Diesel vs Natural Gas Used Generators: Which to Buy

Facility managers shopping for a used standby generator usually start with kilowatts and end up deciding on fuel. Diesel and natural gas units both come through the used market regularly, and both can be a good buy, but they suit different sites. Here is how to think about the choice, and what to check on each.

Where each one shows up on the used market

Diesel generators dominate the used market for larger sizes. Hospitals, data centers, water plants, and industrial facilities have historically specified diesel for standby, so that is what comes out of buildings being demolished or upgraded. Larger diesel sets from the major engine manufacturers are available in every size range.

Natural gas units are common at smaller sizes and in commercial buildings with gas service. Larger natural gas gensets are less common used, and when they appear they are often from cogeneration or prime-power applications with far more hours than a standby unit.

Fuel logistics

Diesel needs a tank, which means a sub-base or day tank with the generator and, for long runtime, a larger bulk tank with its own permitting and containment. The fuel degrades over time and needs treatment or polishing if the generator sits. But the fuel is on site, so the generator runs regardless of what is happening to the gas utility.

Natural gas needs a gas service sized for the generator's demand at full load, which is a meaningful load and sometimes requires a service upgrade or a higher-pressure feed. There is no fuel storage to maintain. The tradeoff is dependency on the gas system during the event you bought the generator for.

In Florida, that tradeoff is real. Hurricanes affect the gas distribution system less than the electrical grid, but extended outages are exactly when fuel delivery to a diesel tank gets hard. Both fuels have failure modes; a site should pick the one it can manage.

Runtime, response, and emissions

Diesel engines pick up load faster and handle block loading better, which matters for facilities that need the generator carrying the building within seconds. Natural gas engines are generally derated compared to diesel at the same displacement and are slower to accept full load.

On emissions, natural gas is cleaner, and in some jurisdictions that affects permitting and allowable run hours for a standby unit. Diesel units of a certain age carry older emissions tiers, which may limit where they can be installed or how they can be used. Check with the local air authority before buying a large older diesel.

What to inspect on a used diesel

Hours, and how they were accumulated. A standby unit with low hours from monthly exercise is ideal; a prime-power unit with many thousands of hours is a different machine. Oil analysis if available. Coolant and hoses. The fuel system for gelling and contamination. The starting batteries and charger. And the alternator, which is often the overlooked half of the set. Then a load test, which we cover in load testing a used generator before you buy.

What to inspect on a used natural gas unit

The same items, with attention to the ignition system, the gas train and regulators, and the engine's hours relative to its service intervals. Natural gas engines typically need more frequent top-end service than diesel, so a unit with high hours and no service record is a risk. Confirm the required gas pressure and volume so you know what the site needs to supply.

The transfer switch and controls

Whatever the fuel, the generator is only half the system. The automatic transfer switch, the paralleling gear if any, and the controller determine how the set actually works in a building. A used generator that comes with its transfer switch and controls intact is worth more than one without, and it is worth confirming what is included before comparing prices.

Frequently Asked Questions

Which is cheaper to buy used?

Diesel sets are more plentiful used, particularly at larger sizes, so there is more choice and more competitive pricing. Small natural gas units are also readily available.

Can a natural gas generator run on propane?

Many can with a conversion kit or a dual-fuel setup. Confirm with the manufacturer's documentation for the specific unit.

What hours are acceptable on a used diesel?

It depends on the application. Standby units accumulate hours slowly and low-hour used sets are common. Judge the unit by its condition and service history, not by hours alone.

Do you have used generators available?

Yes. We buy used generators from facilities across Florida and nationwide, and current units are listed at used generators for sale.

Demo Dynasty buys and sells used industrial generators from Auburndale, FL. Call (689) 323-4676 or contact us about a specific unit.