When people ask me about a diesel generator vs. natural gas generator, the conversation often starts with fuel price. I think that is too narrow. For backup power, the generator that costs less to run is not always the generator that gives you the most resilient system during a major outage.
Diesel and natural gas generator sets can both provide dependable emergency power, but they solve the fuel problem in very different ways. A diesel generator normally carries its energy supply with it in a tank. A natural gas generator is usually connected to a utility pipeline and consumes fuel as long as the gas network can supply it.
In this guide, I will compare reliability, efficiency, fuel storage, startup performance, maintenance, emissions, installation cost, long-duration operation, and the applications where each type makes the most sense. I will also show you how I would think through the choice for a house, commercial building, critical facility, and remote site.
Diesel Generator vs. Natural Gas Generator: Technical Comparison
| Technical factor | Diesel generator | Natural gas generator | Why it matters for backup power |
|---|---|---|---|
| Fuel source | Liquid diesel stored in an on-site tank | Usually utility natural gas through a pipeline | Determines whether your generator depends on fuel deliveries or public infrastructure |
| Energy content | About 137,000 Btu per gallon as a planning value | About 1,036 Btu per cubic foot as a planning value | Useful when comparing fuel consumption on an energy basis |
| Cold start and block-load response | Generally very strong | Modern standby gas sets can be very capable, but model-specific | Large motors, pumps, compressors, elevators and UPS systems can create abrupt load steps |
| Fuel stored on site | Yes | Normally no, unless the installation includes an alternate fuel | On-site fuel can improve independence from external utilities |
| Refueling requirement | Yes during long outages | No routine refueling while pipeline service is available | Natural gas can be convenient during multi-day operation |
| Fuel aging | Diesel requires storage management | No long-term on-site fuel aging in a normal pipeline system | Important for generators that run only during tests and emergencies |
| Combustion CO2 factor | About 163.45 lb CO2/MMBtu of fuel | About 116.65 lb CO2/MMBtu of fuel | Natural gas has a lower direct combustion CO2 factor per unit of fuel energy |
| Local exhaust profile | Typically higher particulates and sulfur-related concerns | Generally cleaner-burning locally | Can influence permitting and siting |
| Typical backup use | Homes, hospitals, data centers, industry, remote sites, critical facilities | Homes, commercial buildings, campuses, industrial facilities, longer-run standby sites | Both technologies now cover a wide range of power levels |
| Primary limitation | Fuel tank, fuel maintenance and refueling logistics | Dependence on gas service and gas-pressure capacity | The weak point is different for each fuel |
The numbers in that table are planning references, not universal generator specifications. Generator efficiency, gas consumption, diesel burn rate, starting capability and maintenance intervals vary substantially by engine family, rating, emissions package and operating load. I always use the exact manufacturer data sheet before final sizing.
Why Diesel Has Traditionally Dominated Standby Power
Diesel has been the traditional choice for serious standby power because the combination is hard to beat: high power density, rapid starting, strong response to sudden loads, and a fuel supply that can be stored at the facility.
That last point is especially important to me. When I install or evaluate a backup system, I ask, what other infrastructure has to remain functional for this generator to operate? A diesel unit with a properly sized tank can operate even if roads, electric utilities and natural gas service are having problems, at least until the stored fuel is exhausted.
Diesel also works well for loads that do not come on gently. A fire pump, large refrigeration compressor, chiller, well pump or motor can demand several times its normal running current during startup. Generator selection has to account for that transient demand. If you are still deciding on capacity, see my diesel generator sizing guide.
The disadvantages are equally real. You need a tank. You need to keep water, microbes and sediment out of stored fuel. You may need polishing or fuel replacement. Large tanks require space and may trigger environmental, fire-code and permitting requirements. During an extended outage, you also need a realistic plan for resupply.
Why Natural Gas Generators Are Increasingly Used for Backup Power
Natural gas eliminates one of the biggest hassles of diesel standby systems: keeping a large volume of liquid fuel on site. If the gas utility remains available, your generator can continue receiving fuel without a delivery truck ever entering the property.
That is attractive for homeowners, commercial sites and facilities where diesel tank placement is difficult. Natural gas also produces less direct CO2 per unit of fuel energy than diesel and generally has lower particulate and sulfur-related emissions.
