Generator Sizing for Factories: A Practical Guide

If you have already worked through the basics of adding up connected loads, you know the drill: list your equipment, note the wattage, apply a safety margin, buy a generator. That works reasonably well for a house, a shop, or a small office. It falls apart on a factory floor.

This post assumes you already understand basic load calculation. If you haven’t, start with our load calculation basics guide first, since we won’t repeat that groundwork here. What follows is specific to industrial loads only: the heavy three-phase induction motors on your compressors, CNC machines, and cranes; starting current spikes five to seven times higher than running current; star-delta and VFD starting methods and how each changes your sizing math; multi-shift duty cycles; and the harmonic distortion VFDs and welding sets introduce into a generator’s alternator. None of this is home, office, or hospital territory. It decides whether your genset trips on start-up or runs your plant for fifteen years without drama.

To make the engineering easier to follow, we’re running your factory’s electrical demand through a Power Fitness Assessment, the kind of physical evaluation a sports physiologist would run on an athlete, applied to your load profile. It’s a framing device, not a replacement for the math. Every section has real formulas and multipliers, and there’s a fully worked numeric example near the end. Treat the metaphor as commentary, not the substance.

Why Factory Loads Are a Different Animal

A home or office load is mostly resistive or lightly inductive: lights, fans, computers, ACs. A factory load is dominated by large three-phase induction motors doing real mechanical work, cyclical duty across multiple shifts, and electronic drives that don’t behave like traditional loads. Three things make industrial sizing genuinely harder:

1. Motor starting current. A motor at standstill behaves like a short circuit until it spins up, drawing several times rated current for a few seconds.
2. Overlapping operation. Machines switch on and off through a shift, so the generator must handle the worst combination of “already running” plus “just switched on,” not a static sum.
3. Non-linear loads. VFDs, welding rectifiers, and electronic ballasts distort the current waveform, stressing an alternator differently than they stress the utility grid.

None of these show up meaningfully in a home load list. All three get their own line item below.

Resting Heart Rate: Baseline Connected and Running Load

Every fitness assessment starts with a resting heart rate, the number your body settles at when it isn’t doing anything strenuous. For a factory, this is the steady-state load: what machines actually draw once running their normal cycle, not their nameplate maximum.

You should already have a connected load list from the basics guide. The difference now is interpretation. Nameplate horsepower is a motor’s rated full-load output, not what it draws every second of a shift. An injection molding machine idles between shots. A screw compressor cycles between loaded and unloaded. A CNC spindle on a light finishing cut draws far less than on a heavy roughing pass. Your resting heart rate for sizing purposes is the realistic average running load per machine, grouped into demand load (covered under VO2 max below), the floor everything else gets added on top of.

The Sprint Test: Motor Starting Current

A sprint test measures anaerobic capacity, a short, explosive burst well beyond what the body can sustain. Motor starting current is the electrical equivalent: a brief spike unrelated to the machine’s normal running draw.

When a three-phase induction motor starts Direct-On-Line (DOL), it behaves like a transformer with a shorted secondary until the rotor turns. Rehlko/Kohler’s own generator-sizing documentation states that “starting current is typically six times a motor’s rated full-load current, and this inrush current stays high until the motor reaches about 75 percent of rated speed.” In practice, factory motors commonly show starting currents in the five to seven times range depending on design, load inertia, and terminal voltage.

This matters because the alternator has to supply that spike without the voltage collapsing so far that contactors drop out or VFDs fault. A generator that comfortably covers steady running load can still stall on a single large motor start if it wasn’t sized for this. It’s also why generator kVA and kW ratings include a separate motor-starting capability figure distinct from the continuous rating, usually published on the same data sheet.

