Either way, a generator that is the wrong size—or even just the wrong size class—will cost you. An oversized unit can cause operational issues when motors start, lead to wasted fuel, void warranties, and create a risk of “wet stacking.” Learning how to correctly size a diesel generator makes the difference between equipment that runs cleanly for decades and equipment that fights against your facility from the very first start-up. The good news is that sizing a diesel generator doesn’t have to be a guessing game. It involves a repeatable process based on your actual load data—a procedure CNWH engineers guide industrial and commercial buyers through 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 systems, lighting, conveyors, and process controls. These establish your baseline demand.
- Intermittent loads cycle on and off: air compressors, pumps, and material handling equipment. They raise peak demand without running continuously.
- High-inrush motor loads are the trap. Motors consume four to seven times their running current during start-up; this “locked-rotor” surge determines the reserve capacity the generator set needs, even if the surge lasts only a few seconds.
Sizing efforts for industrial generators most often fail when this third category is overlooked. If the largest motor starts up, a unit sized only for running loads may experience voltage sag or stall completely.
The Calculation Workflow
Once the loads are categorized, the procurement workflow follows six steps.
- Inventory every load from the panel schedule or facility electrical drawings. Anything not accounted for on paper shouldn’t be connected to the generator.
- Capture both running and starting kW for each motor. List the locked-rotor (starting) figures alongside the running figures so the surge can be visualized.
- Convert amps to kW using the correct three-phase formula. You cannot simply add up amperages across phases or panels and treat the total as the load. Use the formula: kW = (V × I × 1.732 × PF) ÷ 1000. Simply adding up full-load amperes is one of the most common and costly mistakes in commercial projects; a quick check using a 3-phase generator sizing calculator can verify your manual calculations.
- Factor in the power factor. Heavy-duty motor installations typically operate at a 0.8 PF, meaning the generator must supply more kilovolt-amperes (kVA) than the raw kilowatt (kW) figure suggests.
- Layer in voltage drop tolerance to long feeder runs and startup voltage dips remain within equipment limits.
- Apply NEC 445.13(A), which requires conductors to be sized at 115 percent of the generator nameplate current rating. This protects your investment in conductors and ensures the installation is ready for inspection.
Generator sizing formulas, when accurately applied to real-world load data, will almost always land you in the correct size category.
Three real-world examples
Agricultural irrigation and poultry operations: ~25 kW. Center-pivot irrigation pumps combined with poultry house ventilation and lighting create a moderate continuous load, but the pump motor’s inrush current dominates the requirements. After accounting for motor starting headroom, the load stabilizes in the 25 kW range. For these remote farm locations, both our PTO-driven generators and stationary 25 kW diesel generators are suitable for the job.
Small commercial or food service: ~40 kW. Walk-in refrigeration, kitchen exhaust, HVAC, and lighting operate as steady continuous loads, with compressors cycling on top. The standby generator size for this profile must handle refrigeration restart surges to ensure products do not warm up during a power outage. The calculations typically point to the 40 kW range, making our 40 kW diesel generator an excellent choice for standby duty.
Small-scale manufacturing or telecommunications: ~50 kW. Machine shops with multiple motor-driven tools, or telecom sites featuring rectifiers and climate control, generate higher continuous base loads and frequent motor startups. Commercial generator sizing here tends toward the 50 kW level to maintain voltage stability during simultaneous startups. Our 50 kW diesel generator is built specifically for these types of mixed industrial applications.
Avoid the “oversized diesel” trap
It is tempting to aggressively round up the size, but diesel engines suffer when consistently underloaded. When a unit runs for extended periods at loads below approximately 60% of its rated capacity, unburned fuel carbon can accumulate in the exhaust system—a condition known as “wet stacking.” This can foul fuel injectors, glaze cylinder walls, and shorten engine life. Proper sizing for a standby generator involves matching the unit to the actual operating load, rather than to a worst-case peak scenario that may never occur. If your load requirements vary significantly, using multiple smaller units in parallel—rather than one oversized engine—allows you to run the appropriate number of generator sets for the current load.
Choosing the Right Duty Rating
Not all generator ratings are created equal; selecting the wrong rating category can be just as costly as choosing the wrong kilowatt capacity. Standby-rated equipment is designed for backup service during grid outages and is typically limited to a specific number of operating hours per year. Prime-rated units are built for continuous operation as a primary power source and are commonly found at remote job sites and off-grid facilities. Using a standby-rated generator for prime service will shorten engine life and void most warranties. When sizing a commercial backup generator, always ensure the equipment’s rating matches the facility’s actual usage patterns—considering not just the power requirement, but also the frequency and duration of operation.
Planning Your Load Management Strategy
Sizing doesn’t have to be an all-or-nothing decision. Load-shedding controllers allow you to prioritize which circuits the generator picks up and in what sequence, ensuring the generator set never faces the simultaneous startup of every motor. Staggering motor startups—even by just a few seconds—can reduce peak inrush demand by 30% to 40%, potentially allowing for a smaller generator size. CNWH engineers can design a sequential transfer scheme to protect both the generator and the equipment it powers.
Frequently Asked Questions
If our facility’s load increases over the next 5 to 10 years, is it cheaper to upsize now or install a second unit in parallel later?
It depends on the level of confidence in your growth projections. The upfront cost of moving up one rating tier is relatively low; however, if the load remains flat, you will end up running underloaded for years, wasting fuel and risking wet stacking. By paralleling a second unit later, you can match capacity to actual needs as they evolve; two smaller generators also provide redundancy that a single large unit cannot offer. For most facilities, a better long-term investment is to size the system accurately for the current load—plus a modest margin—while designing the switchgear and buswork to accommodate additional parallel units down the line. CNWH engineers can specify both the initial generator set and the electrical infrastructure simultaneously, making the future addition of a second unit straightforward.
Can a diesel generator be sized to run a single critical process line while keeping non-essential loads offline?
Yes,for facilities where only one operation is truly mission-critical, this is one of the most cost-effective sizing strategies. Instead of sizing for the entire building, you size for the critical process line, its support systems (cooling, controls, compressed air), and associated lighting and safety circuits. You size the generator for this subset and install a dedicated transfer switch or load-shedding panel to isolate non-essential circuits during an outage. The result is a smaller, less expensive generator that operates closer to its rated load, thereby improving fuel efficiency and extending engine life.
How do EPA low-load restrictions affect my sizing approach for Tier 4 prime power applications?
Tier 4 diesel engines rely on after-treatment systems—typically Diesel Particulate Filters (DPF) and Selective Catalytic Reduction (SCR)—that require sustained exhaust temperatures to function effectively. If a generator is oversized and the engine runs at light loads for extended periods, exhaust temperatures drop too low for proper regeneration. This leads to DPF clogging, increased maintenance, and potential emissions non-compliance. When sizing for Tier 4 prime power, aim for a load profile that keeps the engine at or above 60% of its rated output during normal operation. If your load profile includes extended periods of low demand, using smaller Tier 4 units in parallel allows you to shut down unnecessary generator sets and keep the active units within their efficient operating range.
Getting the Sizing Right the First Time
Sizing a generator for a commercial building, production line, or off-grid industrial site comes down to accurate load data, correct three-phase conversions, reasonable motor starting headroom, and code compliance. Skip the guesswork and avoid unnecessary costs associated with fuel, maintenance, or downtime. In every market we serve, CNWH engineers customize, design, and build commercial and industrial diesel generators tailored to your exact load profile.
Send us your panel schedule or load list; we will run the numbers with you and recommend the right unit. Call +86 13129092113 or request a quote to get started.