Modern natural gas standby sets are also much more capable than many older comparisons suggest. Purpose-built standby gas generator sets are available in very large sizes, and some modern designs are engineered for fast starting and substantial step-load acceptance.
I would not, however, assume that every natural gas generator can duplicate the transient behavior of an equivalent diesel set. The only defensible way to make that comparison is with the manufacturer’s motor-starting, voltage-dip, frequency-dip and load-step data for the specific models being considered.
Fuel Security: The Most Important Difference
For backup power, I consider fuel security at least as important as fuel price.
A diesel tank gives you a known quantity of energy on the property. If you have 500 usable gallons, you can estimate how many hours of operation that fuel represents. The disadvantage is that once the fuel is gone, your generator stops unless you can arrange a delivery.
Natural gas works in the opposite way. You may have no finite tank to empty, but the system depends on the gas network maintaining sufficient pressure and flow. Gas infrastructure is often highly resilient, but it is not invulnerable. Earthquakes, utility shutdowns, local damage, flooding, fires, pressure restrictions and upstream supply problems can affect service.
How Long Can a Diesel Generator Run From a Tank?
The basic diesel runtime calculation is simple:
Runtime in hours = usable fuel in gallons ÷ fuel burn in gallons per hour
Suppose a generator is actually burning 5.5 gallons per hour at the load you expect. The following table shows the theoretical runtime before adding a fuel reserve.
| Usable diesel | Burn rate | Theoretical runtime | Approximate days |
|---|---|---|---|
| 100 gal | 5.5 gal/hr | 18.2 hr | 0.76 day |
| 250 gal | 5.5 gal/hr | 45.5 hr | 1.9 days |
| 500 gal | 5.5 gal/hr | 90.9 hr | 3.8 days |
| 750 gal | 5.5 gal/hr | 136.4 hr | 5.7 days |
| 1,000 gal | 5.5 gal/hr | 181.8 hr | 7.6 days |
Those figures are examples, not a fuel curve for a specific generator. Real consumption changes with generator size and load. If you want to calculate this in more detail, see my diesel generator burn-rate guide and my article on continuous diesel generator runtime.
Which Fuel Is Cheaper to Run?
Natural gas frequently has a lower fuel cost per unit of energy than retail diesel, but local utility tariffs and diesel prices can change the result. I never compare only the price of one gallon with the price of one therm or one thousand cubic feet because those units contain different amounts of energy.
A better method is to convert both systems to a cost per operating hour or cost per generated kWh at the same electrical load.
Illustrative 75 kW Load Comparison
Assume, only for illustration, that a diesel generator burns 5.5 gallons per hour at the chosen operating point and diesel costs $4.00 per gallon. Fuel cost would be:
5.5 gal/hr × $4.00 = $22.00 per hour
Now assume a natural gas generator delivering the same 75 kW has an electrical efficiency of 32%. Its approximate fuel input would be 0.80 MMBtu per hour. At $4.00/MMBtu, the fuel cost would be about:
0.80 MMBtu/hr × $4.00 = $3.20 per hour
| Illustrative input | Diesel example | Natural gas example |
|---|---|---|
| Electrical load | 75 kW | 75 kW |
| Assumed fuel use | 5.5 gal/hr | 0.80 MMBtu/hr |
| Assumed fuel price | $4.00/gal | $4.00/MMBtu |
| Fuel cost per hour | $22.00 | $3.20 |
| Fuel-only cost per kWh | $0.293/kWh | $0.043/kWh |
I want to emphasize that this is a math example, not a promise about what either generator will cost to operate. The actual gas rate can include demand charges, transport fees and utility-specific tariffs. Diesel price, generator efficiency and load factor also change continuously. For a complete framework, use my generator cost-per-kWh guide.
Efficiency: Diesel vs. Natural Gas
There is no single efficiency number for “a diesel generator” or “a natural gas generator.” A small residential set, a 100 kW commercial machine and a megawatt-class industrial set can have very different heat rates.
In general, modern diesel engines are highly efficient at converting fuel into shaft power, particularly under substantial load. Natural gas reciprocating engines can also be very efficient, especially larger lean-burn designs intended for extended operation.
