity Authority (CEA) publishes the annual average commercial tariff by state, use this to check if your customer's quoted tariff is in range.

Step 2, Calculate system size from consumption

The core sizing formula for commercial solar in India:

System size (kW) = Monthly consumption (kWh) ÷ Monthly generation factor

The monthly generation factor is how many kWh a 1 kW solar system generates in your geography per month, averaged over the year. This ranges from 110–130 kWh/kW/month across most Indian locations:

Source: Solargis India irradiance data and MNRE Solar Resource Atlas. Annual generation values include a 10–15% system efficiency derating for inverter losses, wiring losses, soiling, and temperature. The solar payback period by state guide contains city-level generation benchmarks used in ROI calculations.

Step 3, Validate against roof area

Roof area is the second constraint on commercial system size. The rule of thumb is:

Roof area required = System size (kW) × 10–12 sq m/kW

This assumes standard 400–450 Wp monocrystalline panels (roughly 2.0–2.2 sq m each) with a 20–25% inter-row spacing allowance to prevent shadow from the row in front during low-angle winter sun.

Step 4, Check the net metering cap

India's net metering framework has a 25 kW cap for LT (low-tension) commercial connections in most states. Systems above 25 kW on LT connections either export under gross metering arrangements or must shift to captive consumption (zero export) design. The cap situation by connection type:

• LT Commercial (single-phase up to 11 kV): Net metering typically capped at 25 kW in most states. Some states (Gujarat, Maharashtra) have been liberalising this. Check current DISCOM regulations, they change frequently.
• HT Commercial (11 kV and above): Net metering or net billing at larger scales; regulations vary significantly by state and DISCOM. In some states, HT consumers must use gross metering or captive models above 1 MW.
• Captive model: For systems above the net metering cap, all solar generation is consumed on-site (no export). This requires careful load analysis to ensure the system does not generate more than the building consumes, otherwise generation is wasted.

For the detailed net metering cap rules and state-by-state situation, read our guide on net metering 25kW cap in India.

Commercial customers with owned premises (factory owners, warehouse owners, hospital management) should also evaluate accelerated depreciation, 40% in Year 1 under the Income Tax Act's Section 32. According to IEEFA (Institute for Energy Economics and Financial Analysis), accelerated depreciation consistently ranks as the top financial lever that tilts commercial solar investment decisions in India. Read our detailed analysis in the accelerated depreciation for solar guide.

Step 5, Inverter sizing and DC:AC ratio

Commercial string inverters are sized using a DC:AC ratio, also called the inverter loading ratio. The formula:

DC:AC ratio = Total panel capacity (kWp DC) ÷ Inverter rated capacity (kW AC)

For Indian commercial installations, the standard DC:AC ratio is 1.1–1.3. This means you can connect 110–130 kWp of panels to a 100 kW inverter. The rationale: solar panels rarely produce 100% of rated output in real Indian conditions (temperature derating, dust, spectral mismatch), so the inverter is sized to the realistic output range, not the peak nameplate DC figure. Oversizing panels relative to inverter capacity also improves generation in low-irradiance morning and evening hours when panels are below inverter clipping threshold.

For commercial systems above 30 kW, use multiple inverters of 25–30 kW each (the common commercial string inverter size range from Growatt, Solis, and others) rather than a single large inverter. Multiple inverters provide redundancy, if one fails, the others continue producing. This is critical for commercial customers where generation downtime translates directly to lost savings.

Step 6, String design

String design determines how many panels are connected in series per string, and how many strings connect to each inverter MPPT input. The key constraints are:

Voc check: Maximum open-circuit voltage (Voc) of the string at minimum expected temperature must not exceed the inverter's maximum input voltage. For Indian conditions, minimum temperature is typically 5–10°C in northern India, 10–15°C in central India. Panel Voc increases as temperature drops, calculate worst-case: Voc_string = Panel_Voc × (1 + Voc_TC × (T_min − 25°C)) × N_panels_per_string.

