Factory Solar System Design: Three-Shift Loads

Factory solar goes wrong when it is sized to the roof instead of the load. A plant that runs three shifts uses much of its energy at night, when solar cannot help, so the design question is how much of the array’s output the plant can use in daylight.

Take a hypothetical 4 MW automotive parts plant that runs 24 hours a day, six days a week. The roof could host about 3.2 MWp, and the plant manager assumes that array will offset most of the bill. Once night load, a midday output peak above daytime demand, and a low export rate are counted, the real offset can be far smaller. That is the central design problem of factory solar, and solving it has little to do with choosing better panels. All figures in this guide’s examples are assumptions for teaching, not data from a real plant.

Last updated: November 2026

This guide covers the design workflow for matching photovoltaic generation to three-shift manufacturing loads: load profile analysis, sizing math, inverter and storage decisions, tariff effects, and a worked example whose inputs are all stated assumptions.

What Makes Factory Solar Different From Other Commercial PV

Most commercial solar guides describe a building that runs from 8 AM to 6 PM, five days a week, such as an office, a school or a warehouse with daytime distribution. The load is a plateau from morning to late afternoon with weekend troughs, and solar fits that shape well.

A factory running three rotating shifts is different. Production lines do not stop. HVAC serving cleanroom or process-controlled areas runs around the clock. Compressed air, chilled water and process water loops are continuous. Lighting drops between shifts but never reaches zero. The result is a load profile that looks like a slightly wavy plateau across the whole week, with shift-changeover bumps and troughs during maintenance windows.

That shape changes the design assumptions. The daytime-to-night ratio of consumption matters more than the building’s capacity factor. The load floor matters more than the peak. Demand charges can matter more than energy charges. And tools that size commercial PV to peak demand or a typical commercial load shape can produce arrays that overgenerate at midday and undergenerate during shift transitions.

A poorly sized factory array does not just underperform, it can destroy value. As arithmetic only, assume the plant pays 0.14 USD per kWh for grid energy and is paid 0.04 USD per kWh for export. Every exported kilowatt-hour then gives up 0.10 USD compared with using it on site.

Load Profile Analysis: The Foundation of Factory Solar

Every factory PV design starts with one artifact: a 12-month, 15-minute-interval log of plant-level kW demand. This load profile shows most of what matters before a single panel is placed.

What the Load Profile Reveals

Five metrics come out of interval data. The percentile and time window are design choices, so state them in your calculation.

Daytime baseline (kW): a low percentile of demand during daytime hours on weekdays. This is the load your solar array can serve without exporting.

Daytime average (kW): the mean demand over the same window. It sits above the baseline.

Daytime peak (kW): a high percentile of the same window. Use it for inverter and switchgear sizing, not array sizing.

Nighttime average (kW): mean demand overnight. Compare it with the daytime average to see how much the plant really runs at night.

Weekend ratio: weekend daytime average divided by weekday daytime average. It is the best single predictor of weekend export risk.

Reading a Three-Shift Profile

This table shows the general shape of a three-shift load, not measured data. Real profiles differ by plant, so build yours from interval data.

Shift-change overlaps matter because the plant momentarily approaches its connected load. An array sized to the daytime average will undersupply during those windows, which is fine because the grid covers the gap. An array oversized to the peak will export surplus during calmer mid-shift hours, which is expensive when the export rate is low.

The 15-Minute vs Hourly Trap

A common mistake is designing factory solar from monthly bills or hourly averages. Both miss the granularity that matters. Many demand charges are based on the highest interval average in a month, often over 15 or 30 minutes, so check your tariff. Hourly data smooths out the real peaks, which means PV-plus-storage designs built on hourly data can underdeliver on demand charge reduction.

For a deeper treatment, see our guide on solar system sizing using 15-minute interval load data.

Sizing Methodology: From Load Profile to Array Size

Once you have clean 15-minute interval data, sizing follows five steps. The worked numbers use the hypothetical plant described later and are assumptions.

Step 1: Establish the Daytime Energy Pool

Sum the kWh consumed during daytime hours on the days the plant runs, for 12 months. This is the daytime energy pool. In the hypothetical plant below, assume 9.5 GWh of weekday daytime use for this illustration.