Starting current is not fixed, though. How you start a motor changes the number dramatically:

• DOL: Simplest and hardest on the generator, full 5x to 7x starting current at once.
• Star-delta (wye-delta): Starts in a reduced-voltage star configuration before switching to delta. Per Kohler, this cuts starting current to roughly a third of the DOL value, around two to two-and-a-half times running current.
• Autotransformer starters: Tap voltage down (commonly 50, 65, or 80 percent), reducing current roughly in proportion, though torque falls with the square of that reduction.
• Soft starters: Thyristor-based, ramping voltage with an adjustable current limit, often 2x to 4x.
• VFDs: Ramp voltage and frequency from zero, so there’s no hard inrush, starting current stays close to running current.

Don’t assume DOL for every big motor. Check your compressor, crane, and pump panels. If they already use star-delta, soft starters, or VFDs, you can size against a much smaller spike than the worst-case DOL figure, sometimes the difference between a 400 kVA and a 550 kVA genset.

One-Rep Max: Your Single Highest Momentary Demand Point

A one-rep max is the single heaviest lift you can complete once, not what you can sustain. For a factory generator, it’s the worst moment the electrical system will ever demand: typically the largest motor starting while the rest of the plant is already near its normal peak.

This figure, more than average running load, sets minimum genset capacity. A generator sized only for steady-state demand can be dangerously undersized for the transient spike, since starting current is additive on top of whatever else is already drawing power.

The approach: identify the largest motor by starting kVA (not running kVA), assume everything else is at a realistic running peak, and add that motor’s starting kVA on top. If your largest motor is VFD-started, check the next-largest DOL or star-delta motor too, since a smaller hard-started motor can sometimes spike harder. We’ll run full numbers below, but the point is that a genset’s momentary capability, not just its continuous rating, has to clear this bar with margin.

Sustained VO2 Max: Full-Load Running Capacity

VO2 max measures the maximum oxygen the body can use during sustained, intense effort, what you can hold, not just spike to. For a factory, this is the genset’s sustained capability during peak production hours, arrived at through demand factor and diversity factor.

If you added up the full nameplate rating of every motor and machine, you’d grossly oversize the generator, since not everything runs at full nameplate simultaneously. Demand and diversity factors correct for that.

Demand factor is the ratio of actual maximum demand to total connected load, always less than one. Electrical Contractor Magazine notes that demand factor is used to calculate what portion of a system’s connected load will actually be drawn at once, which is why feeder and generator capacity can legitimately be sized below the sum of every connected device. A multi-shift line with machines cycling on and off will have a demand factor well under 1.0 for most categories.

Diversity factor works across sections rather than within one machine group. Electrical4U defines it as the ratio of the sum of individual maximum demands of separate parts of a system to the simultaneous maximum demand of the whole, typically greater than one because different sections peak at different times. Your press shop, paint line, and packing area rarely all peak in the same window, so the plant’s coincident peak is smaller than the sum of each section’s own peak.

In practice: calculate demand load section by section with realistic demand factors, then apply a diversity factor across sections for one coincident maximum demand figure. Converted to kVA at your power factor, that’s the sustained load the genset must hold at full production, not the theoretical maximum if everything ran flat out together, which rarely happens.

Recovery Time Between Sets: Duty Cycle and Staggered Starting

Athletes don’t do heaviest lifts back to back with no rest; recovery is part of the plan. The same applies to starting large motors: stagger them.

If your compressor, chiller, and largest CNC machine all auto-restart together after a power interruption or a scheduled genset test, you stack multiple starting spikes into a demand worse than any single motor’s one-rep max, a common and avoidable mistake.

The fix is sequential motor starting, usually via the AMF panel or PLC-based load sequencing, restarting large motors one at a time with a short delay so voltage and frequency recover before the next load hits. This connects to load factor and duty cycle: a genset run near its continuous rating for long stretches, with badly timed simultaneous restarts on top, runs hotter and trips more than one with properly staggered starts.

Power Factor: The Efficiency of Your Effort

Two athletes can lift the same weight with very different efficiency. Power factor is the electrical version, and it changes how big a generator you need for a given amount of usable work.

