Short answer: a diesel engine is made to work. If your generator runs for long periods at light load, the cylinders do not get hot enough, the fuel burns poorly and the cylinder walls “glaze”. Our rule: choose a generator whose real load will be 70 to 80 % of its rated output.
What happens in an engine that runs almost unloaded
- The cylinders do not reach their proper temperature: the fuel does not burn completely.
- That poorly burned fuel fouls the cylinders and the exhaust (“wet stacking”: a black, oily exhaust).
- Oil bakes onto the cylinder walls and forms a smooth varnish: that is glazing. The rings no longer seal properly.
- Compression then escapes into the crankcase (“blow-by”): the engine burns oil, smokes blue and loses power when you ask for it.
Below about 30 % load for long periods, the risk becomes real. Caught in time, the engine can be saved; too advanced, glazing can require a rebuild.
What we saw at a customer’s site
A generator loaded at about 20 % of its output: the symptoms, fouled cylinders and compression escaping into the crankcase, started within the first 200 hours. A new engine, damaged only because it was too big for its job.
The right output: a 70 to 80 % load
Choose a generator your real load will fill to about 70 to 80 % of its output. Enough margin for an electric motor’s starting current, without running the diesel unloaded.
Example: a house whose equipment to keep running needs about 12 kW. A 15 kW generator will work at 80 %: that is the right choice. A 30 kW would run at 40 %, and even less at night.
To find your real load: the DMW value on your Hydro-Québec meter (how to read it) and the list of your equipment. The full method: choosing your generator size.
Is your generator already too big?
Run it regularly under a real load: during the test run, plug in more equipment (heating, water heater, workshop) instead of letting it idle. An engine that works burns off its deposits.
Symptoms already showing? Caught in time, we put the generator on a load bank for several hours at about 90 % of its output: the engine gets fully hot, burns off its deposits and the rings start sealing again. We have done it before, and it worked. Monthly test and maintenance: standby generator maintenance.
At Solution NRJ
We never push you toward a bigger unit. We look at your DMW and your equipment, and we recommend the output that will make the engine work properly. Every generator is also load-bank tested for 2 hours, from 10 % to 110 %, before delivery.
Frequently asked questions
Does a bigger generator last longer?
No. An oversized generator runs at light load, builds up carbon and glazes: it wears out faster than a correctly sized one.
Is it good to idle my generator every week?
A short no-load run is not enough and fouls the engine over time. Do the test with a real load connected.
What load should I aim for?
About 70 to 80 % of the generator’s output for normal use, and avoid long periods below 30 %.