Vmp operating range: The string voltage at maximum power point (Vmp) must fall within the inverter's MPPT voltage window across the full temperature range in India (from 5°C minimum to 45°C+ in peak summer).

String count per MPPT: Each MPPT input on a commercial inverter accepts a defined number of strings in parallel. Confirm from the inverter datasheet.

For a 100 kW commercial system in Gujarat using 440 Wp panels (Voc 38V, Vmp 31V, Isc 12A) with a 100 kW inverter (MPPT range 200–850V DC, max input 1000V DC):

• Series string of 22 panels: Vmp = 22 × 31V = 682V (within MPPT range); Voc at 5°C = 22 × 38 × 1.025 = 856V (below 1000V limit)
• 22-panel strings are valid for this configuration

This level of design detail should be documented in every commercial proposal before submission. An EPC that shows string design calculations builds far more credibility with commercial customers than one that just names the inverter brand and capacity. This is also important for the commissioning process, see our solar commissioning process guide for what documentation is required at DISCOM inspection.

Stats, commercial solar market in India

Step 7, Battery vs grid-tie decision for commercial

The majority of commercial solar installations in India are grid-tied without batteries. The financial logic is clear: at commercial scale, battery storage is expensive per kWh, and the payback on the battery portion is long unless the customer has a specific need.

When to recommend battery for commercial:

• Site has significant grid outages during business hours (tier-2 industrial estates with 4+ hours daily load shedding)
• Business has critical loads that cannot tolerate outages (hospitals, cold storage, data centres)
• The DISCOM grid tariff has a high demand charge structure, batteries can reduce peak demand charges significantly, improving ROI

When battery is NOT needed for commercial:

• Grid is reliable (metro urban areas)
• Business operates daytime hours (office building, school), generation and consumption align well
• The additional capital cost would push payback beyond the customer's acceptable threshold

For commercial HT customers with demand charges, the battery sizing logic is different from residential, you are sizing to flatten the demand curve, not just to provide backup. This is a specialised design that requires load profile data (15-minute interval data from the smart meter) to execute properly. The general rule: if demand charges exceed 30% of the monthly bill, battery storage deserves serious analysis.

Worked example, 100 kW factory in Pune

Let's walk through the complete sizing exercise for a real-world scenario: a manufacturing factory in Pune, Maharashtra.

The client brief: 12,000 sq ft factory building. Monthly electricity bill: ₹1,20,000. One shift operation (8 AM–6 PM). Wants to maximise solar savings. LT commercial connection, sanctioned load: 150 kW.

1. 1

   Read the bill

   Monthly bill ₹1,20,000 at blended LT commercial tariff of ₹7/kWh = 17,143 kWh/month consumed. 12-month average used; summer peak months hit 20,000 kWh, winter low months 14,000 kWh. One-shift operation means 90%+ of consumption falls between 8 AM and 7 PM, excellent solar overlap.

2. 2

   Calculate system size

   Pune generation factor = 118 kWh/kW/month. System size = 17,143 ÷ 118 = 145 kW. However, the net metering cap for LT commercial in Maharashtra is 25 kW for standard connections; above 25 kW goes to gross metering/captive. Since this is a single-shift factory consuming most energy during solar hours, sizing for 90–95% captive self-consumption is the right approach. Captive target: 17,143 × 0.90 = 15,430 kWh/month. System size = 15,430 ÷ 118 = 130 kW. Round to 100 kW given roof constraint (see next step).

3. 3

   Validate roof area

   12,000 sq ft ≈ 1,115 sq m. Usable area after setbacks, AC units, and water tanks = approximately 750 sq m (67% utilisation). At 10 sq m/kW: 750 ÷ 10 = 75 kW. At 12 sq m/kW (with conservative inter-row spacing): 750 ÷ 12 = 62 kW. Site visit confirmed 800 sq m usable (20-degree tilt, east-west ridge): maximum ~80 kW on flat mounting or ~70 kW with tilted mounting. Final design: 100 kW DC (245 units × 415 Wp panels) on low-tilt mounting (5–10 degree) to maximise density.

