Buying a Used Diesel Generator: Hours, Load Tests and Prices
Quick answer. A used towable diesel generator typically sells for $7,000 to $20,000 at 20 to 45 kW and $15,000 to $45,000 at 56 to 150 kW. Hours matter less than service records and a load test: a well-kept rental set with 10,000 hours often beats a low-hour standby unit that sat.
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Price bands by type
| Type | Typical used range | Who buys it |
|---|---|---|
| Portable gas or inverter (2 to 10 kW) | $300 to $2,500 | Trades, small crews, home backup |
| Towable diesel (20 to 45 kW) | $7,000 to $20,000 | Contractors, events, small job sites |
| Towable diesel (56 to 150 kW) | $15,000 to $45,000 | Rental yards, larger sites, temporary power |
| Standby, natural gas or propane (15 to 50 kW) | $2,500 to $12,000 | Shops, offices, small facilities |
| Diesel standby (150 kW and up) | $20,000 and up | Plants, hospitals, data rooms |
These are typical asking prices on the used market. Hours, emissions tier, enclosure condition and whether a recent load test report comes with it move a unit up or down within its band.
What do engine hours really tell you?
Less than most buyers think. A maintained industrial diesel often runs 20,000 hours or more before a major overhaul, so how the hours were put on matters more than the number.
Rental and job-site sets run under real load and get regular oil changes, which is what a diesel wants. Standby sets often show a few hundred hours after ten years, but many were only exercised at no load, which glazes cylinders and builds carbon.
Ask for the service log, the date of the last oil and coolant service, and any load bank reports. An hour meter can be replaced, so check that wear on pedals, panels and the engine matches the reading.
How do you load test a used generator?
Run it under real load for at least 30 minutes and watch voltage, frequency, temperature and exhaust. A unit that only idles for you has not been tested.
- Cold start it. It should crank briefly, start clean and settle without hunting.
- At no load, check voltage and frequency at the panel: 60 Hz in the US, within about 1 Hz.
- Apply load with a load bank, or the biggest motor or heater the seller has, building toward 50 to 100 percent of the rating.
- Watch the frequency dip when load hits and how fast it recovers. A long sag points to governor or fuel problems.
- Read the exhaust: brief haze on a load step is normal, steady black smoke means overload or fuel trouble, white means unburned fuel or coolant, blue means it is burning oil.
- After the run, check for oil or coolant leaks and blowby at the crankcase breather.
Prime, standby and continuous ratings
Buy on the rating that matches how you will run it: prime for daily job-site use, standby only for emergency backup.
The standby rating is the maximum output for emergency use a limited number of hours a year. The prime rating is lower and covers variable load all day. A 100 kW standby set is roughly a 90 kW prime set, so size a job-site unit on the prime number.
Emissions tier and where you can run it
Newer mobile diesels meet EPA Tier 4 Final and carry a diesel particulate filter and often DEF, which adds maintenance but keeps them legal to run almost anywhere. Older Tier 2 and Tier 3 units cost less and are simpler to fix.
Some states and air districts limit where older portable diesel engines can operate or require registration. Check your state and local air rules for where you will run the unit before you buy an older tier.
Voltage, phase and connections
- Rated voltage and phase on the data plate: 120/240 single phase, or 120/208 and 277/480 three phase.
- Whether the alternator is reconnectable or the unit has a voltage selector switch, so it can serve more than one job.
- Receptacles and cam-lock connections that match your cords and panels.
- For building backup, a transfer switch sized to the service, installed by a licensed electrician.
The enclosure and trailer
- Enclosure rust, door latches and sound insulation, which matters on night and residential jobs.
- Fuel tank condition, fuel age and a clean water separator.
- Coolant condition, radiator fins and belts.
- On towables: tires, bearings, lights, the hitch and a title in the seller's name.
- Common makes to look for: Caterpillar, Cummins, Generac Mobile, Kohler, Multiquip, Doosan, Atlas Copco and Wacker Neuson.
Questions buyers ask
- How many hours can a diesel generator run?
- With regular oil, filter and coolant service, an industrial diesel generator often runs 20,000 hours or more before an overhaul. Service history and how the unit was loaded predict its life better than the hour meter.
- What is wet stacking?
- Wet stacking is unburned fuel and carbon building up in the exhaust of a diesel that runs too lightly loaded. Look for black oily residue at the stack, and avoid running any diesel set for long periods at very low load.
- Can I use a used generator to back up my building?
- Yes, through a transfer switch installed by a licensed electrician. Never connect a generator to a building panel without one: it can send power back into the utility lines and endanger line workers.
- Is a Tier 4 generator worth more used?
- Usually, because it can run in more places, but the particulate filter and DEF system add cost if they need repair. If you will only run it where older tiers are allowed, an older unit can be the better value.
Have one to sell? List it free and hand it off locally. Looking up a specific model? See specs and used prices by model.