How Much Rooftop Solar Does Your Factory Need? A Simple Sizing Guide

Most factories size rooftop solar the wrong way round. They measure the roof, fill it with panels, then hope the power gets used.

The better question is not "how much can my roof hold?" It is "how much solar can my factory actually use, and how much will the rules let me connect?"

The answer comes from four numbers. Find all four, take the smallest, and you have a sensible starting size. Here is how to do it in about an hour, with your electricity bill and a tape measure.

How much rooftop solar does my factory need?

Your right plant size is the smallest of four limits: your daytime load, your shadow-free roof area, your sanctioned load or contracted demand, and your state's rooftop capacity rule. The smallest number wins.

Start with load, not with the roof. Solar units used on site are usually your most valuable units.

How do I work out my factory's daytime load?

Look at what your factory draws between morning and late afternoon, on a normal working day and on a weekly off. Your base daytime load is the level you stay above on most working days. That is your first sizing number.

Monthly units on your bill are a useful start, but they hide the shape of your day. A factory that uses 2 lakh units mostly at night needs a very different plant from one that uses them on a day shift. So ask for interval data:

• Ask your discom for your load profile, if your meter records it.

• If not, have an electrician fit a logger at the main incomer for a few representative weeks.

• Note your weekly offs, shutdowns and any seasonal peaks.

• Note any planned new line or expansion, because your load may rise.

Avoid sizing to your peak. Peaks are short, and solar generation above your load has to be exported or wasted.

How much roof area do I need per kW of solar?

Plan for about 10 square metres of shadow-free roof per kW. That is the rule of thumb in the MNRE (Ministry of New and Renewable Energy) rooftop FAQ. Another industry guide, EAI, uses 100 to 130 square feet, or about 12 square metres, per kW.

So 7,000 square metres of clear roof would hold about 580 to 700 kW. Measure only the area that is truly usable. Take out:

• Shadows from tanks, parapets, chimneys and nearby buildings.

• Walkways, vents, skylights and space kept for maintenance.

• Roof sections due for replacement soon.

• Sections that may not carry the extra weight.

Weight deserves its own check. Industry guides put the added load from panels and mounting at about 15 to 20 kg per square metre, while another says pre-engineered sheds usually take 12 to 15 kg per square metre. Because estimates differ, get a structural engineer to check older sheds, especially corroded purlins, before you finalise the size. Our old factory roof solar readiness checklist covers the full check.

How many units will a rooftop plant generate per kW?

A reasonable planning range is about 1,400 to 1,650 units per kW a year, depending on location. The MNRE FAQ says a 1 kWp plant can generate 4 to 5.5 units on a clear sunny day, but not every day is clear, so do not multiply that figure by 365.

One industry guide gives these annual ranges for 1 kW:

The same guide says panels lose about 0.5% to 0.7% of output a year, and regular solar plant monitoring shows whether yours is keeping up. Ask any developer for a site-specific simulation, and ask what shading and losses it assumes. A number with no assumptions behind it is just a sales number.

What limits does my sanctioned load and state put on plant size?

In most cases, your connection and your state's metering rules put a hard ceiling on what you can connect. These rules change often, so check the current notification before you decide.

Three examples show how much the answer can vary:

Net metering sets off exports against imports. Net billing and gross metering value exports and generation differently, so read how each option works in your state before you compare them.

Two more checks often catch buyers out:

• Local grid capacity. Maharashtra's draft caps cumulative rooftop capacity at 70% of a transformer's or feeder's rated capacity, unless a load study allows more. In Karnataka, Power Peak Digest reports an 80% limit for plants under 150 kW on existing transformers. Your size may be limited by your neighbours' solar.

• Storage triggers. Maharashtra's draft proposes storage of 50% of capacity for 2 hours, or 25% for 4 hours, above 100 kW. Gujarat, as reported, asks for storage when consumers above 100 kW contract demand install beyond their sanctioned demand. These conditions can change a project's cost, so price battery storage into the plan wherever it applies.

What happens to solar power on Sundays and holidays?

It depends on your state's rules. Your plant keeps generating when your factory stops, so a six-day factory produces about one-seventh of its yearly solar on its weekly off. Whether that power has value is a state-rule question.

Compare two cases:

• Karnataka: exports set off against imports under net metering, with any net surplus bought monthly at the tariff KERC sets. Energetica reports a generic tariff of ₹2.34 a unit for non-DCR plants and ₹3.11 for DCR plants (DCR means Domestic Content Requirement). A factory with enough monthly imports can absorb more weekend exports.

• Gujarat: SolarQuarter reports that non-residential adjustment is capped at 30% of imports in each billing cycle, with banking charges, and that excess is treated as lapsed. So a month with 1,50,000 units imported would allow at most 45,000 units of adjustment. Export beyond that is lost. More in our guide to Gujarat's new rooftop solar rules.

