Who This Checklist is For (and What It Solves)
If you're a facilities manager, an electrical contractor, or a procurement lead tasked with buying a backup generator for a commercial or industrial site—and you're not a generator engineer—this is for you. Specifically, this covers the 50 to 1000 kVA range, which is where most of our projects land.
The problem I see over and over is simple: the generator itself is a well-engineered piece of equipment. The failure almost always happens in the gap between what a spec says and what actually gets delivered. A vendor says 'yes, we can do that' to a request for a 300 kW SDMO diesel generator, but the fine print on the fuel consumption or the transfer switch rating tells a different story.
I've been a quality inspector for over four years, and I review roughly 200+ deliverables annually. In Q1 2024 alone, I rejected about 15% of first deliveries for spec mismatches. This checklist isn't theory—it's the four-step process I use to keep that number low.
Step 1: Define Your Non-Negotiables Before You Talk to Any Vendor
Don't start with price. Start with the three things your site cannot live without.
1. Power requirement (kW or kVA): This isn't just 'what's the biggest motor we need to start?' It's the total load, plus a margin for future growth. I've seen people spec a 650 kVA Kohler SDMO generator based on a quick guess, only to find out later that their HVAC upgrade a year prior pushed them over the limit. Measure, don't assume.
2. Fuel type and runtime: Are you on natural gas, diesel, or propane? For industrial sites, diesel is still the workhorse. But if your local regulations have emissions limits, that's a non-negotiable filter. Also: how long do you need to run without refueling? 24 hours? 72? This determines the size of your day tank and whether you need a sub-base tank.
3. Transfer switch requirements: This is the part people forget. A generator is only as good as the switch that transfers the load. Is it open transition (a brief power blip) or closed transition (no blip)? For a hospital or a data center, that's the difference between a restart and uninterrupted operations. Write this down.
I don't have hard data on how often people skip this step, but based on my 200+ reviews, my sense is roughly 30% of initial RFP documents are missing at least one of these three specs. That's where the rework starts.
Step 2: Don't Just Read the Brochure—Verify the 'Standard' Specs
This is where the 'assumption failure' almost always lives. A vendor says '300 kW SDMO diesel generator' and hands you a datasheet with a standard rating. Sounds great. But 'standard' can mean different things.
What to ask:
- Is it prime power or standby power? A generator rated for 300 kW as standby (running for a few hours during an outage) cannot run continuously at that load. For continuous use, you might need to derate it to 80%. I assumed 'same specs' meant identical performance across vendors once. Didn't verify. Turned out one vendor's '300 kW' was standby, the other's was prime. Reject and redo.
- Altitude and temperature derating. A generator rated at sea level loses roughly 3% of its capacity for every 500 meters above that, and more for high ambient temperatures. If your site is in Denver or a high-elevation facility, the spec sheet from the factory in Florida might not apply.
- Fuel consumption at full load vs. 75% load. The brochure always shows full load. But most generators run at 70-80% load. Ask for the fuel curve. A vendor who lists all the numbers upfront—even if the total looks higher—usually costs less in the end.
I recall a project where we needed a 650 kVA Kohler SDMO generator for a manufacturing line. The vendor quoted a standard model. But when I checked the technical manual, the standard alternator couldn't handle the harmonic load from the VFDs on the line. That discovery cost us a week of re-spec'ing but saved a $22,000 redo after installation.
Step 3: Ask About What's NOT Included
I've learned to ask 'what's NOT included' before 'what's the price.' This is the transparency step, and it's where trust gets built or destroyed.
The hidden checklist:
- Delivery and positioning: Is it drop-shipped to the curb, or does the vendor place it on a concrete pad? A 5-ton generator is not a weekend DIY project.
- Installation materials: Are the battery cables, fuel hoses, and vibration isolators included? Or will you need to run to the supply house on installation day for 'little things' that add up to $800?
- Commissioning and testing: This is the biggest gap. I'd say 70% of the RFPs I see don't specify a load bank test. The vendor installs the generator, starts it with no load, it runs fine. Then when a real outage hits and the load turns on, a loose connection or a voltage regulator issue becomes apparent. For our $18,000 project last year, the commissioning fee was $1,200. It caught a misadjustment before it became a problem. Worth every penny.
- Warranty and support: Who do you call at 3 AM when the generator fails an automatic weekly test? Is the vendor local, or do you wait for a tech from three states away?
The vendor who lists all these upfront—even if the total looks higher—usually costs less in the end, because you don't pay for surprises. We were using the same words ('full install') but meaning different things. Discovered this when the order arrived and the pad was not included.
Step 4: The Site Visit and the Blind Test
This is the step most people skip, and it's the one that catches the most problems.
Go see the actual equipment. Not a showroom model. Not a photo. The actual unit that will be shipped to you, or one identical to it. Check the build quality: welds, panel fit, paint consistency. I ran a blind test with our team: same generator specs, from two different vendors, one with a slightly higher price point. 80% of the team identified the higher-priced vendor's unit as 'more professional' without knowing the cost difference. The difference was in the wiring harness routing and the label quality. On a 50-unit fleet, the perception of quality matters to your brand.
Test under load if possible. Ask the vendor to demonstrate the generator running at 80% load for 30 minutes. Watch for voltage stability, exhaust smoke (black smoke = overfueling), and temperature rise. A 'working' generator in a showroom is not the same as a 'reliable' generator on your site.
Check the documentation. Is there a bill of materials that matches the quote? Are the electrical diagrams clear and as-built? I've rejected shipments where the control panel drawing didn't match the actual wiring (a manual ranging multimeter found a 12V discrepancy). That issue cost the vendor a redo on their dime, and we updated our contract to require digital copies of all diagrams before shipment.
Common Mistakes to Watch For
Here's what I see most often, in order of frequency:
- Skipping the load bank test. I said this above, but it's worth repeating. It's the single most common source of 'the generator failed when we needed it.'
- Ignoring the fuel system. A dirty fuel tank, wrong fuel hose material, or a day tank that's too small are guaranteed to cause a start failure. I recall a site where the diesel fuel had been sitting in a tank for three years before the generator was even connected. The generator started fine on fresh fuel in the test; during the outage, the old fuel clogged the filter in 10 minutes.
- Assuming 'standard' means 'compatible.' A 300 kW SDMO diesel generator from one vendor may have a different footprint, different exhaust outlet location, or different control voltage than a unit from another vendor. If you're replacing an existing generator, measure twice. I wish I had tracked the number of times we've had to modify concrete pads because of dimension differences—it's more than zero.
- Not planning for regular maintenance. The generator itself is a machine that needs care. Who will change the oil, test the battery, and clean the radiator? A backup system that isn't tested weekly is a backup system that might not work. I've seen a $10,000 engine damage because a coolant level sensor was disabled to stop an alarm. That's not a vendor problem—that's a maintenance plan problem.
In the end, the best approach is simple: be specific about your needs, verify the specs that matter, and don't be shy about asking for the hidden costs upfront. The extra time you spend before the purchase is the time you'll save when the lights go out.