Power factor (PF) is the ratio of real power (kW) to apparent power (kVA), the total power the generator supplies including the reactive component motors demand but don’t convert into work. Generator Source explains that PF equals kW divided by kVA, and most diesel generators are rated at 0.8 PF, so kVA = kW / 0.8. A motor-heavy floor, especially with lightly loaded or oversized motors, tends to run at a lower power factor than a resistive load, needing more kVA to deliver the same real kW.

This is where power factor correction earns its keep. Capacitor banks near large motors reduce the reactive power the generator must supply, improving overall PF and shrinking the kVA needed for the same real output, sometimes the difference between needing a bigger genset and comfortably fitting an existing one. Check your billed power factor (Indian utilities commonly penalize poor PF) as an early diagnostic before finalizing a size.

Harmonic Load: VFDs, Welding Sets, and Alternator Strain

Harmonic distortion is like an arrhythmia: a disruption that stresses the whole system even when the average number looks fine. VFDs, welding rectifiers, and other non-linear loads don’t draw current as a smooth sine wave; they draw it in pulses containing frequencies well above the base 50 Hz supply. A utility grid, with very low source impedance, absorbs this reasonably well. A generator’s alternator has much higher internal impedance, so the same harmonic currents produce disproportionately larger voltage distortion.

A white paper from the IEEE Houston Section on alternator reactance and non-linear loads explains that harmonic currents interact with system impedance to induce harmonic voltages, and because generator impedance is much higher than utility impedance, the same current produces significantly larger distortion. It notes that generators serving heavy non-linear loads have traditionally been oversized, with rules of thumb from 1.4 to as much as 5 times the load’s kVA depending on the equipment, and that alternator subtransient reactance (X”d) is the key spec driving how well a machine handles this.

For a floor with multiple VFDs and welding sets on the same bus, this isn’t theoretical. Practical steps:

• Ask your supplier for the alternator’s X”d and THD capability for non-linear loads, not just the continuous kVA rating.
• Where harmonic load is a large share of demand, budget a margin, commonly 10 to 25 percent, on top of the base kVA figure.
• Consider line reactors or harmonic filters on the largest VFDs, often cheaper than oversizing the whole genset.
• Understand how synchronous vs asynchronous generators differ here, relevant to industrial-grade harmonic handling.

This is part of what separates a genset built for factory duty from one built for light commercial use; see what makes a generator industrial-grade for the broader construction and control differences.

Training Progression: Headroom for Future Expansion

No athlete trains to their exact current one-rep max and stops; a program builds in progression. Your generator needs the same forward margin, since lines get added and machines get replaced with bigger ones far more often than removed.

A generator sized precisely to today’s coincident peak, with no growth allowance, is one you’ll replace again in two or three years the moment you add a line or a new CNC cell. Common practice is 15 to 25 percent headroom above calculated present demand for plausible near-term expansion, more if a new line is already budgeted.

This is also where standby versus prime power matters. If the genset runs only during outages, a standby-rated unit with reasonable headroom is usually appropriate. If you’re running significant daily hours across shifts, a prime-rated set is the right starting point, and headroom matters even more since prime units run harder for longer.

The Worked Example: A Mid-Size Auto-Ancillary and Plastics Unit

Here’s a fully worked, illustrative example: a mid-size auto-ancillary and plastics component unit near Noida, running two shifts, producing injection-molded components with in-house CNC machining and assembly. All figures are illustrative for demonstration, not measurements from an actual facility.