4. 4

   Inverter configuration

   DC:AC ratio 1.15: 100 kWp DC ÷ 1.15 = 87 kW AC inverter capacity. Use 4 × 25 kW string inverters (100 kW AC total), this gives DC:AC ratio of 1.0 with room for future panel additions. Alternatively: 3 × 30 kW string inverters = 90 kW AC, DC:AC ratio = 1.11. Both are valid; 4 × 25 kW offers better redundancy for a factory setting.

5. 5

   ROI calculation

   Annual generation: 100 kW × 118 kWh/kW/month × 12 = 1,41,600 kWh/year. Annual savings: 1,41,600 × ₹7 = ₹9,91,200 (~₹9.9 lakh/year). System cost (100 kW commercial, installed): approximately ₹50–60 lakh. Simple payback: ₹55 lakh ÷ ₹9.9 lakh = 5.6 years. With accelerated depreciation (30% tax bracket, 40% Year 1 AD): effective Year 1 tax saving of ₹6.6 lakh, bringing effective net cost to ₹48.4 lakh. Adjusted payback: 4.9 years. Lifetime savings over 25 years: ₹2.5 crore+ (at current tariff, pre-escalation).

Pros and cons, commercial sizing approaches

Shadow analysis for commercial rooftops

Shadow analysis is non-negotiable for commercial systems above 25 kW. Unlike residential installations where a quick site visit can identify obvious obstructions, commercial rooftops often have HVAC units, ventilation structures, smoke exhaust stacks, and overhead cable trays that create complex shading patterns.

The standard tools used by professional EPCs:

• PVsyst, the industry standard simulation software used for bankable generation reports for large commercial and industrial projects. PVsyst reports are required by banks and financiers for project loans.
• HelioScope (cloud-based), faster to use than PVsyst for 50–500 kW systems; used by many commercial EPCs for initial proposals before committing to full PVsyst simulation.
• Google Earth + shade analysis plugins, adequate for initial desktop assessment but should not be used as the sole basis for a ₹50 lakh commercial proposal.

The shadow analysis determines the optimal panel layout, tilt angle, and row spacing to minimise inter-row shading while maximising panel density. For flat RCC rooftops (most Indian commercial buildings), the optimal tilt angle is typically 10–15 degrees for captive commercial designs (lower tilt = more panels per square metre; slightly less generation per panel but more total panels in the space). For south-oriented tilt optimisation, use tilt = latitude − 5° as the starting point.

How QuickEstimate handles commercial sizing automatically

When Rohit's EPC team receives a commercial inquiry, a factory owner wanting to know the cost and ROI for solar, the traditional workflow is: collect the electricity bill, do the sizing manually in Excel, build a proposal in Word, send a PDF. Each proposal takes 2–4 hours. With 10–15 commercial inquiries per month, that is 20–60 hours of engineering and proposal writing, before a single site visit.

• Proposal Generator, enter the monthly electricity bill, DISCOM, tariff rate, and roof area; QuickEstimate auto-calculates system size using the India-specific generation factors by city, applies the correct DC:AC ratio, and generates a branded commercial proposal with the worked ROI and payback calculation in minutes, not hours.
• Lead Capture, tag commercial leads separately from residential; track monthly bill value (a proxy for project size) directly in the pipeline so Rohit knows which commercial inquiries are worth prioritising for site visits.
• Pipeline Management, commercial projects have longer sales cycles and more stakeholders (finance manager, promoter, and sometimes a bank if project financing is involved). The pipeline tracks each stage, inquiry, site survey, proposal sent, negotiation, PO received, with assigned team member and follow-up dates so large commercial deals never slip through the cracks.

For the broader question of when a solar CRM transforms your commercial quoting workflow, read when to buy a solar CRM. If you are scaling your EPC business beyond residential into commercial, the solar EPC guide covers the business model transitions involved. Start your free QuickEstimate account and send your first automated commercial proposal today.

Frequently asked questions

How do I size a solar system for a commercial customer from just the electricity bill?

