Generator Set Sizing: Bigger Is Not Better

In June 2018, a Nigerian customer named Chidi emailed asking for a generator set — 200 kilowatts, straight off. He said the site was big, buy big, safe bet.

I asked what loads he'd be running. He reported: three cold storage units, eight kilowatts each; two water pumps, five kilowatts each; lighting plus a welder, around ten kilowatts. I worked it out: maximum simultaneous load, sixty kilowatts.

He even attached site photos, including the nameplates of all three cold storage units — careful of him. The nameplate power matched what he reported, so the list was usable.

Sixty kilowatts of load on a two-hundred-kilowatt set is a thirty percent load factor. I told him you can't size it like that. He replied: doesn't a big horse pull a small cart with ease?

A big horse on a small cart hurts a diesel most

I've run into this misconception more than ten times. A diesel engine is not an electric motor — it hates light loads.

Run a diesel set at low load for a long time and cylinder temperature never comes up, so the diesel doesn't burn clean. Unburned diesel mixes with carbon particles — part exits the exhaust, part stays in the cylinder. The trade calls it wet stacking.

Wet stacking's consequences arrive in order: first white smoke from the exhaust, then carbon-clogged injectors, abnormal wear gaps in liners and piston rings, diluted oil. Further down the road: turbo oil leaks, then overhaul.

There's a folk way to judge light loading: check exhaust temperature. A 200 kW set carrying 60 kW runs exhaust over a hundred degrees cooler than at full load — fuel in the chamber doesn't burn through. That's where wet stacking starts.

Manufacturers generally draw the floor at no less than thirty percent load factor; for long-term running, keep it at sixty to eighty percent.

The lesson of the Wenzhou customer

I told all this to Chidi. He wasn't fully convinced, so I told him about a domestic customer.

In 2017, a seafood cold-storage owner in Wenzhou ran a 200 kW set on a 50 kW load — same thinking, bigger is safer. After four months the set started blowing white smoke and felt sluggish.

At first he thought it was the fuel. He changed diesel filters twice — useless. The smoke thickened; even the temperature probes in the cold rooms started alarming before he finally had the engine opened up.

Inside: four of six injectors clogged, piston rings seized with carbon, the turbo center-section seal leaking oil. One overhaul — parts plus labor, 36,000. After the overhaul he sold the set, bought a 100 kW one, and hasn't had a problem since.

That 36,000 was the premium for "buy big, play safe."

The owner later told me over dinner that what hurt most wasn't the 36,000 — it was the seven overhaul days when the cold storage lost power twice and two tons of frozen stock thawed, another ten-thousand-plus gone.

The right calculation

I ran the numbers for Chidi, and I'll write the method here — anyone can follow it.

One: add up every load that can run simultaneously — that's your running power. Watch motor loads: pumps and compressors draw three times running current at startup, so don't start several at once.

Two: multiply running power by 1.25 — that's the floor for the set's rated power. The headroom covers future equipment and load swings, but don't oversize it.

Three: the resulting load factor should land between sixty and eighty percent of rated power. That's the sweet spot.

Common examples: loads under 30 kW, choose a 50 kW set; 50–60 kW of load, choose 75–100 kW; around 80 kW, choose 100–120 kW. Any plan with a load factor under fifty percent we won't sell — we talk the customer out of it.

One more detail: impact loads like welders multiply their draw the instant they strike — either calculate them separately or fit a soft starter. Chidi's welder was ten kilowatts; I counted it as fifteen.

Chidi's math: 60 kW times 1.25 is 75 kW. He said he'd add two more cold storage units next year; even after that, the load factor sits around seventy percent. Final choice: a 100 kW set.

What happened after

The set shipped to Lagos at the end of July. In September Chidi messaged: the set runs ten hours a day, burns less fuel than he expected, sounds steady. He'd worried 100 kW wouldn't be enough; measured load factor came in at sixty-some percent — right in the sweet zone.

A neighboring site had a 250 kW set running under thirty percent load — two overhauls in a year. Chidi brought the owner to see his own set, and that man later came to us for a replacement too.

Fuel burn has numbers too. Generating the same 60 kW, a 100 kW set burns over twenty percent less than a 200 kW set. Chidi's set runs ten hours a day; the monthly diesel savings cover a watchman's wages.

About sizing: buy too small and you burn the set; buy too big and you also burn the set — just in a different way.

If you can't run the calculation, that's fine. Send us your load list and mark which loads run together. We'll compute it at 1.25 times and show you the process — keep a copy and check it against anyone you like.

The sizing worksheet for Chidi's set is on our shared drive. The file is literally named "100 kW." Ask, and I'll send it to you.