Step 1: Connected Load and Realistic Demand Factors

Equipment	Qty	Rating (each)	Total connected load	Demand factor	Demand load
Injection molding machines	5	40 HP (29.8 kW)	149.2 kW	0.75	111.9 kW
Screw air compressors	2	60 HP (44.8 kW)	89.5 kW	0.85	76.1 kW
CNC machining centers (VFD-driven)	4	15 HP (11.2 kW)	44.8 kW	0.60	26.9 kW
EOT crane (5-tonne)	1	25 HP (18.7 kW)	18.7 kW	0.20	3.7 kW
Conveyor motors	6	5 HP (3.7 kW)	22.4 kW	0.80	17.9 kW
Mold-cooling chillers	2	25 HP (18.7 kW)	37.3 kW	0.90	33.6 kW
MIG welding sets	3	15 kW	45.0 kW	0.35	15.8 kW
Utility and water pumps	2	10 HP (7.5 kW)	14.9 kW	0.50	7.5 kW
Lighting, office, utilities	–	–	40.0 kW	0.90	36.0 kW
Total			461.8 kW		329.4 kW

Notice the spread: continuous-duty equipment like chillers and compressors sits at 0.85 to 0.90, while intermittent equipment like the crane and welding sets sits much lower, reflecting real duty cycle rather than nameplate capacity.

Step 2: Plant-Wide Diversity Factor

Different sections don’t peak simultaneously, so a diversity factor is applied to the summed demand load. Using an illustrative 1.15 across the plant’s functional sections:

329.4 kW / 1.15 = 286.4 kW coincident maximum demand

Step 3: Convert to kVA

At a plant power factor of 0.8 (typical for a motor-heavy floor without capacitor correction):

286.4 kW / 0.8 = 358.0 kVA sustained running demand, the VO2 max figure the genset must comfortably hold during peak production.

Step 4: The One-Rep Max, Motor Starting Scenario

The largest motors are the two 60 HP screw compressors, each drawing roughly 57 kVA at full running load (using the rule of thumb that a mid-size three-phase motor draws close to 1 kVA per horsepower). Assume star-delta starters, standard practice at this size.

Star-delta reduces inrush to roughly 2.5 times running kVA instead of 6x DOL, so one compressor starting cold draws approximately 57 x 2.5 = 142.5 kVA versus its own 57 kVA running.

Worst case: Compressor 1 and the rest of the plant are already at the Step 3 coincident demand, and Compressor 2 starts cold. Removing its average running share (roughly 47.6 kVA, since both compressors together contribute about 95 kVA of the 358 kVA total) and adding its starting kVA instead:

358.0 – 47.6 + 142.5 = 452.9 kVA momentary peak demand

That momentary figure, not the 358 kVA sustained figure, is the true one-rep max the alternator must clear without excessive voltage dip.

Step 5: Margin for Harmonics and Expansion

With four VFD-driven CNC machines and three welding sets on the bus, this floor carries a meaningful non-linear load share. Budgeting 10 percent for harmonic handling and a further 15 percent for planned expansion (a sixth CNC cell already on the roadmap):

358.0 kVA x 1.10 x 1.15 = 452.9 kVA

This lands almost exactly on the motor-starting one-rep max from Step 4, a common pattern: harmonic and expansion margins on sustained load, and the pure starting transient, often converge on a similar target, a reasonable confirmation the number is in the right zone.

Step 6: Final Recommendation

Rounding up from the ~453 kVA figure with reasonable headroom, a 500 kVA prime-rated genset is a defensible single-unit recommendation, or two 250 kVA units in parallel, adding redundancy (service one unit without a full plant outage), modularity for expansion, and better harmonic handling since loads split across both. Either way, confirm CPCB IV+ compliance before purchase, covered next.

The Fitness Report Card: Summary

Fitness Test	What It Measures	Calculated Result	What the Genset Must Deliver
Resting heart rate	Baseline connected load	461.8 kW connected	Realistic floor for the calculation
VO2 max (sustained)	Coincident demand after demand and diversity factors	358.0 kVA	Continuous full-load running capability
Sprint test	Motor starting current multiplier	2.5x (star-delta) vs 6x (DOL)	Alternator absorbs the spike without excessive voltage dip
One-rep max	Single highest momentary demand	452.9 kVA	Peak transient capability without tripping
Harmonic and expansion margin	VFD/welding load and future growth	+10% and +15%	Included in final unit sizing
Final recommendation		500 kVA (or 2 x 250 kVA parallel)	Prime-rated, CPCB IV+ compliant

Compliance: Don’t Separate Sizing From Regulatory Approval

Sizing and compliance aren’t separate conversations in India, they’re one. A factory installing a new DG set, or resizing an existing one, needs to check both the engineering fit and the regulatory fit before finalizing a purchase.