Step 2: Set the Self-Consumption Target

Pick a self-consumption target and keep exported energy small. This illustration assumes 92 percent. The math:

Maximum array generation = Daytime energy pool ÷ self-consumption target
                        = 9.5 GWh ÷ 0.92
                        = 10.3 GWh per year

Step 3: Convert kWh to kWp Using Specific Yield

Specific yield (kWh per kWp per year) depends on the site, the layout and the losses. Take it from a simulation for the actual site, such as a run in an energy-yield tool, not from a regional table. For the illustration, assume 1,550 kWh/kWp/year:

Optimal array size = 10.3 GWh ÷ 1,550 kWh/kWp/year
                   = 6,645 kWp DC

This is the largest size that holds the assumed 92 percent self-consumption. Installed size depends on roof availability, capex and grid export rules.

Step 4: Validate Against Roof Area

Use the module datasheet area plus row spacing for your layout. For this illustration, assume a 600 Wp bifacial module needs 2.4 m² including spacing. Read the bifacial solar panel design guide for layout considerations.

Roof area required = 6,645 kWp ÷ 0.6 kWp per panel × 2.4 m² per panel
                   = 26,580 m²

Usable roof area is the roof area left after skylights, HVAC mounts, fire setbacks and structural exclusion zones. Measure it for your building. For large plants, the load profile is often the binding constraint rather than the roof, but check both.

Step 5: Validate Against Grid Connection Limits

Many jurisdictions limit behind-the-meter or net-metered solar relative to contracted demand or by size band, and rules differ by country and utility. Ask the utility or regulator for the written rule before you fix the size. For deeper coverage, see our guide on grid export limitation rules by country.

Inverter and DC/AC Ratio Design for Continuous Loads

The inverter loading ratio (ILR), the DC array kWp divided by inverter AC kW, is one of the most consequential design choices in factory PV.

Why Factories Can Justify a Higher ILR

A higher ILR clips a little output at midday and raises output in the morning and afternoon, and it spreads the inverter’s fixed cost over more panels. For a factory whose daytime load stays above the inverter’s AC rating, midday clipping may cost little, because the plant can absorb every AC kilowatt the inverter produces and the clipped DC energy would have been exported at a low rate anyway. The shoulder-hour gain is added self-consumption.

This guide does not publish a recommended ILR range, because the right value depends on the load, the tariff and the module and inverter data sheets. Simulate clipping against your 15-minute load profile and compare several ratios.

Inverter Selection: String vs Central

Factors that can push a large factory toward central inverters:

• Large, uniform roof segments suit larger inverter blocks.
• Reactive power capability varies by model and grid code, so compare datasheets.
• A single point of medium-voltage connection can simplify interconnection when the factory already operates at medium voltage.

String inverters can win when:

• The roof has several orientations or is segmented by skylights and equipment platforms.
• The factory wants modular fault tolerance, so a single failure takes less capacity offline.
• The interconnection is at low voltage and the array is modest in size.

Compare installed cost per watt from real quotes for your roof. This guide does not publish a cost difference between the two.

Storage: When and How Much

Battery storage is often the most overspecified component in factory solar. Many proposals add it by default to support critical loads or provide backup. For many three-shift factories, the right answer is no storage, or a small system aimed only at demand charge reduction. Read adding battery storage services for the service side.

When Storage Can Make Sense

Storage on a factory PV system can pay back in three situations:

Scenario 1: High demand charges. If the demand charge is a large part of the bill, peak shaving with a short-duration battery can lower the billed peak. Estimate the value from your interval data and tariff.

Scenario 2: Wide time-of-use spreads. Where peak-hour energy costs much more than off-peak energy, the battery can charge from midday solar and discharge in the peak window.

Scenario 3: Export curtailment or near-zero feed-in. Where export is limited or unpaid, storage can absorb output that would otherwise be curtailed and use it later.

Sizing Storage for Three-Shift Factories

Unlike residential or small commercial solar, factory storage rarely covers the night shift, because that would require a battery far larger than most financial models support. Factory storage is usually sized for one of two jobs.

Demand charge reduction: battery energy equals the longest peak-event duration in hours times the peak-shaving target in kW, plus allowances for round-trip efficiency and depth-of-discharge limits. As arithmetic only, a 200 kW peak lasting 90 minutes needs 300 kWh of usable energy before allowances.

