SDMO Generator FAQ: 400 kW vs 800 kW Kohler SDMO Sizing, Propane vs Gas, and Surge Protection Lessons

Six years ago, I thought sizing a generator was simple: add up the kW, add 20 percent, pick the next size up. Then I specified a 400 kW Kohler SDMO generator for a site that tripped on start-up because two chillers started at the same time. I've personally made and documented fourteen significant mistakes like that, totaling roughly $210,000 in wasted budget. Now I maintain our team's pre-purchase checklist. This FAQ is the short version of that checklist.

Here are the questions I would answer if we were standing in front of a generator:

• What is a Kohler SDMO generator?
• 400 kW or 800 kW—how do you pick?
• Whole house generator with propane tank: how big should the tank be?
• Gas vs propane generator: which is better for backup?
• Do I need a surge protector for portable generator?
• Why do two quotes for the same generator look so different?
• Can an SDMO generator work for a house?

What is a Kohler SDMO generator?

SDMO is a French generator brand that Kohler acquired in 2013. In many export markets you'll see SDMO and Kohler SDMO on the same type of nameplate. They're not competing product lines—it's one engineering and support organization under Kohler Power.

What matters more than the badge is how the set is built. The Kohler SDMO range runs from roughly 20 kVA up to 1,250 kVA, with 400 kW and 800 kW models covering a lot of industrial and data-center applications. The spec sheet should show the generating set performance class (ISO 8528) and the alternator standard (IEC 60034). If a quote doesn't list those, ask why.

400 kW or 800 kW Kohler SDMO generator: which one do I need?

From the outside, a 400 kW and an 800 kW unit look similar. The reality is that machine sizing is not just running kW. I made this mistake early: I added up the nameplate loads, added 20 percent 'safety,' and picked a 400 kW Kohler SDMO generator. Motor starting, UPS harmonics, and voltage dip on load acceptance matter just as much.

A site with roughly 390 kW of continuous load had two chillers that started together. Actual demand jumped to more than 510 kW for a couple of seconds. The generator didn't trip—the VFDs faulted on undervoltage. We upgraded to an 800 kW Kohler SDMO generator, and the problem went away. But the re-engineering, crane, and restocking costs were on me because I hadn't asked one question: what's the largest motor, and can it start while other loads are already running?

That's why I tell people to do a load study before talking price. If the application is critical, NFPA 110 defines Level 1 vs Level 2 classifications, and those drive transfer switch requirements and start-up time. Also, don't run a diesel under about 30 to 40 percent load for long periods; it can lead to wet stacking. A 400 kW set running a 120 kW building all day isn't 'efficient'—it's a maintenance problem waiting to happen.

Whole house generator with propane tank: how big should the tank be?

People assume a whole house generator with propane tank is all about the generator. The tank is often the bottleneck. A homeowner once saved $300 on a smaller 120-gallon tank, then ended up paying $650 for an emergency propane delivery during an ice storm. The generator was fine; the fuel math was wrong.

Size the tank backward from outage duration and expected load, not from the generator peak rating. Ask your installer for a fuel consumption curve at your typical load, then size for at least three days of runtime. NFPA 58 covers tank placement and fill procedures; NFPA 54 covers fuel gas piping. If an installer can't answer those questions, get a second opinion.

Gas vs propane generator: which is better for backup?

From my perspective, natural gas is the best choice when the local gas grid is reliable and pressure is sufficient. No tank, no refill calls, unlimited runtime. But there is a catch: if the storm takes out gas distribution, the 'unlimited' fuel is gone.

Propane gives you an on-site fuel stock that doesn't degrade the way gasoline does, but it has its own issues. The generator needs a different jet or orifice, the regulator can freeze in cold weather if the tank is undersized, and you need a delivery plan before an event, not after.

For industrial 400 to 800 kW units, I rarely see gas vs propane as a question. It's diesel. Propane doesn't fit well at that power level in most applications. For a whole-home unit, I'd choose natural gas in town and propane in a rural area with reliable delivery. That's a practical rule, not a scientific one.

Do I need a surge protector for portable generator?

Yes. Portable generators can produce voltage spikes when loads cycle off, and the power is sometimes not as clean as utility power. A standard power strip isn't designed for that. A surge protector with a UL 1449 rating will clamp a spike before it reaches your electronics.

Last year a contractor powered his temp office from a portable generator. When the mini-split cycled off, the voltage spiked and killed the variable-frequency controller—no, wait, the controller took the spike; the mini-split itself was fine. $760 for the repair plus a service call. The surge protector would have cost around $40. It's cheap insurance.

Also, never connect a portable generator directly to household wiring. Backfeeding is dangerous. Use an interlock or a transfer switch, and if you have a home network or appliances with circuit boards, add a whole-house surge protector at the panel.

Why do two quotes for the same generator look so different?

The most frustrating part of quote comparison is that the same kW number can produce wildly different totals—not because someone is dishonest, but because nobody defines the scope. The hidden costs I've paid for, or rather, the ones I should have asked about, show up like this:

• Delivery and lifting: a 400 kW diesel set is not a pallet-ship item.
• Coolant, fuel, and first fill, plus the battery heater.
• Automatic transfer switch and control wiring.
• Commissioning, load bank testing, and documentation.
• Exhaust, sound attenuation, and remote monitoring.

A transparent quote lists these separately. A quote that looks $18,000 cheaper often leaves them as allowances. I've learned to ask 'what's NOT included' before asking 'what's the price.' The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. At least, that's been my experience with industrial projects.

Can I use an SDMO generator for a whole house?

If you mean a Kohler SDMO industrial set, it's usually overkill. The smallest SDMO generating sets are around 20 kVA, which is roughly 16 kW of real output—bigger than the typical 7 to 14 kW air-cooled residential unit. A purpose-built whole-home unit is quieter, easier to permit, and simpler to maintain.

That said, for a large estate, farm, or workshop with heavy motor loads, a 20 to 60 kVA SDMO set can make sense. Just understand that you're moving into industrial territory: concrete foundation, exhaust silencing, fuel system, and annual load testing. In my first year (2018), I quoted a small backup unit for a farm and forgot the remote coolant heater. The generator started fine in the spring. In December, it didn't. Lesson: accessorize for the site, not just the kW.