How I Learned That "More kW" Isn't the Answer
I still cringe thinking about the 2021 install. A new factory wing, clearly needed backup power, and I was the guy who signed off on the spec. We put in a 250 kW SDMO generator. Looked good on paper. Handled the lights, the computers, and the main HVAC units. Perfect.
Then they fired up the production line.
The line didn't just flicker. It stalled. The motor starting current—locked rotor amps—dwarfed our calculated load. The generator tripped. The whole wing went dark. That shutdown cost $12,000 in lost production before we got a rental unit on site. We ordered the wrong machine.
I'm not an electrical engineer. I handle operations and maintenance for industrial facilities, and I've been ordering backup power for 8 years. That mistake taught me a lesson I now build into every spec: kW rating is just the starting point. Here's the checklist I use for every SDMO generator quote to make sure I don't repeat 2021.
The SDMO Generator Sizing Checklist (4 Steps)
Who this is for: Facility managers, maintenance leads, or anyone responsible for specifying an industrial standby generator (20–1250 kVA range). Not for residential installs. Not for portable units. If you're wiring a Predator 5500 into your home, this checklist will over-complicate things.
Total steps: 4. The third step is where most people trip up.
Step 1: Build the Real Load Profile (Not Just the Sum of Nameplates)
Everyone adds up watts. That's Table Stakes 101. But a load profile factors in when things run and how they start.
- Continuous loads: Lights, servers, HVAC (steady state). Add them up.
- Non-continuous loads: Pumps, compressors, some production equipment. Factor in their duty cycle. Don't just sum full load amps for something that runs 20% of the time.
- Future expansion: Add 30-40% buffer. Period. Cheap insurance.
On that failed project, I skipped the load profile entirely. I used a spreadsheet from a different site and adjusted the total kW. Lazy. Costly.
Step 2: Identify the Worst-Case Starting Load
This is where my 2021 mistake lived. A motor starting can pull 6x to 8x its running current for a few seconds. A single large motor—or several smaller ones starting together—can overwhelm a generator's transient response.
Check this: What's the largest motor? What's the largest group of motors that could start simultaneously? Your generator needs to handle the voltage dip (usually around 20-30% dip is acceptable for industrial).
For industrial SDMO units, I always ask the supplier for the transient response curve. If they can't provide it, that's a red flag.
Step 3: The "Don't Size at 100%" Rule (The One Everyone Skips)
Here's the thing: you don't want to run a generator at its full rated capacity for extended periods. It's bad for fuel efficiency, it shortens engine life, and it leaves zero margin for error.
Industry best practice: size the generator so the total running load is no more than 75-80% of the generator's prime power rating. For standby applications, you can push it closer to 90%, but I don't recommend it.
Real talk: a 250 kW SDMO generator should handle a running load of around 190-200 kW. If your load profile says 225 kW, you need to step up to the 325 kW model. Or the 550 kW unit, depending on motor starting requirements.
I ignored this. I had a 250 kW generator running at 210 kW steady state. It worked for a month. Then a compressor kicked in, and the transient load spike tripped the unit. The engine was fine. My reputation wasn't.
Step 4: Verify kVA vs kW and Power Factor
This gets into electrical engineering territory, which isn't my expertise. I'd recommend consulting an electrical engineer for this. But I can tell you what I check:
- kW (real power): What does the work (motors, lights, heaters).
- kVA (apparent power): The total power the generator must supply, considering power factor.
Most industrial loads have a power factor of 0.8 to 0.9. A 250 kW load with a 0.8 power factor requires 312.5 kVA of generator capacity. A 250 kW generator? It's typically rated at 312.5 kVA at 0.8 PF. So they're the same thing, but only if you understand the relationship.
Simple rule from my experience: always compare kVA ratings, not kW. It's the more direct measure of what the generator can deliver.
Common Mistakes & Notes
- Don't trust the nameplate alone. Motor nameplates show full load amps at specific conditions. Your actual load may be different. Measure it if you can.
- Derate for altitude and temperature. A generator that works fine at sea level will lose about 3-4% capacity per 1,000 feet above 5,000 feet. I learned this from a site in Colorado.
- Don't forget the ATS specs. The automatic transfer switch needs to match the generator's capacity. A 250 kW generator @ 480V 3-phase requires a switch rated for at least 400 amps.
- Fuel supply matters. For a 550 kW SDMO generator running at full load, you're looking at roughly 30-35 gallons of diesel per hour. Tank sizing isn't my expertise, but it's critical.
When to Call a Pro
I'm not an electrical engineer, so I can't speak to detailed power system studies or harmonic analysis. What I can tell you from a maintenance perspective is: get a professional engineer to review your load profile and generator sizing for any installation over 150 kW. It's a few hundred dollars for a review that can save you thousands in downtime.
This checklist? It's what I use to get the preliminary spec right before handing it to the EE. It catches 90% of the basic errors. The last 10%? That's why experts exist.
Prices as of early 2025; verify current DOE compliance dates for diesel generator emissions standards (Source: EPA).