Self-consumption uplift on low-load days: a low-load Sunday creates hours of surplus. A battery can capture that surplus and use it early on Monday before solar ramps up. Size it from your interval data.

For a treatment of commercial storage sizing, see our guide on commercial battery storage sizing.

LFP vs NMC for Industrial Storage

Lithium iron phosphate (LFP) chemistry is widely used in stationary storage. Do not rely on a generic chemistry comparison for your project. Compare each candidate’s own datasheet on these points.

For industrial use, footprint often matters less than for vehicles, so cycle life, safety and warranty terms usually decide. As arithmetic only, one cycle a day for 10 years is 3,650 cycles, so compare that with each warranty’s cycle count.

Roof Considerations for Industrial PV

Factory roofs fall into three broad structural classes, each with its own PV implications. A structural engineer must set the allowable added load for the actual roof.

Metal Deck (Standing Seam or Trapezoidal)

The dominant roof type for modern factories. Standing seam roofs can accept clamp-on rail systems that avoid roof penetrations, which can help preserve the roof warranty. Trapezoidal profiles need either standing seam adapters or fasteners through the rib. Check the roofing manufacturer’s position on attachments before you design.

Wind uplift is a key engineering risk on metal-deck roofs. Tilt, attachment type and the row position all change the load on each clip, so ask the racking supplier for the uplift rating for your profile and have a structural engineer check it. See our flat roof ballasted solar systems guide for a related treatment.

Concrete Slab

Common in older factories and warehouses. Slabs can carry more dead load, but penetrations are expensive and waterproofing is a risk, so designs often lean toward ballasted systems. Ballasted designs need a wind and slope analysis, and the added ballast weight needs a structural review before any layout is fixed.

Asbestos Cement Sheet

Common in factories built before certain bans. PV mounting on such roofs creates hazardous-material handling issues. Many EPCs recommend addressing the roof before installation, sometimes by replacing the sheet and then installing PV. Follow local hazardous-material rules and ask a qualified surveyor.

Power Quality and Reactive Power Compensation

A frequently overlooked factor in factory PV design is the inverter’s ability to supply reactive power.

The Power Factor Problem

Industrial loads are heavily inductive. Motors, transformers and discharge lighting consume reactive power, which can lower the plant power factor below the level a tariff requires. Some tariffs penalize low power factor.

Factories often install capacitor banks for correction. These are passive and switched in steps, so they cannot follow fast changes in motor load.

How Solar Inverters Can Help

A grid-tied PV inverter can deliver real power and also supply or absorb reactive power, within limits set by its datasheet and the local grid code. Some models can supply reactive power even when the array is dark. Capability differs by model, so read the datasheet and confirm that the grid code and the utility permit that operating mode.

That can reduce, but may not remove, the need for a capacitor bank. Compare the avoided penalties and the avoided capacitor cost against any added inverter cost using real quotes.

Configuration Notes

Power factor correction from the inverter generally requires:

• Inverter firmware that supports the control mode you need, such as a fixed power factor or a voltage-dependent reactive power mode.
• A communication link to the plant SCADA or the main incomer meter.
• Control logic that prioritizes real power during the day and reactive power during shoulder and night hours.

Not all inverters offer this as standard. Specify it in the tender.

Tariff Structures and Their Design Impact

Three tariff structures dominate factory solar economics, and each pushes the design differently. For comparisons by country, see our net metering by country page.

Time-of-Use Energy Charges

Energy charges differ by hour of day, usually with peak, shoulder and off-peak blocks. Some industrial tariffs have many time periods and wide gaps between peak and off-peak prices, so read your own rate schedule.

Design implication: PV is most valuable in peak and shoulder hours. East-west split arrays, with morning and afternoon output peaks, can outperform south-facing arrays where the peak periods fall on both sides of noon. See our guide on east-west vs south-facing solar layouts.

Demand Charges (kW)

Demand charges bill the highest interval average power draw each month. As arithmetic only, a 2 MW billed peak at an assumed 20 USD per kW per month costs 2,000 × 20 × 12 = 480,000 USD a year on capacity, regardless of energy used.

Design implication: Solar must reliably reduce the billed peak. An array helps on a clear day at noon. On an overcast afternoon it does not, and the billed peak is set by the worst interval, not the average. Storage is the more reliable peak-shaving asset for demand charges.