What often matters more in real backup service is load factor. If you buy a 500 kW generator and usually operate it at 60 kW, its fuel efficiency per generated kWh can be poor regardless of fuel. Oversizing also affects engine temperature, maintenance patterns and long-term operating cost.
| Load condition | What I expect | Why it matters |
|---|---|---|
| Very light load | Higher fuel cost per kWh | Fixed engine losses become a larger share of output |
| Moderate load | Usually better efficiency per kWh | Engine operates closer to its efficient working range |
| High load within rating | Good utilization but less reserve | Motor starts or new loads can cause larger transient dips |
| Overload | Unacceptable operating condition | Can produce frequency/voltage problems, overheating or shutdown |
Startup Time and Load Acceptance
Diesel’s reputation for backup power comes partly from its ability to start quickly and accept substantial load. For emergency systems serving life-safety loads, that matters more than a small difference in fuel price.
Natural gas technology has improved considerably. Caterpillar, for example, currently markets large natural gas standby systems capable of very fast starting and substantial single-step load acceptance. That means I would not dismiss natural gas simply because someone says “gas generators are slow.”
At the same time, I would not size either fuel by nameplate kW alone. I want to know the largest motor, the sequence in which loads connect, whether there is a UPS, whether variable-frequency drives are used, and what voltage and frequency dip the connected equipment can tolerate.
For a home, the largest transient load may be an air conditioner, well pump or heat pump. For a commercial building, it could be an elevator, chiller, fire pump or refrigeration compressor.
Diesel vs. Natural Gas Emissions
If emissions are part of your decision, natural gas usually has the advantage at the point of combustion. The U.S. Energy Information Administration lists a direct CO2 combustion factor of about 116.65 pounds per MMBtu for natural gas compared with about 163.45 pounds per MMBtu for distillate fuel oil, the category that includes diesel.
| Fuel | Direct CO2 factor | Difference |
|---|---|---|
| Natural gas | 116.65 lb CO2/MMBtu | Lower direct combustion factor |
| Diesel/distillate | 163.45 lb CO2/MMBtu | Higher direct combustion factor |
That table does not by itself tell you total lifecycle emissions per kWh. Generator efficiency matters, and upstream fuel production and methane leakage are separate issues. But for local standby permitting, natural gas can also offer advantages in particulate matter and sulfur-related emissions.
If you want a broader discussion, I have a separate guide to the environmental impact of diesel generators.
Maintenance Differences
Diesel and natural gas generators both need scheduled maintenance. Neither is a “install it and forget it” machine.
With diesel, I pay particular attention to the fuel system because backup units may sit for long periods. Water accumulation, microbial contamination, degraded fuel, clogged filters and tank sediment can turn an otherwise healthy engine into a no-start generator during an outage.
Natural gas avoids long-term liquid-fuel storage problems, but it introduces different components: gas regulators, valves, ignition systems, spark plugs on spark-ignited engines, air-fuel controls and gas-pressure requirements. The gas supply must also be able to deliver the required flow at the pressure specified by the manufacturer while other gas appliances may be operating.
| Maintenance area | Diesel | Natural gas |
|---|---|---|
| Engine oil | Required | Required |
| Cooling system | Required on liquid-cooled units | Required on liquid-cooled units |
| Starting battery | Critical | Critical |
| Stored fuel quality | Major concern | Usually not applicable with pipeline gas |
| Spark ignition service | Normally not applicable | Applicable to spark-ignited gas engines |
| Fuel filters/regulators | Diesel filters and water separation | Gas filtration/regulation components |
| Exercise and load testing | Important | Important |
Installation Cost: Which One Costs More?
I would not give you a universal answer because the generator itself is only part of the installed price.
A diesel installation can require a base tank or remote tank, containment, fuel piping, fill connections, venting, spill control and sometimes substantial site work. Natural gas may eliminate the storage tank but require a larger gas service, new gas piping, a regulator, utility coordination or even a meter upgrade.
The electrical side is similar for both: transfer switch, conductors, overcurrent protection, grounding and bonding, concrete pad, controls, commissioning and permits. For a permanent system, see my detailed guide to the cost of installing a permanent diesel generator.