Divide the monthly kWh consumption by the monthly generation factor for your city (use 110 for lower-irradiance states like West Bengal; 120 for Gujarat, Maharashtra; 125 for Rajasthan). The result is the approximate system size in kW. Validate this against available roof area (10–12 sq m/kW) and the customer's net metering cap (typically 25 kW for LT commercial). For a captive design above 25 kW, size to 80–90% of the daytime consumption to avoid export waste.

What is the rule of thumb for roof area required for commercial solar in India?

Plan for 10–12 square metres of usable roof area per kW of solar capacity. This includes inter-row spacing to prevent winter shading between panel rows. For a 100 kW system, you need 1,000–1,200 sq m of usable roof area. The usable area on a typical commercial building is 60–70% of the gross rooftop footprint, account for setbacks from parapets, HVAC equipment, water tanks, and staircase structures.

Should a commercial factory choose net metering or captive (zero-export) solar?

For systems up to 25 kW, net metering is generally better, surplus power earns credits and the system can be sized to full consumption without waste. For systems above 25 kW, most states require moving to gross metering or a captive model. Single-shift factories with 90%+ daytime consumption are ideal captive solar customers, nearly all generation is self-consumed. Multi-shift or 24/7 operations need careful analysis because nighttime consumption cannot be solar-served without battery storage.

What DC:AC ratio should I use for a commercial solar system in India?

Use a DC:AC ratio of 1.1–1.3 for commercial string inverters in India. Start at 1.15 as your default: it captures the benefit of panel overclocking in low-irradiance morning and evening hours without causing significant midday clipping. For high-irradiance locations (Rajasthan, Gujarat), stay at 1.1–1.15 to minimise clipping. For moderate-irradiance locations (coastal Maharashtra, Bengal), 1.2–1.25 is acceptable.

Is accelerated depreciation available for all commercial solar installations?

Yes. Under Section 32 of the Income Tax Act, solar energy generating equipment qualifies for 40% accelerated depreciation in Year 1. This benefit is available to any company or firm that owns the solar asset (not available for leased or third-party owned systems). For a profitable company in the 30% tax bracket, the Year 1 tax saving from accelerated depreciation effectively reduces the net system cost by 12% (40% × 30%), a significant financial benefit that should be highlighted in every commercial proposal. Read the full analysis in our accelerated depreciation guide.

How long does a commercial solar project take from inquiry to commissioning?

A typical commercial rooftop solar project in India takes 60–120 days from signed purchase order to commissioning. Key timeline drivers: equipment procurement (2–4 weeks), structural design and DISCOM net metering application (2–6 weeks), installation (1–3 weeks depending on system size), and DISCOM inspection and net meter installation (1–4 weeks). The DISCOM approval and net meter installation step is the biggest variable, some DISCOMs process in 2 weeks; others take 3+ months. See our solar commissioning process guide for the stage-by-stage breakdown, and the payback period guide for state-level generation context for your ROI presentations.

What is the minimum roof area needed for a 100 kW commercial solar system?

A 100 kW system requires approximately 1,000–1,200 sq m of usable roof area using standard 400–450 Wp panels at low tilt (5–10 degrees). If the roof is smaller, consider higher-efficiency panels (500–550 Wp bifacial) which need less area per kW but cost 15–20% more per panel. For a 12,000 sq ft (1,115 sq m) factory building, plan for 60–75 kW as a realistic cap when accounting for setbacks and rooftop equipment, not 100 kW. Always verify usable area on site before quoting.

Does PM Surya Ghar scheme apply to commercial solar projects?

PM Surya Ghar Muft Bijli Yojana is a residential rooftop scheme, it applies to domestic consumers with residential electricity connections. Commercial consumers (LT commercial, HT industrial) are not eligible for PM Surya Ghar central subsidy. Commercial solar incentives in India come from separate channels: MNRE's CFA scheme for government and institutional buildings, state-level commercial solar programmes, and accelerated depreciation under the Income Tax Act. For the detailed comparison of which scheme applies to which consumer type, see our PM Surya Ghar vs CFA scheme guide.