Gas or Diesel Generator?

Facilities with a natural gas connection, such as hotels, need to choose between a gas and a diesel generator. The right choice depends on whether the generator will be used for short, infrequent outages or for long outages, and on how likely the gas supply is to be cut off in events such as an earthquake or a flood.

There is no “better” answer between gas and diesel. The two technologies are strong at different jobs. In this article we compare the two technologies impartially, from an engineering point of view, not any particular product.

The key difference: where is the fuel?

In a diesel generator, the fuel sits on site, in a tank. It is independent of the mains and the gas network. The bigger the tank, the longer the set runs. As long as fuel is delivered, there is no time limit.

In a gas generator, the fuel comes through a pipeline. No tank filling, no fuel quality monitoring, no risk of theft. But the set depends on the pressure and continuity of the gas supply.

Comparison table

Criterion	Diesel	Gas
Fuel source	Tank on site	Pipeline (or LNG/CNG storage)
Independence from the grid	High; fuel is on site	Depends on the gas line
Sudden load acceptance	Usually strong	Usually needs stepped loading
Coming online	Fast; common for standby power	Varies by engine
Fuel storage	Tank; fuel needs looking after	No storage (if there is a pipeline)
Long-duration running	Refuelling must be planned	Uninterrupted as long as the line holds
Exhaust particulates	Higher; after-treatment may be needed	Usually lower
Size and initial investment per kW	Usually more compact	Usually larger and more expensive
Typical use	Standby power, construction sites, remote sites	Continuous generation, cogeneration

The table shows general trends. For a specific engine, the figures are in the manufacturer’s data sheet.

Load acceptance: the difference you notice most on site

When load jumps, a diesel engine quickly increases fuel injection. That is why a large pump starting, or the ATS handing over the whole load at once, is usually manageable for a diesel set.

Many gas engines run on an air-fuel mixture. The mixture has a limited response to sudden change. That is why gas sets are mostly loaded step by step. This difference matters in places like hospitals and data centres, where the whole load has to be picked up within seconds after an outage. The acceptable load step is assessed against the ISO 8528-5 performance classes and the engine data.

Running hours and cost

If the generator will run only a few hours a year, the initial investment and reliable starting come first. Diesel is common for this profile.

If the generator will run thousands of hours a year, fuel cost outweighs the initial investment. Depending on local gas and diesel prices, gas sets can be attractive in this scenario. Bring exhaust and jacket water heat into the picture, and the subject becomes cogeneration.

On the diesel side, you can estimate fuel cost with the fuel consumption calculator.

Installation differences

For a diesel set, installation means a tank location, fuel line, overflow and leak protection, exhaust and ventilation. For a gas set, the tank is replaced by gas pipework: a pressure-reducing and metering station, a gas leak detector, a safety shut-off valve and an approved installation design. The gas distribution company’s approval may be needed, and that takes time. In both cases, exhaust, ventilation and sound insulation are planned in a similar way.

Fuel security

In a diesel tank, the longer fuel sits, the higher the risk of water and microbial contamination. The fuel must be checked periodically. For tank capacity and autonomy, see our article on fuel tank sizing.

On the gas side, the risk is the pipeline itself. If the gas is cut off because of an earthquake, a flood or maintenance, the set stops too. That is why some critical facilities assess the reliability of the gas line separately.

Maintenance and service

Regular maintenance is essential for both engine types, but the content differs. On the diesel side, the fuel filters, the water separator and the condition of the fuel in the tank come first. Gas engines need regular checks of the ignition system (spark plugs, ignition coils), the gas pressure regulator and the mixture setting. Whether technicians and parts are available locally for each engine type matters at least as much as the fuel price. On a remote site, choosing a technology whose nearest service is hours away gets expensive at the first fault.

Common mistakes on site

• Choosing a gas set because there is a gas line. The line’s pressure and capacity may not be enough for the set’s full-load demand.
• Loading a gas set in one go, like a diesel. The frequency dips and the set drops the load.
• Forgetting the fuel on a diesel set. The tank is full, but the fuel is old, and the filters clog at the first outage.
• Looking only at the initial investment. Deciding for a facility that will run many hours without working out the fuel cost.
• Checking regulations afterwards. Rules on emissions, fuel storage and gas installations vary by location.

Decision checklist

• How many hours a year will the generator run?
• Within how many seconds must the whole load be picked up during an outage?
• How large is the biggest load step?
• Is there a reliable gas supply on site? Are the pressure and flow rate sufficient?
• What happens if the gas line is cut off?
• Is diesel storage and refuelling possible?
• Is heat recovery needed?
• What are the local rules on emissions and storage?

Next step

If you would like to assess diesel options for standby power together, share your load profile. You can estimate your power requirement with the power calculator, and for your project, contact us.