Example Installation Cost Categories
| Cost category | Diesel system | Natural gas system |
|---|---|---|
| Generator set | Model and rating dependent | Model and rating dependent |
| Fuel infrastructure | Tank, fill, vent, containment, piping | Gas line, meter/regulator capacity, utility work if required |
| Transfer equipment | Required for automatic standby | Required for automatic standby |
| Concrete/site work | Usually required | Usually required |
| Permits/engineering | Site-specific | Site-specific |
| Fuel maintenance | Ongoing storage concern | Minimal stored-fuel maintenance |
Which One Is Better During a Long Outage?
This is where the answer becomes very location-specific.
If roads are blocked and fuel trucks cannot reach you, a natural gas generator may have a major logistical advantage as long as the gas network keeps operating. You do not have to ration the last 100 gallons in a tank or wait for a delivery.
But if the emergency itself can disrupt gas service, the advantage reverses. A diesel generator with enough on-site storage may be more independent because you already possess the fuel needed for a defined number of operating hours.
I therefore size backup systems around the failure modes of the location, not only around generator horsepower. A coastal hurricane site, wildfire zone, earthquake region, floodplain, rural farm and urban hospital can have completely different fuel-security priorities.
Residential Backup: Diesel or Natural Gas?
For many homes connected to a reliable natural gas utility, gas is attractive because it is convenient. You avoid storing hundreds of gallons of diesel, there is no fuel polishing, and the generator can automatically start during an outage without anyone arranging a delivery for the first few days.
Diesel becomes more attractive when the property does not have natural gas, when the owner already stores diesel for equipment, when very long autonomous operation is important, or when the site needs a heavy-duty generator with strong load response.
If you are evaluating a house specifically, my home backup diesel generator guide goes deeper into residential sizing and use cases.
Commercial Buildings and Small Businesses
For offices, stores, restaurants and small industrial facilities, I compare the fuel decision against the actual outage cost. If the generator is protecting refrigeration, servers, security systems, communications or production, reliability is more important than saving a few cents per generated kWh.
Natural gas may be attractive in urban sites where tank placement is difficult. Diesel may be attractive where the building owner wants a known quantity of fuel on site and strong motor-starting performance.
I also check whether the gas service has enough spare capacity. A building may already have boilers, water heaters, kitchen equipment or process burners. The gas generator has to operate at the same time as whatever loads remain in service unless the system is intentionally designed to shed them.
Hospitals, Data Centers and Critical Infrastructure
Critical facilities need more than a simple “diesel versus gas” comparison. Codes, authorities having jurisdiction, fuel-duration requirements, redundancy, start-time requirements and reliability studies all affect the system.
Historically, diesel has been widely used because on-site fuel storage makes the backup supply visibly independent of a public gas network. Modern gas systems can meet demanding standby requirements in some applications, but code acceptance and fuel-supply requirements must be checked before choosing the technology.
I would never replace that engineering process with a generic web recommendation. For mission-critical power, the fuel system, electrical topology, transfer scheme, load sequence and maintenance program have to be designed as one system.
Remote Sites and Rural Properties
If there is no natural gas pipeline, the comparison ends quickly. A diesel generator is usually the more practical option unless propane or another stored gaseous fuel is available.
Diesel is particularly useful where the same fuel is already used by tractors, construction equipment, pumps or fleet vehicles. The site can consolidate fuel logistics instead of maintaining a completely separate energy source.
The tradeoff is storage quality. A remote tank that sits for months needs disciplined inspection, water control and fuel management.
Noise: Is Natural Gas Quieter Than Diesel?
People often assume natural gas will always be quieter. The engine combustion characteristics are different, but enclosure design, fan speed, exhaust silencing, engine rpm, load and installation geometry often have more influence on the measured sound at a property line.
I compare the manufacturer’s published dBA value at a stated distance, not the fuel name. Two enclosed generator sets with different fuels can be closer in sound level than two diesel sets with different enclosure packages.
Cold Weather Performance
Both systems need cold-weather planning.
