How to Size A Diesel Generator for Industrial and Commercial Applications

A generator that is wrong by even one size class costs you either way. Undersize it and the unit nuisance-trips the moment a motor starts; oversize it and you bleed fuel, void warranties, and risk wet stacking. Learning how to size a diesel generator correctly is the difference between a unit that runs clean for decades and one that fights your facility from the first startup. The good news is that diesel generator sizing is not guesswork. It is a repeatable engineering sequence built on your actual load data, and PDG engineers walk industrial and commercial buyers through it every day.

Start By Classifying Your Loads

Before any math, sort every load on the panel into three buckets, because each demands different headroom.

• Continuous loads run steadily for long stretches: HVAC, lighting, conveyors, and process controls. These set your baseline demand.

• Intermittent loads cycle on and off: Air compressors, pumps, and material handling. They raise peak demand without running constantly.

• High-inrush motor loads are the trap. A motor draws four to seven times its running current at startup, and that locked-rotor surge dictates how much reserve capacity the genset needs even though it lasts only seconds.

Industrial generator sizing fails most often when this third category is ignored. A unit sized for running load alone will sag or stall when the largest motor kicks in.

Work the Calculation Sequence

Once loads are classified, the procurement workflow follows six steps.

1. Inventory every load from the panel schedule or facility electrical drawings. Nothing should reach the generator that you have not accounted for on paper first.

2. Capture both running and starting kW for each motor. List the locked-rotor or starting figure beside the running figure so the surge is visible.

3. Convert amps to kW using the correct three-phase formula. This is where most spec errors happen. You cannot add amperage across phases or panels and treat the total as load. Use kW = (V × I × 1.732 × PF) ÷ 1000. Adding amps directly is one of the most common and most expensive mistakes in commercial work, and a quick pass through a 3-phase generator sizing calculator confirms the hand calculation.

4. Factor in the power factor. Motor-heavy facilities often run at 0.8 PF, so the generator must supply more kVA than the kW number alone suggests.

5. Layer in voltage drop tolerance so long feeder runs and starting dips stay inside equipment limits.

6. Apply NEC 445.13(A), which requires conductors to be sized at 115 percent of the generator nameplate current rating. This protects your conductor investment and keeps the installation inspection-ready.

That generator sizing formula, applied honestly to real load data, lands you within one size class nearly every time.

Three Worked Examples From Real Markets

Agricultural irrigation and poultry, about 25 kW. A center-pivot irrigation pump plus poultry-house ventilation and lighting carries a modest continuous base, but the pump motor's inrush dominates. After adding starting headroom for that motor, the load settles into the 25 kW class. For these tractor-adjacent and remote farm sites, our PTO generators and stationary 25kW diesel generators both fit the duty.

Small commercial or food service, about 40 kW. Walk-in refrigeration, kitchen exhaust, HVAC, and lighting run as steady continuous load, with compressors cycling on top. Backup generator sizing for this profile has to cover refrigeration restart surge, so the product never warms during a transfer. That math typically lands in the 40 kW range, well served by our 40kW diesel generators configured for standby duty.

Small manufacturing or telecom, about 50 kW. A machine shop with several motor-driven tools, or a telecom site with rectifiers and climate control, builds a higher continuous base plus repeated motor starts. Commercial generator sizing here favors the 50 kW class to hold voltage during simultaneous starts. Our 50kW diesel generators are built for exactly this kind of mixed industrial duty.

Avoid the Diesel Oversizing Trap

It is tempting to round up aggressively, but diesel engines punish chronic underloading. Run a unit below roughly 60 percent of rated load for long periods and unburned fuel carbons up the exhaust system, a condition called wet stacking that fouls injectors, glazes cylinders, and shortens engine life. The right move in standby generator sizing is to match the generator to the actual operating load, not to a worst-case peak you may never see. If your demand genuinely varies, paralleling smaller units beats one oversized engine, because you can run the right number of gensets for the load present.

Choose the Right Duty Rating

Not every generator is rated the same way, and picking the wrong rating class is as costly as picking the wrong kW. Standby-rated units are designed for backup duty during grid outages, typically limited to a set number of hours per year. Prime-rated units are built for continuous operation as the primary power source, common on remote job sites and off-grid facilities. Putting a standby-rated generator into prime service shortens engine life and voids most warranties. Commercial backup generator sizing should always confirm that the unit's duty rating matches how the facility will actually use it, not just how much power is needed but how often and for how long.

Plan Your Load Management Strategy

Sizing does not have to be all-or-nothing. Load shedding controllers let you prioritize which circuits the generator picks up and in what order, so the genset never faces every motor starting at once. Staggering motor starts by even a few seconds can reduce peak inrush demand by 30 to 40 percent, which in some cases drops the required generator size by an entire class. PDG engineers can design a sequenced transfer scheme that protects both the generator and the equipment it feeds.

Frequently Asked Questions

If our facility's load grows over the next 5 to 10 years, is it cheaper to oversize now or to install a paralleled second unit later?

It depends on how confident the growth projection is. Oversizing by one class is relatively low cost upfront, but if the load stays flat you will run underloaded for years, wasting fuel and inviting wet stacking. Paralleling a second unit later lets you match capacity to actual demand as it develops, and two smaller generators also give you redundancy that a single larger unit cannot. For most facilities, the better long-term investment is to size accurately for current load plus a modest margin, and design the switchgear and bus to accept a paralleled unit down the road. PDG engineers can spec both the initial genset and the electrical infrastructure so adding a second unit later is straightforward.

Can a diesel generator be sized to run a single critical process line while leaving non-essential loads offline?

Yes, and it is one of the most cost-effective sizing strategies for facilities where only one operation is truly mission-critical. Instead of sizing for the entire building, you identify the critical process line, its support systems (cooling, controls, compressed air), and the associated lighting and safety circuits. Size the generator to that subset and install a dedicated transfer switch or load shedding panel that isolates non-essential circuits during an outage. The result is a smaller, less expensive generator that runs closer to its rated load, which improves fuel efficiency and engine longevity.

For Tier 4 prime power applications, how does the EPA's underload restriction change my sizing approach?

Tier 4 diesel engines rely on aftertreatment systems, typically a diesel particulate filter (DPF) and selective catalytic reduction (SCR), that need sustained exhaust temperature to function. If the generator is oversized and the engine runs at a light load for extended periods, exhaust temperatures drop too low for proper regeneration. That leads to clogged DPFs, increased maintenance, and potential emissions non-compliance. When sizing for Tier 4 prime power, target a load band that keeps the engine at or above 60 percent of rated output during normal operation. If your load profile has long periods of light demand, paralleling smaller Tier 4 units allows you to shut down unneeded gensets and keep the running units in their efficient operating range.

Get the size right the first time

Sizing a generator for a commercial building, a manufacturing line, or an off-grid industrial site comes down to honest load data, the right three-phase conversions, sensible motor headroom, and code compliance. Skip any one of those and you pay for it in fuel, repairs, or downtime. PDG engineers size, design, and build custom commercial generators and industrial diesel generators to your exact load profile, across every market we serve. 

Send us your panel schedule or load list and we will run the numbers with you and recommend the unit that fits, the first time. Call (229) 671-9171 or request a quote to start.