If you're like me—an office administrator or facility manager who suddenly gets told "we need a backup generator for the new office"—you probably start with a Google search. I did. And I made some expensive mistakes. This isn't a textbook about power systems. This is a checklist I've built over the last four years, after processing roughly 180 orders across 15 equipment categories, including three major generator purchases. I wrote it so you don't have to learn what a 'load bank test' is when a vendor is already on site and your Finance Director is asking why the invoice is 20% higher than the quote.
This guide is specifically for sourcing industrial diesel generators in the 50-1000 kVA range (think SDMO, Kohler-SDMO units) for commercial or industrial facilities. It's not for buying a portable unit for a construction trailer.
Before You Start: The Two Questions
I used to jump straight to pricing. Bad idea. Before you contact your first vendor, answer these two:
- What are we actually powering? (The entire building? Just the server room? Critical lighting and HVAC?)
- Who is signing off on this? (Operations? Finance? An external consultant? Each has different priorities.)
Knowing this prevents half the headaches later.
Step 1: Calculate Real Load (Not Just Peak)
This is the step most people skip. A vendor asks, "What's your peak load?" and you guess 200 kVA. But a generator sized for peak only is a generator that will fail under sustained load.
What I do now:
- Get a list of all critical equipment from the facility manager.
- Calculate running load (what's on 24/7) and starting load (motors, HVAC compressors which need 3-6x their running amps to start).
- Add a 20-25% buffer. A generator running at 80-85% load is more efficient and lasts longer than one pushed to 95%.
Example: If your running load is 150 kVA, look at 188-200 kVA units. That's why you see so many 188 kVA Kohler-SDMO generators in B2B specs—it's a sweet spot for medium commercial. Not too big, not too small. I learned this the hard way after specifying a 150 kVA unit that couldn't handle the starting surge for the building's chiller.
Step 2: Verify Specs, Not Just the Brochure
Brochures are marketing. Technical data sheets are truth. If I remember correctly, we had a quote for a 200 kVA unit that claimed 'full rated power at 40°C ambient.' When I dug into the spec sheet, the fine print said 'derating required above 30°C.' That's common for air-cooled radiators in hot climates. The vendor confirmed: expected output dropped to roughly 170 kVA in summer. We had to upsize. Better to catch that before purchase.
Specific things to check:
- Rated power vs. standby power: Standby rating (for emergencies) is typically higher but for limited annual hours. Don't spec a standby-rated unit if you plan to run it for peak shaving.
- Fuel consumption at 75% load: This is your real-world operating cost, not the no-load figure some vendors quote.
- Sound levels: At 1 meter and 7 meters. A 75 dBA unit at 7m is very different (louder) than a 75 dBA unit at 1m.
The conventional wisdom is that big brands always outperform budget brands. My experience with three different generator vendors suggests otherwise. For certain applications, a well-spec'd mid-tier unit from a reputable assembler (like an SDMO unit with a Kohler engine) can deliver better value than an overpriced 'premium' brand with the same engine block.
Step 3: Don't Forget the Transfer Switch (The $3,000 Oversight)
Honestly? This was my biggest rookie mistake. I was so focused on the generator itself—kW rating, engine type, warranty—that I completely forgot about the automatic transfer switch (ATS). The ATS is what detects a power outage and signals the generator to start. You need one rated for your full load. I didn't. The vendor quoted me one for $3,200 after the fact. Finance was not happy. I ate some of that.
Now I ask: "Is the ATS included? What brand? Is it rated for open transition or closed transition?" If you have sensitive electronics (medical imaging, data centers), you might need a closed transition switch (make-before-break) to avoid micro-interruptions. That's a different spec entirely.
Step 4: Clarify Installation Scope (The 'Hidden' Costs)
The generator price is rarely the final price. What else is needed?
- Concrete pad: Dimensions and reinforcement requirements.
- Fuel tank: Sub-base tank included? Double-walled? Capacity to run 24, 48, or 72 hours?
- Electrical connections: Wiring from generator to ATS, ATS to main panel.
- Exhaust system: Spark arrestor? Silencer grade (industrial vs. residential).
- Permitting: Who handles the city permit? That can take 2-6 weeks in some areas.
I didn't fully understand the value of a detailed installation scope until a $30,000 order came back with a change order for an additional $12,000 because the site required a 500-gallon buried fuel tank instead of the standard 300-gallon sub-base tank. (The AHJ had strict fire code requirements I hadn't asked about.)
Step 5: Get a Commissioning Plan in Writing
This is the step most people forget until the generator arrives. A generator sitting on a pad isn't a backup power system—it's an expensive lawn ornament. You need someone to commission it: test it under load, verify the ATS sequence, check for fuel leaks, and program the controller.
Key questions for the vendor:
- Is commissioning included in the price? If not, what's the cost?
- What is the lead time for a commissioning tech? (In 2023, some manufacturers had 4-6 week backlogs.)
- Do you provide a load bank test as part of commissioning? (This verifies the generator can handle its rated load. It's an additional cost, usually, but worth it for critical applications.)
Approved the purchase and immediately thought, 'Did I confirm the commissioning schedule?' Hit 'confirm' and didn't relax until the test was done and the report was signed.
One Last Thing: The Two-Week Worry
Even after choosing the 188 kVA Kohler-SDMO unit for our project, I kept second-guessing. What if the 15% buffer wasn't enough? What if we had a heat wave that pushed ambient temps to 45°C? The two weeks between order confirmation and delivery were stressful. I didn't relax until the commissioning tech ran the load bank test and the generator ran at 190 kVA for 2 hours without a hiccup.
Looking back, I should have asked for the ambient temperature derating curve during the quote stage. At the time, I didn't even know that data existed.
If someone has insight on how often generators fail load bank tests due to undersized radiators, I'd genuinely love to hear it. I've never found solid industry data on that specific failure mode.
This checklist isn't exhaustive, but it covers the five steps I wish someone had handed me in 2021. Use it, adapt it, and hopefully you'll avoid my $3,200 mistake.