Maximum Demand or Capacity Charges

Common in parts of Europe and Asia. The contracted demand, in kVA, sets the maximum simultaneous load and a fixed monthly charge. Exceeding it can trigger penalties or contract upgrades.

Design implication: Solar that reduces utility-supplied demand may support a contracted demand reduction at renewal, which turns a one-time renegotiation into lasting fixed-charge savings if the utility allows it. Quantify it and include it in the solar proposal platform output.

For European factories, see our guide on solar self-consumption rules across Europe.

A Worked Sizing Example: Hypothetical 4 MW Auto Parts Plant

This is a worked example for a hypothetical plant archetype. Every input is an assumption for teaching. It is not a real project, and the cost and tariff inputs are not market data.

Plant Profile (All Assumed)

Sizing Calculation

Daytime energy pool (weekdays + Saturday): 14.2 GWh

Self-consumption target: 92%

Maximum array generation at that target: 14.2 ÷ 0.92 = about 15.4 GWh

Assumed specific yield: 1,575 kWh/kWp/year

Array size for 15.4 GWh: 15,430 ÷ 1,575 = about 9,800 kWp

Roof constraint: 32,000 × 0.70 = 22,400 m² usable. At 2.4 m² per 600 Wp panel, 9,333 panels = 5,600 kWp. The roof binds.

Net metering constraint: 1,000 kWp is eligible for net metering in this example. Above that, export is paid at 0.045 USD/kWh.

Final array size: 5,600 kWp, limited by roof, with the first 1 MW under net metering.

Inverter Configuration (Assumed)

• AC rating: 4,400 kW (ILR = 5,600 / 4,400 = 1.27)
• Topology: set in the tender after comparing string and central options
• Estimated noon clipping: 2.1% of annual DC production (assumed)

Annual Production and Self-Consumption (Assumed)

• AC delivered after all losses: 8,150 MWh per year (assumed)
• Daytime energy pool, weekdays + Saturday: 14,200 MWh, so output fits inside it
• Sunday export: about 230 MWh per year, or 2.8% of generation (assumed)

Financial Outcome (Illustrative)

Battery Time-Shift Modeling for Factories with Demand Charges

For factories where demand charges dominate the bill, battery time-shift modeling becomes the central design exercise. The method:

Step 1: Identify the Top Demand Events

From 12 months of 15-minute data, extract the highest-demand events. These are usually moments when production lines, HVAC and compressed air peak together, so look at the time of day and the season.

Step 2: Quantify Peak Shaving Value

For each high-demand event, calculate:

Peak shaving value = (event peak kW - baseline kW) × demand charge rate × 12 months

As arithmetic only, if the top events average 350 kW above baseline at an assumed demand charge of 22 USD per kW per month, peak shaving value = 350 × 22 × 12 = 92,400 USD a year.

Step 3: Size the Battery to the Event

For events lasting 1.5 hours at 350 kW of peak shaving, with assumed efficiency and depth of discharge:

Battery energy required = 350 kW × 1.5 hours ÷ (round-trip efficiency × usable DoD)
                       = 525 kWh ÷ (0.92 × 0.90)
                       = 634 kWh nameplate

Battery power rating must cover the 350 kW continuous target, with margin for short-duration peaks.

Step 4: Validate Solar-Charged vs Grid-Charged

A battery charged from solar avoids exporting at a low rate. A battery charged from off-peak grid energy creates tariff arbitrage. Many factory designs use a hybrid dispatch, charging from solar when surplus exists and from off-peak grid energy when needed for next-day peak coverage.

Check the Payback From Quotes

Divide the quoted installed cost of the battery by the annual savings you computed to get a simple payback. This guide publishes no installed battery price, so use written quotes only.

Compressed Air, Chillers, and HVAC: The Hidden Solar Leverage

Three load categories dominate many factories. Each interacts with solar differently.

Compressed Air

Compressors are often a large share of plant electricity use. They are largely steady-state: they cycle on and off but the duty cycle is consistent across shifts. That makes compressed air a reasonable solar match, because the load is tied to production and the demand profile is predictable. Air receivers hold some stored energy, which can smooth short interruptions, but check your receiver size and the compressor controls.