New and resized DG sets above applicable capacity thresholds generally need to meet CPCB IV+ emission norms, and older generators that predate these norms typically need a Retrofit Emission Control Device fitted to continue running legally, particularly in Delhi-NCR where GRAP restrictions can limit or suspend DG set operation during high-pollution periods. Our explainer on why RECD is mandatory in India covers the background if you’re evaluating an older genset rather than buying new. Separately, factory installations typically require approvals tied to your factory license; see factory license requirements for DG set installation and the DG set emission regulations reference page.

Sequence it this way: finalize kVA sizing first, then check compliance requirements at that specific capacity and location, since they vary by capacity slab and rules like GRAP staging.

Common Sizing Mistakes on Factory Floors

• Sizing off nameplate connected load with no demand factor applied. Routinely produces a genset two to three times larger, and pricier, than needed.
• Ignoring starting method when calculating motor starting kVA. Assuming DOL for a star-delta or VFD-started motor causes oversizing; assuming the reverse causes dangerous undersizing.
• Treating three-phase motor loads like single-phase resistive loads. Starting behavior and protection requirements differ; see single-phase vs three-phase generators if your plant runs mixed loads.
• Skipping harmonic assessment with multiple VFDs on the floor. A generator correctly sized on kW and kVA alone can still misbehave if harmonic load wasn’t factored in.
• No staggered starting logic on the AMF panel or PLC. Turns several manageable spikes into one unmanageable one.
• No expansion headroom at all. The single most common reason factories replace a genset before its rated service life ends.

These fundamentals, plus broader buying pitfalls, are covered in common mistakes to avoid when buying a diesel generator, worth reading alongside this one.

Frequently Asked Questions

How much bigger should a factory generator be than the connected load? Rarely should it match connected load at all. After realistic demand and diversity factors, most floors need a genset closer to 60 to 75 percent of connected load, adjusted upward for starting transients, harmonics, and expansion headroom, as in the worked example above.

What is the difference between star-delta starting and VFD starting for generator sizing? Star-delta cuts starting current to roughly a third of DOL, typically two to two-and-a-half times running current. VFD starting ramps voltage and frequency gradually, so starting current stays close to running current, the gentlest option for generator sizing.

Do VFDs really require a bigger generator? Not because of their own running load, usually modest, but because of harmonic distortion, which stresses an alternator more than it stresses the grid. Heavily VFD-loaded floors commonly need a sizing margin, or a generator with better harmonic-handling reactance, on top of the base kW and kVA calculation.

Should a multi-shift factory use a prime-rated or standby-rated generator? If the genset runs only during occasional outages, standby rating is typically sufficient. If it runs extended daily hours across shifts, prime-rated is the right starting point, since the two ratings assume different duty cycles and running hours.

Is a CPCB IV+ compliant generator mandatory for a new factory DG set in India? Yes, for new and resized DG sets above applicable capacity thresholds, and factories in Delhi-NCR face additional GRAP-linked restrictions during high-pollution periods. Older non-compliant sets typically need a retrofit emission control device to continue legal operation rather than requiring outright replacement.

Getting the Sizing Right the First Time

Factory generator sizing rewards patience with the numbers more than any other DG set purchase, because the penalty for getting it wrong, a genset that trips on motor starts, overheats under harmonic load, or gets outgrown within two years, is expensive to fix after installation. Work through the connected load, apply honest demand and diversity factors, check starting methods before assuming worst-case DOL multipliers, account for harmonic load if VFDs and welding equipment are part of your floor, and build in real expansion headroom.

If you’d like a second opinion on your load list, starting-method assumptions, or the compliance requirements for your capacity and location, get in touch with our team before you commit to a purchase.