A generator that's too big for the job doesn't just cost more to buy. Left running under light load for long enough, it can quietly damage itself, and the first sign most people notice is that it doesn't seem to have the grunt it used to.
That condition has a name. It's called wet stacking, and it's one of the more common ways a diesel generator gets worn out well before it should.
What's actually happening inside the engine
A diesel engine needs to get hot enough, and work hard enough, to fully burn the fuel going into it. When it's running at a small fraction of what it's rated for, it never reaches that point. Some of the fuel goes through unburned instead, and it ends up in the exhaust system rather than being converted into power.
You can usually see it before you diagnose it. A black, oily residue builds up around the exhaust manifold, the piping, and the turbocharger if the unit has one. It's often mistaken for an oil leak, but it's actually the heavier parts of the diesel fuel that never combusted properly.
Why it happens on site more than people realise
This is the part that matters for construction and trade work specifically. It's common to see a generator sized for the biggest possible draw on a job, tools, lighting, everything running at once, and then most of the time it's only running a fraction of that load. A few power tools here and there on a generator sized for the whole crew is exactly the kind of light, sustained running that causes this.
It's not really about generator quality. It happens to well-built units the same as cheap ones. It's about the mismatch between what the engine's rated to do and what it's actually being asked to do most of the time.
What it costs you if it's ignored
Wet stacking doesn't just sit there being cosmetic. Left alone, it:
- Fouls the injectors, which affects how well the engine runs even under a proper load later
- Builds carbon on the exhaust valves and turbo, which restricts airflow over time
- Reduces the maximum power the generator can actually produce
- Can require a full engine overhaul if it's allowed to build up for long enough
A generator that's been quietly wet stacking for months might start fine and run, but not put out what it's supposed to when you actually need the full capacity, which is often exactly the moment you can't afford it to fall short.
The fix is simpler than the problem sounds
The standard answer is to run the generator under enough load, often for a set period, to get it up to proper operating temperature and burn off what's built up. Some maintenance regimes use a set standard for this: a 30-minute run each month at a minimum of 30% of rated load is a commonly used benchmark, though the exact figure depends on the engine and manufacturer.
If there isn't enough real load on site to do that, a load bank can apply an artificial load to get the same result. It's a normal part of generator servicing, not a sign something's gone badly wrong.
The better fix is getting the size right from the start
The cleanest way to avoid this is sizing the generator to what it's actually going to run most of the time, not the rare moment everything's plugged in at once. If your load genuinely varies a lot between quiet stretches and full crew days, that's worth flagging when you're sizing it, since the right answer might be a smaller unit than the peak load alone would suggest, or a plan for running it under proper load periodically if a bigger unit is unavoidable.