Chillers and Process Cooling

Chiller loads correlate with ambient temperature, so summer afternoon chiller demand peaks when solar output peaks. A chiller with thermal storage, such as an ice tank or chilled water tank, extends the match: it can run at higher capacity during solar peak and reduce load during shift changes. For cooling-only applications, thermal storage can be a cheaper option than batteries, so compare quotes for both.

HVAC

HVAC loads correlate with ambient temperature and occupancy. Three-shift factories have steady occupancy, so HVAC is more weather-driven than shift-driven. Solar matches well in cooling season and poorly in heating season, when more HVAC demand falls at night. In mixed-climate factories, HVAC alone is rarely the dominant solar argument. It is one of several daytime loads that solar covers efficiently.

Roof Mounting Methods for Industrial-Scale Arrays

The mounting system on a factory roof affects installation speed, structural risk and long-term maintenance access.

Clamp-On Standing Seam

Best for: standing seam metal roofs in good condition. Pros: no penetrations, fast installation, can preserve the roof warranty. Cons: limited tilt options, sensitivity to wind uplift, and the need to confirm the seam profile fits the clamp.

Through-Deck Fasteners on Trapezoidal

Best for: trapezoidal profile metal roofs with good substrate condition. Pros: proven structural attachment and flexible panel layouts. Cons: penetrations need flashing, installation is slower, and watertightness depends on correct sealing.

Ballasted on Concrete Slab

Best for: flat concrete roofs with enough structural capacity. Pros: no penetrations and a reversible installation. Cons: heavy, needs structural review, limited to flat or near-flat substrates, and ballast can shift over time.

Tensioned Cable Systems

Best for: sawtooth or arched factory roofs with high vertical clearance. Pros: adapts to non-flat geometries and can avoid roof contact. Cons: specialized engineering, higher capex and a narrow supplier base.

For most modern factories with metal-deck roofs, the choice is between clamp-on standing seam and through-deck on trapezoidal. The seam profile and the roof age usually decide which fits.

Working with Long-Term Power Purchase Agreements

For factories with strong credit, a third-party-owned power purchase agreement (PPA) can convert a large capex into an operating expense.

How Industrial PPAs Work

A renewable energy developer or financier installs, owns and operates the array. The factory commits to buy the output for a fixed term at a fixed or escalating rate. Term and price are negotiated, so compare offers against your own tariff.

When PPAs Can Make Sense

• The factory has limited capex or competing capital priorities.
• The tax structure does not reward direct ownership, for example no investment tax credit or depreciation benefit.
• The factory wants a fixed energy rate to hedge tariff increases.
• Operations and maintenance is outside its core competency.

When PPAs May Not Make Sense

• The factory has cheap access to capital and a long planning horizon.
• Local depreciation or tax incentives favor direct ownership.
• The factory’s credit is not strong enough to attract competitive PPA pricing.
• The factory plans to relocate within the PPA term.

Compare direct ownership and a PPA on the same assumptions, including tax treatment, financing cost and O&M. This guide publishes no typical IRR for either route.

Common Design Mistakes in Factory Solar

The same mistakes recur in industrial PV designs.

Sizing to Roof Area Instead of Load

Engineers see a large roof and propose an array the roof can hold. If the plant’s daytime baseline is far below that array’s output, a large share of generation is exported at a low rate. Size to the load, even if part of the roof stays empty.

Designing Without 12 Months of Data

Single-month or three-month load data hides seasonal variation. A plant with cooling-driven load is higher in summer. A plant with process heat can be higher in winter. Solar sized to one season oversizes for another.

Ignoring Process Variability

Factories rarely run at constant production. Order books change, model changeovers cut throughput, and annual maintenance shutdowns reduce load for a period. A design that assumes full production for every working week overestimates self-consumption. Include your actual shutdown calendar.

Underspecifying the Inverter Communication Layer

A factory PV system must often integrate with plant SCADA, the main incomer meter and the energy management system. Inverters that ship without the protocol your plant uses, such as Modbus TCP, OPC-UA or IEC 61850, can force costly retrofits. Specify it in the tender.

Skipping the Structural Engineer

A large rooftop array imposes loads that few existing roofs were designed for. Skipping a structural review to save engineering fees is a common cause of roof damage and warranty disputes.

What Does Solar for a Factory Cost in India?

