When someone asks me to compare a 45 kVA SDMO generator with a 35 kVA Kohler-SDMO generator, they usually expect a simple "this one wins" answer. I don't give that answer, because it's the wrong question. I've managed procurement for a mid-sized manufacturer for six years, and every generator invoice lands in my cost-tracking system. When I first started buying backup power, I assumed the lowest quote was the best choice. Three budget overruns later, I stopped buying spreadsheets and started buying total cost of ownership.
You're comparing the wrong numbers
From the outside, choosing a generator looks like a nameplate matching exercise. Add up the loads, pick a kVA, compare prices. The reality is that kVA without a duty rating tells you very little. ISO 8528 defines standby, prime, and continuous ratings, and I've never met a salesperson who volunteered which one they were quoting.
A 45 kVA SDMO generator quoted at standby rating might only be 41 or 42 kVA prime. The 35 kVA Kohler-SDMO generator could be quoted at prime and actually handle a 38 kVA overload for one hour. These are different machines, but the comparison only works if you line up the same rating.
There's also the kVA-vs-kW trap. A 45 kVA generator at a standard 0.8 power factor produces 36 kW of real power. That's fine for motors, but electronic loads behave differently. Non-linear loads create harmonic currents that heat up alternator windings. The nameplate rating assumes a certain harmonic content; your actual load may be worse.
What nobody notices before buying
The hidden variable is load profile. A site with 30 kVA of steady load might be fine on a 35 kVA generator. The same site with one large air compressor that starts under load? Not fine. The 45 kVA unit is barely enough. The bigger unit doesn't always save you; oversizing causes wet stacking. Unburned fuel and carbon buildup produce maintenance costs that never show up on the original quote.
I only believed this after ignoring it once. We bought a cheaper unit based on price per kVA. It tripped at 60% load twice in the first quarter. The local dealer couldn't find a fault because, technically, there wasn't one. The machine didn't handle harmonic loads the way the spec sheet implied. The "cheap" option ended up costing 30% more than the reputable unit we replaced it with.
Diesel generator inverter is another rabbit hole
During the same audit, I kept hearing about "diesel generator inverter" technology. Instead of running at 1500 or 1800 rpm all the time, the engine speed adjusts to match load. That's a legitimate way to reduce fuel consumption at partial load, and it's not just for portable machines anymore.
But it's not automatically a no-brainer. The fuel saving depends on how many hours you run at 30-60% load. If the machine spends most of its life at 80-90% load for short outages, a variable-speed diesel generator inverter package can mean more control complexity and a longer repair path. It's a system decision, not a brochure decision.
What the wrong decision costs
Here's a real example from my records. In 2023, one of our production sites needed backup power. We got three quotes. The lowest was for a 45 kVA SDMO generator at $18,400 installed. The highest was for the 35 kVA Kohler-SDMO generator at $21,900, including an automatic transfer switch, remote monitoring, and a five-year maintenance schedule.
On paper, the SDMO quote looked like the smart procurement choice. But after I calculated fuel at expected partial load, oil changes, filter kits, load bank testing, and the probability of a service visit after year three, the higher quote was cheaper by about $3,700 over five years. That's the number I had to explain to a CFO who had already visually approved the lower price. The data won.
That experience also made me pay attention to suppliers who treat small buyers as an afterthought. The company that patiently answers questions on a one-unit order is the company you need when a machine fails during a storm. Small orders have a habit of becoming large orders, but only with suppliers who understand the word "relationship."
The concise answer
Do a load audit before you ask for three quotes. Identify actual running loads, motor starting peaks, and expected outage hours. Ask vendors to specify ISO 8528 duty ratings. Ask for fuel consumption at 50% and 75% load, not just full load. Ask what happens to the transfer switch and controller if the project schedule slips. Then compare total cost of ownership, not purchase price. If this is for a life-safety application, NFPA 110 will dictate transfer time and testing long before your budget does.
And for the side questions: if you're considering a natrual gas generator, check gas pressure stability and local fuel storage before falling in love with the fuel price. If someone asks me "what is a plug-in hybrid electric vehicle (PHEV)?", I explain that it's a vehicle with both a battery and an internal combustion engine, which can be plugged in to charge. It's great for a field office emergency light, but it's not a 24/7 power source for a production line. Bottom line: choose the generator based on the problem you're actually solving. In my experience, that's usually a 35 kVA Kohler-SDMO generator with the right service contract, not a 45 kVA SDMO-generator with a lower invoice and higher lifetime cost.
