I was in my office last March when a client called. Not a routine check-in. The kind of call where the voice is tight, and you can hear the hum of an empty building in the background. They had a 1000 kVA Kohler-SDMO unit on site—a beast of a machine. A week before a major facility audit, and the thing wouldn't take load. The local service tech said it was a fuel issue. The client was panicking.
Most people assume that when you buy a generator that big, the problem is solved. You plug it in, it runs, you're covered. That's the surface-level thinking. The real issue, the one that eats projects alive, usually isn't the machine itself. It's the sizing. Not the KVA rating, but the real sizing logic that nobody talks about.
The Question Everyone Asks vs. The Question They Should Ask
The first thing a buyer asks me is always, "What size generator do I need for a 50,000 sq ft building?" They're looking for a rule of thumb. They want a chart. The truth is, that question is almost useless.
The better question is, "What is the starting load?" Most buyers focus on the running power (KW) and completely miss the starting surge. An industrial chiller, for example, can draw 3-4 times its running current for a split second. If your generator isn't sized for that peak, it will trip its breaker every single time. Or worse, it will stall under load and refuse to start.
That client with the 1000 kVA unit? Their issue wasn't the generator. It was a massive HVAC compressor that had a locked-rotor amp rating of nearly 600 amps. The generator had the running capacity to handle it, but the starting surge was right on the edge of the generator's transient response. We ended up having to install a soft starter on the compressor. That cost them an extra $4,000 and a rushed delivery fee (note to self: always verify motor specs before the final spec). The delay meant their audit had to be rescheduled. Not a cheap mistake.
The Deep Reason: We Confuse 'Capacity' with 'Capability'
Here's the insight that changed how I spec generators: Generators are not just fuel converters. They are torque-limited machines. A diesel engine has a specific power band. The alternator needs to see a certain voltage and frequency to maintain stability.
This is where the KW vs. KVA confusion really hurts. KVA is the 'apparent power'—what the generator can produce in terms of voltage and current. KW is the 'real power'—the actual work being done. The difference is the power factor (PF). A site with a lot of motors (bad PF) will load the generator differently than a site with mostly lighting and computers (good PF).
I'm not an electrical engineer, so I can't speak to the reactive power mathematics. But what I can tell you from a procurement and commissioning perspective is: Never size a generator based on KVA alone. You need to know the starting load profile and the site's power factor.
Most buyers focus on the per-unit price and the KVA rating. They miss the setup fee—the cost of the soft starter, the automatic transfer switch (ATS) that's appropriately rated for the surge, and the fuel line sizing (which is a whole other problem). These can add 30-40% to the total cost of installation.
The Cost of Getting It Wrong (It's Not Just the Generator)
The direct cost of a wrong-size generator is obvious: you buy a unit that's too small, and it fails. But the hidden cost is much larger. A generator that's too large for the load can suffer from 'wet stacking'—where unburnt fuel accumulates in the exhaust system because the engine isn't running at a high enough load to burn it off. This is a huge issue for telecom sites or data centers that run a 1000 kVA generator for a 50 kW load. It sounds crazy, but it's incredibly common.
We once had a client who bought a 500 kW SDMO unit thinking they needed 'the biggest just in case.' The average load on site was only 80 kW. Within six months, the engine was fouled with carbon deposits. The warranty repair (which took three weeks) cost the manufacturer a lot, but the client's downtime? That was the real penalty.
Last quarter, I dealt with a rush order for a 50 kW SDMO diesel generator that was supposed to power a new restaurant's kitchen. The owner had calculated the load based on nameplate ratings from the equipment (always wrong). The actual starting load for the fryers and refrigeration was 40% higher than he estimated. We swapped the generator the day before opening. The shipping cost was an extra $800 (this was a rush). The client's alternative was a $12,000 penalty for lost revenue on opening day.
The Real Solution: A Simple, Honest Audit
So what actually works? Don't guess. Don't use a 'home generator sizing calculator' for a commercial job—those are built for refrigerators and a few lights, not for an industrial load. And don't assume that a solar generator and panels will handle the surge (they usually can't, not without a massive battery bank).
The right approach is to do a site audit. Measure the actual starting load of the biggest motor. Understand the type of load (motors vs. resistive). Then, and only then, select the generator. A 1000 kVA Kohler-SDMO unit is a fantastic machine, but it's a fantastic machine for a specific job. For a 50 kW load, a smaller, correctly sized 50 kW SDMO diesel generator will be more reliable, cheaper to run, and less likely to cause headaches.
I've been doing this for a decade. I've seen more problems from people trying to 'future-proof' by oversizing than from people sizing correctly for the current load. The simplest solution is often the most honest one: match the machine to the load, not to your fears about what might come. In my role coordinating emergency power for commercial clients, the ones who sleep best are the ones who know exactly how many amps their main compressor draws at startup. Everything else is just noise.
