SDMO Generator Buying Guide: Lessons From a Buyer After Three Installations

If you're researching backup power for a business, let me save you some time: the generator's brand and kW rating matter less than the dealer's ability to deliver, install, and support the whole package on your schedule. I know that because I've bought three diesel generators since 2020—a 50 kW SDMO diesel generator for our office, a 300 kVA Kohler-SDMO generator for our main plant, and a third unit after we consolidated locations. Almost every expensive lesson came from the parts around the generator, not from the generator itself.

I'm not an electrical engineer. I'm an office administrator who manages purchasing for a three-location company. I handle facility orders totaling roughly $250,000–300,000 a year across eight vendor categories, and I'm the one who gets called when something doesn't work. So this article is practical, not textbook, and I'll tell you where I'm unsure.

How I ended up with this job

I took over purchasing in 2020, when our long-time facilities manager retired. We had two offices and a small production site, and none of them had backup power. That June, the transformer serving our main building failed and shut us down for six hours. My boss said, 'fix it.' In admin language, that means get three quotes, check references, and recommend something without looking like an idiot.

I didn't know what a contactor was back then. I didn't know kVA from kW. And I had no idea that the least important part of an emergency power project is often the generator itself. I learned all of that the slow way.

Start with the numbers that matter: kW vs. kVA

Searching for a '50 kW SDMO diesel generator' is simple. Deciding whether 50 kW is actually the right size is the hard part.

kW is real power, the power that runs lights, computers, and motors. kVA is total electrical load, including the reactive power that doesn't do useful work but still has to be generated and carried through cables. Most industrial diesel generator sets are rated using ISO 8528 as a reference, and 0.8 power factor is the common basis for kVA ratings. So the rough conversions are:

• 50 kW ≈ 62.5 kVA
• 300 kVA ≈ 240 kW

The difference matters because breakers, cables, and transfer switches are sized from amps, and amps come from kVA, not kW. You can pick a generator with plenty of kW and still mismatch it with everything around it.

In 2021, I added up every breaker rating in an office panel schedule and concluded we needed around 180 kW. A dealer suggested a paid load study. I thought it was overkill. When the study came back, the building's actual simultaneous demand was about 40 kW. We bought the 50 kW unit and it's been fine since. My rule now: if you're guessing, you're gambling, and the load study is cheap compared to the iron it keeps you from buying.

What a '400 amp breaker panel' actually did for us

At our main plant, the distribution voltage is 480 V, and the essential loads panel is exactly what the name says: a 400 amp breaker panel. Why 400 amps? Because at 480 V three-phase, 400 A × 480 V × √3 equals about 332 kVA. Our 300 kVA Kohler-SDMO generator fits under that with a little headroom, which is what you want. Generator output goes through the automatic transfer switch, lands in that panel, and from there feeds whatever has to stay alive during an outage.

That '400 amp' number looked huge to me the first time I saw it. It's just the standard frame size that matches the load. The panel is a metal box with a main breaker rated for 400 A and branch breakers for individual circuits. Nothing magical. But it's where the whole project gets real, so I won't skip over it in a spec review anymore.

The contractor who wanted to save us money with a contactor

Here's my most expensive lesson. A bidder on the office project offered to save us about $3,500 by patching a small critical-load panel into the generator with a switch built around a 2 pole 30 amp contactor with a 120 volt coil. He said the 30 A branch was all that truly needed backup: the server closet and the emergency lights. He also said it was 'basically the same thing.' It wasn't. A bare contactor isn't a listed transfer switch assembly, and one without mechanical interlocking can let the utility and generator connect at the same time—which is a safety issue, not just a paperwork issue.

We proceeded because management wanted the savings. The inspector rejected it. The rework cost about $7,800, plus six weeks of delay and a very unpleasant conversation with my boss. (Which, honestly, I deserved.)

After that, I asked our generator service tech what that part is actually for. On a large SDMO enclosure, a 2 pole 30 amp contactor with a 120 volt coil is often used to switch radiator cooling fans. The 120-volt coil is energized by the control transformer, and the contactor's contacts carry the fan circuit. That's a reasonable use. Using it to patch around code-required transfer equipment is not.

If you're looking at phrases like '2 pole 30 amp contactor' and '120 volt coil' because a component failed, remember this: a contactor does not have built-in overcurrent protection. The breaker or fuse lives upstream. And a replacement should match the coil voltage, pole count, and contact rating—but the bigger question is why it failed in the first place. (Mental note: check coil voltage under load, not just with a meter.)

What is an inverter generator? (And why people keep asking me)

When I tell people what I do, half of them ask whether our backup units are inverter generators. So let's answer that directly.

An inverter generator creates AC power, converts it to DC, and then uses electronics to rebuild clean AC power. That gives you stable voltage and frequency, quieter operation, and better fuel economy at light load. It's an excellent option for small portable loads, RVs, tailgating, or running sensitive electronics at a campsite.

That doesn't mean 'inverter' is automatically better for commercial backup power. At the scale of a 50 kW SDMO diesel generator or a 300 kVA Kohler-SDMO generator, what matters more is three-phase output, motor-starting capability, fuel supply, and how the unit works with an automatic transfer switch. I own a small inverter generator for the travel trailer and I like it. But I would never try to run a building's essential loads on it, and that's not an insult to the product. It's just the wrong tool for that job.

Why I now pay for certainty

In March 2024, the coil on a fan contactor at our main plant failed. The replacement part cost less than $40, but standard shipping meant the backup system would be down for four or five days. I paid $86 for overnight shipping and didn't think twice. Finance questioned the line item. I told them: that's not shipping, that's insurance.

That's the mindset I'd bring to any generator purchase. The cheapest proposal is not the cheapest if the vendor can't deliver on the day you need them. Emergencies don't wait for standard freight, and neither should your backup power. When a bid looks much lower than the others, ask what was taken out. Usually, it's the load study, the commissioning test, the transfer switch, or the service agreement.

Where I'd push back on 'buy a generator'

Not every business needs a diesel generator. If you only need to keep a few computers and a modem running for a couple of hours, a UPS plus a small inverter generator is probably enough. If you have a one-time event with no ongoing need, renting is smarter. And if your outages are rare and short, the maintenance costs of a diesel might not make sense. Those are legitimate decisions, and I've recommended against buying in two cases since 2020.

Also, don't oversize. A diesel generator that's too large for its load runs cool and can wet-stack, which means soot and unburned fuel build up in the exhaust and cause maintenance problems. The goal is to size for actual demand plus some margin, not for the total of every breaker you can find.

I don't have hard data on failure rates across the whole SDMO and Kohler-SDMO fleet, and I won't invent any for you. What I can say from our three installations is that the engines have started every weekly exercise since their install dates. The parts that gave us trouble were small control components—a contactor coil, a controller board, a loose connection in the breaker panel. That's why I keep coming back to the same advice: pay attention to the parts around the machine, and buy from a dealer who will actually show up when the power doesn't.