Load Testing a Used Generator Before You Buy

Standby generators are among the most commonly resold industrial equipment, and among the easiest to misjudge — because a generator that starts and runs unloaded tells you almost nothing.

Load testing is the test that matters

A generator can start, run smoothly and sound perfect with no load on it, and fail under load. Load bank testing applies a real electrical load and reveals what a no-load run cannot:

• Whether the engine holds rated output.
• Voltage and frequency stability under step loads.
• Cooling system performance at temperature.
• Exhaust condition under load, which reveals combustion problems.
• Fuel system performance at demand.
• Regulator and control response.

Test at or near rated load for a meaningful duration, not for five minutes. Ask for the results in writing, and be sceptical of a seller unwilling to load test a machine of any real value.

Hours in context

Standby generators typically accumulate few hours, which is why hours alone are a poor guide. A unit with very low hours that sat for a decade with no maintenance may be in worse condition than one that exercised regularly and ran through outages.

What matters more:

• Exercise history. Regular loaded exercise is the best indicator of a healthy standby unit.
• Maintenance records. Oil and filter changes, coolant service, fuel polishing, battery replacement.
• Storage conditions. Indoors versus outdoors, and whether it was maintained while idle.
• Age. Rubber, hoses, seals and gaskets degrade on a calendar, not on an hour meter.

The engine

Oil analysis history if available. Look for leaks, check coolant condition, inspect hoses and belts, check the radiator and cooling package, and look at the exhaust system.

Start it cold if possible. A cold start tells you more than a warm one — hard starting, smoke that does not clear, and unusual noise all show up then.

The alternator

Insulation resistance testing is the key measurement, and it is inexpensive. Low readings indicate moisture or insulation degradation and are a serious finding on a machine that has been stored outdoors.

Also check bearing condition, the exciter, and the voltage regulator.

Fuel system

Diesel fuel degrades. A unit that sat with old fuel in the tank frequently has contaminated fuel, fouled filters and a tank with growth in it. That is manageable — fuel polishing and tank cleaning — and it is a cost to know about.

On natural gas units, check the fuel train and confirm the gas pressure the unit requires is available where you intend to install it.

Controls and switchgear

Control panel generation and whether it is supported. Transfer switch, if included, and its condition and rating. Paralleling gear on multi-unit installations.

Obsolete controls are a common replacement on older units and they are not cheap.

What comes with it

A generator without a transfer switch, a base tank, an enclosure or a proper exhaust is a partial purchase. Packages sold complete from a single facility are generally the better value and the easier installation.

Installation is substantial

Foundation or pad, rigging, fuel supply, exhaust, cooling and ventilation air, electrical connection, transfer switch, permits and inspection. On smaller units the installation frequently costs more than the generator.

Emissions requirements also apply depending on the engine tier, the application and the jurisdiction, and they are worth confirming before purchase rather than after.

Where good used generators come from

Facility closures, data centre upgrades, hospital and institutional replacements, and consolidations. Units removed from well-maintained critical facilities are the best buys in the market, because they were exercised, serviced and documented.

That provenance is worth asking about, and it is worth paying for.