Same plant, very different result. This is why a size that works in one state can waste power in another.

Check time-of-day (ToD) tariffs as well. ToD tariffs can change what each solar unit saves you. In Chandigarh, for example, a February 2026 report said industrial users get a 20% rebate from 9 am to 5 pm. So value solar at your actual daytime rate, not your average rate. Here is how to read your factory's electricity bill to find it.

Can you show me a worked example?

Yes. This is an illustrative factory, not a real client. Say it is an HT (high tension) unit in Bengaluru, running six days a week, using 30 lakh units a year:

The smallest number is 450 kW, so that is the starting size. At 1,500 units per kW a year, it would generate about 6.75 lakh units, or 22.5% of annual use.

At the HT-2a energy charge of ₹6.70 a unit, those units would be worth about ₹45 lakh a year in avoided energy charges. That is before what you pay for the solar power, and it assumes all units are set off at the full rate.

Could this factory go larger? Filling the 700 kW roof would generate about 10.5 lakh units, or 35% of annual use. In Karnataka, net metering may make that workable if monthly imports stay high enough, so it is worth modelling. Under a rule like Gujarat's reported 30% cap, the same choice could lose real power. Test both sizes against hourly data before you decide.

Does the size calculation change under CAPEX and OPEX?

The technical limits stay the same, but the risk moves. Under CAPEX, you buy the solar plant outright, so you carry the cost of any oversizing. Under OPEX or RESCO (Renewable Energy Service Company), a developer funds and owns the plant, and you pay per unit with no upfront cost.

Under OPEX, check these points:

• Whether the agreement sets a minimum offtake or a deemed-generation charge.

• What happens on Sundays and during shutdowns, and who bears the cost of unused units.

• Whether the plant can be expanded if your load grows.

• What assumptions the developer used for your load, and whether you can see the hourly model.

What should I ask a developer before accepting a proposed size?

Ask them to show how they reached the number. A good proposal should explain the size, not just state it.

1. What load data did you use, and for which months?

2. What share of generation will I use directly, and what share will be exported?

3. How is exported power treated under my state's current rules?

4. Does this size trigger a storage requirement?

5. Has local transformer capacity been checked?

6. What yield did you assume per kW, and what are the shading losses?

7. Has the roof been structurally checked?

8. How does the saving change if I shut down for two weeks a year?

FAQ

How much area does a 1 MW rooftop solar plant need?

Using the MNRE rule of thumb of 10 square metres per kW, about 10,000 square metres of shadow-free roof. EAI's figure of about 12 square metres per kW would make it about 12,000.

Can my rooftop solar plant be bigger than my sanctioned load?

Usually not under net metering, though rules vary by state. Karnataka's limit is the lower of sanctioned load or contracted demand and 1,000 kW. Gujarat, as reported, adds storage conditions for going beyond sanctioned demand at larger connections.

Should I fill my whole roof with solar?

Only if you can use or get value for the power. In states with strong export limits, a full roof can waste units, so check against your load and the current rules first.

How many units does 1 kW of rooftop solar generate per day?

The MNRE FAQ says 4 to 5.5 units on a clear sunny day. For annual planning, use a location-specific yearly figure, as actual days vary.

About SustVest

SustVest (SolarGridX Ventures Pvt Ltd) is a CRISIL Top-10 rooftop solar developer in India, running 80+ distributed solar plants (about 30 MW) across 13 states for factories, hospitals, hotels, tea estates and institutions. Under its OPEX/RESCO model, SustVest funds, builds, owns and maintains the plant, and you pay a lower per-unit tariff with zero upfront cost. Want a size based on your own load data? Talk to the SustVest team.

Sources

• MNRE: FAQ on Grid Connected Solar Rooftop System (via JBVNL)

• EAI: Steps to size your rooftop PV plant

• Bridgeway Power: Solar panel generation in India, units per kW

• Secured Engineers: Solar for factories

• KERC (Grid Interactive Distributed Solar Photovoltaic (DSPV) Plants) Regulations, 2026

• Power Peak Digest: Karnataka opens solar net metering to all consumers (26 August 2026)

• Energetica India: KERC revises solar tariffs in Karnataka (28 August 2026)

• SolarQuarter: GERC notifies 2026 Distributed Renewable Energy Regulations (2 November 2026)

• Energetica India: MERC proposes new rooftop renewable energy rules (23 September 2026)

• ThePrint (PTI): KERC increases electricity tariffs for commercial, industrial consumers (4 March 2026)

• ET EnergyWorld via Pulse: Peak-hour power to cost more in Chandigarh soon (27 February 2026)