There is no responsible single per-kilowatt answer for an Indian factory before the EPC has inspected the site and reconciled interval load data with the proposed electrical scope. A quote for a structurally ready shed with spare low-voltage capacity is not comparable with one that includes roof strengthening, a dedicated transformer, high-tension interconnection, shutdown work, or export control.

Ask every bidder to price the same schedule: modules and inverters; mounting structure; structural survey and strengthening; DC and AC cabling; protection, earthing, lightning protection, and monitoring; transformer or switchgear changes; metering and DISCOM work; access and fall protection; testing; taxes; warranties; O&M; and every exclusion. The CEA’s 2023 safety regulations set solar-specific protection and testing requirements, while the BIS compulsory-registration list identifies regulated PV equipment categories. The project engineer must confirm the standards and approvals applicable on the quotation date.

Do not accept a payback number based only on annual units. Model 15-minute factory demand, production shutdowns, Sunday load, self-consumption, export treatment, contract demand, demand charges, degradation, downtime, O&M, inverter replacement, financing, tax treatment, and commissioning delay. Run a downside case with lower yield and delayed approval before comparing CAPEX, lease, RESCO, or PPA proposals.

When Heaven Green Energy may fit

Heaven Green Energy currently describes turnkey commercial and industrial EPC for factories and businesses, including design, procurement, installation, and maintenance. Its published service scope covers Gujarat and broader C&I work, but the company must confirm site serviceability, DISCOM experience, equipment, exclusions, and contractual guarantees for the specific factory.

Disclosure: SurgePV is promoting Heaven Green Energy through a commercial relationship. Apply the same engineering, financial, reference, and contract checks to every EPC bidder.

Indian factory owners can review Heaven Green Energy’s current commercial and industrial EPC scope and request a site-specific, itemized proposal.

Conclusion

• Start with 12 months of 15-minute interval data, not roof area or panel count. Every meaningful design decision flows from the load profile.
• Size the array to the daytime baseline and keep exported energy small. Increase size only if export economics justify it, and check weekend load.
• Add storage only when demand charges or time-of-use spreads create a clear payback. Defaulting to storage on every project can destroy returns.

To model these choices for a specific plant, book a SurgePV demo or read how the generation and financial tool handles payback inputs.

Frequently Asked Questions

How do you size a solar system for a factory running three shifts?

Start with 15-minute interval load data for at least 12 months, find the weekday daytime baseline, and size the array so nearly all generation is used on site. Night-shift load is served by the grid whatever the array size. Check weekend load too, because that is where oversized arrays export.

Should a factory with continuous production add battery storage?

Add storage only if the tariff pays for it, through demand charges or a wide time-of-use spread. Three-shift factories already use most solar on site, so battery value comes from peak shaving and tariff arbitrage, not from time-shifting energy. Model both against your interval data and tariff before you commit.

What is the right inverter loading ratio for a factory rooftop?

There is no single right ratio, because it depends on the daytime load, the tariff and the equipment datasheets. A higher ratio adds output in the morning and afternoon and clips a little at noon. Simulate clipping against 15-minute load data and check each inverter’s maximum DC input.

How much rooftop space does a factory need for meaningful solar coverage?

Roof need is array size divided by module power, times the area per module including row spacing. Use the module datasheet and your layout instead of a rule of thumb. For example, if a 600 Wp module needs 2.4 square metres including spacing, a 1 MWp array needs about 4,000 square metres.

Do factory solar systems need to be designed differently for night shifts?

Yes. The system itself is not different, but the financial model is. Night-shift consumption is supplied by the grid whatever the solar size, so the target shifts from maximizing generation to maximizing daytime self-consumption. Size from the daytime load, not the roof.

How does a factory’s power factor affect solar design?

Inductive loads lower the power factor, which can trigger penalties on some industrial tariffs. Some inverters can supply reactive power, so they may help correct power factor, but capability varies by model and grid code. Check the inverter datasheet and the utility rules before you drop a capacitor bank.

What payback period is realistic for factory solar in 2026?

Payback depends on installed cost, tariff, self-consumption and export treatment, so there is no standard figure. In this guide’s illustrative example, simple payback is about 3.9 years on assumed inputs that are not market data. Model your own tariff, shutdown weeks and degradation, and run a downside case.