A second-hand diesel generator can be a smart investment for businesses, farms, or homeowners looking to secure reliable backup power without paying the full price of a brand-new unit. With careful buying, it’s possible to pick up a high-quality generator at a fraction of the cost. However, purchasing used equipment always comes with risks—especially if you’re unsure what to look out for.
Diesel generators are built to last, and many models are designed for decades of use with proper maintenance. But even the toughest units can suffer from neglect, overuse or hidden damage. Whether you’re buying from a dealer, auction, or private seller, carrying out thorough checks before handing over any money is essential to avoid expensive mistakes and ensure the generator you buy will serve your needs for years to come.
Checking the Service History and Hours Run
The first thing to ask for when considering a used diesel generator is the service history. A well-maintained generator should have clear records of oil changes, filter replacements, coolant checks and regular servicing intervals. These details give a good indication of how the generator has been treated over its lifetime.
Also important is the number of hours the generator has run. This is the diesel equivalent of mileage on a vehicle. A low-hour generator that’s been run only occasionally—such as for backup in a small business—is likely to be in better shape than a heavily-used unit from a construction site running daily loads. That said, a low-hour generator with no service record can also be a red flag, as sitting idle without proper care can be just as damaging as overuse.
Inspecting the Physical Condition
A full visual inspection of the generator is essential. Look for signs of rust, oil leaks, fuel stains or coolant residue on or around the engine block. These could indicate underlying issues that may not have been addressed. Surface corrosion isn’t necessarily a deal-breaker, particularly on older units stored outdoors, but it should be limited to non-critical components and not structural or engine-related areas.
Inspect wiring and hoses for cracking, fraying or signs of tampering. Wiring that’s been repaired with tape or temporary fixes could point to electrical issues or amateur modifications. Check that the control panel is intact, responsive, and not displaying any error codes or faults. On larger units with soundproof canopies or weather housings, inspect the casing for dents or water ingress, especially around vents and seals.
Starting the Generator and Listening Carefully
Never buy a used diesel generator without seeing it running. A live test will tell you far more than pictures or paperwork ever could. The engine should start easily and settle into a smooth, consistent idle. Excessive cranking or visible smoke on startup could indicate issues with fuel injectors, glow plugs or the compression system.
Listen for knocking, rattling or irregular noises that could point to engine wear or loose internal components. Diesel generators should run with a steady tone, and while some vibration is expected, it shouldn’t shake excessively. When connected to a load, the generator should maintain its output without faltering or tripping. Watching the generator perform under load is the best way to judge how it might cope with your real-world power demands.
Verifying the Power Output and Load Capacity
Make sure the generator’s output capacity in kVA or kW matches your requirements. Overloading a generator will lead to poor performance, system stress and potential failure. If you’re planning to power your home, site, or business operations, calculate your total power needs before buying and make sure the generator has enough capacity to handle them, with some margin for future expansion or startup surges.
It’s also wise to ask whether the generator has been load tested recently. A proper load test involves simulating different demand levels and confirming the generator delivers steady voltage and frequency across its rated output range. A dealer or experienced technician should be able to carry out or provide evidence of a load test prior to sale.
Looking at the Engine and Alternator Brands
Diesel generators are made up of two main components—the engine and the alternator. Well-known engine manufacturers like Perkins, Cummins, Yanmar and Deutz are generally seen as more reliable and easier to source parts for. Alternators from brands like Stamford, Leroy Somer or Mecc Alte also tend to offer greater longevity and support.
Check for matching nameplates on the engine and alternator, which should confirm their make, model and serial number. If the branding looks suspicious or the generator appears to be a mix of components from different eras or sources, it may have been rebuilt or heavily modified. While some rebuilds are perfectly legitimate, others may hide reliability concerns.
Ensuring Compliance and Emissions Standards
Depending on your location and intended use, your generator may need to meet certain compliance or emissions standards. In the UK, many commercial applications require that generators meet Stage V emissions regulations, particularly for urban or site-based use. Older diesel generators may not be compliant and could be restricted or fined if used in low-emission zones.
Ask the seller for documentation or certification confirming compliance with relevant standards. If buying for commercial use or resale, make sure the generator comes with CE marking and appropriate safety labelling. Non-compliant imports or modified units could lead to issues with insurance, planning regulations or environmental controls.
Investigating the Fuel System and Battery
A close look at the fuel tank, lines and filters is also important. Contaminated diesel—often due to water ingress or microbial growth—is one of the most common causes of generator issues. Check for signs of sediment, sludge or discoloured fuel. Clean tanks and filters suggest the generator has been maintained properly. If in doubt, arrange for a sample to be taken for testing, particularly on larger or higher-value units.
Don’t forget to check the battery and starter system. A flat or failing battery can stop a generator from starting when you need it most. Make sure the battery is holding charge, that connections are clean and tight, and that the generator has been run recently to keep it in good working order.
Asking About Spare Parts and Support
Before you commit to a second-hand generator, make sure you can get parts, service and support for the specific model. Even if the generator runs perfectly now, it will eventually need filters, belts, oil and general maintenance—and older or obscure models can be more difficult to support.
UK-based dealers will often stock or supply parts for common engine and alternator brands. If the generator has been imported or is a lesser-known make, you may struggle to find replacements without relying on overseas suppliers. It’s a good idea to search for parts availability before you buy, especially if you’re expecting the generator to be in regular use.
Confirming Transfer Switch Compatibility
If you plan to use the generator as a standby power source for your home or business, you’ll likely need to connect it to an automatic or manual transfer switch. This allows safe switchover from mains to generator power without backfeeding into the grid. Check whether the generator is compatible with your existing switchgear, or if you’ll need additional installation work.
Some second-hand generators come with a transfer switch included, which adds value. Others may require a new panel or rewiring to meet current electrical safety standards. Always consult a qualified electrician or generator installer before making a final decision, especially if integration with your building’s wiring is involved.
Knowing the Reputation of the Seller
Buying from a reputable seller—whether it’s a dealership, a well-known auction platform or a commercial fleet disposal—gives you a greater level of protection. Look for sellers who provide warranty options, load testing, and professional servicing prior to sale. If possible, visit the seller in person, inspect the generator onsite and see it running before committing.
Private sellers offering low prices with vague descriptions or limited information should be approached with caution. A second-hand generator is a significant investment, and it’s worth paying a little more for peace of mind than chasing a bargain that could cost you far more in repairs or replacement.
Investing in Reliability When It Counts
Buying a used diesel generator can be a brilliant way to secure reliable backup power or support off-grid operations without breaking the bank. But it pays to do your homework. With a proper inspection, clear service records, and a seller you trust, a second-hand generator can perform just as well as a new one at a fraction of the cost.
Take the time to check each detail, ask the right questions, and test the generator properly before making your decision. With the right unit, you’ll have the confidence that your power supply is secure—whatever comes your way.