Diesel fuel can gel or become difficult to flow at low temperatures, and cold engines may require jacket-water heaters, battery heaters or properly specified seasonal fuel. Natural gas does not gel like diesel, but the engine still has batteries, lubricating oil and cooling-system components that must function in the cold. Gas regulators and supply systems also have specified pressure and temperature limits.
I therefore treat winter readiness as an installation issue rather than assuming one fuel is automatically immune to cold-weather problems.
How I Size Either Generator
The sizing method is essentially the same whichever fuel you choose:
- List every load that must operate during an outage.
- Separate running loads from motor-starting or inrush loads.
- Identify the largest load step the generator will see.
- Determine whether loads can be sequenced or shed.
- Check kW and kVA requirements.
- Check voltage, phase and frequency.
- Apply altitude and temperature derating if required.
- Use the manufacturer’s transient-performance data before final selection.
Example Load Calculation
| Backup load | Running demand | Special consideration |
|---|---|---|
| Lighting and controls | 5 kW | Usually low surge |
| Refrigeration | 12 kW | Compressor starting surge |
| Well or process pump | 15 kW | Motor starting current |
| HVAC | 20 kW | Compressor/fan starting |
| IT and communications | 8 kW | UPS and power-quality considerations |
| Total running load | 60 kW | Generator still has to handle the largest transient |
A 60 kW running total does not mean I automatically buy a 60 kW generator. The required size depends on load sequencing, motor starting, allowable voltage dip and reserve capacity.
Total Cost of Ownership Matters More Than Purchase Price
If you compare only the generator price, you can miss the larger economic picture. I include fuel infrastructure, fuel cost, annual testing, service, tank maintenance, gas-service upgrades, permitting, emissions equipment, expected runtime and major overhauls.
| Ownership factor | Diesel impact | Natural gas impact |
|---|---|---|
| Initial generator price | Often competitive for standby duty | Model/size dependent |
| Fuel storage investment | Can be substantial | Usually avoided with pipeline gas |
| Fuel price exposure | Retail/commercial diesel pricing | Utility gas tariff |
| Long-run fuel cost | Can be relatively high | Often lower where gas rates are favorable |
| Fuel maintenance | Tank testing, water control, filtration/polishing as needed | Little or no stored-fuel maintenance |
| Utility dependency | Lower while stored fuel remains | Higher because pipeline service is normally required |
What About Dual-Fuel or Bi-Fuel Systems?
Some larger systems combine diesel and natural gas so the site can use gas when it is available while retaining diesel capability. The exact architecture varies. Certain systems start on diesel and substitute a portion of the diesel energy with natural gas under suitable load.
That can improve fuel flexibility, but it also adds controls, piping and commissioning complexity. I would evaluate it when a facility has a strong business case for extended run hours or fuel diversity, not simply because two fuels sound better than one.
Diesel Generator vs. Natural Gas Generator: Which Is Better?
I would lean toward diesel when on-site fuel independence, heavy motor starting, rapid load acceptance, remote operation or established critical-power practice is the priority.
I would lean toward natural gas when a reliable gas utility is already available, long outage duration is a concern, you want to avoid a large fuel tank, local emissions are important, or the generator may run enough hours that natural gas fuel economics materially affect ownership cost.
| Your priority | Fuel I would investigate first | Reason |
|---|---|---|
| Known on-site fuel reserve | Diesel | You control the stored quantity |
| No liquid-fuel tank | Natural gas | Pipeline supply avoids bulk diesel storage |
| Large abrupt motor loads | Diesel, then compare specific gas models | Diesel has traditionally been strong in transient response |
| Long outage with reliable gas grid | Natural gas | No routine truck refueling |
| Remote property without gas service | Diesel | Fuel can be delivered and stored |
| Lower direct combustion CO2 per fuel energy | Natural gas | Lower EIA CO2 coefficient per MMBtu |
| Existing bulk diesel infrastructure | Diesel | Can share established site logistics |
Questions I Would Ask Before Buying
- How many kW and kVA do I actually need?
- What is my largest motor or load step?
- How many hours or days of autonomous operation do I require?
- Can my natural gas service deliver enough pressure and flow at full generator load?
- How much usable diesel can I legally and practically store?
- Can fuel trucks reach the property after the disasters I am planning for?
- Can the gas network be interrupted by those same disasters?
- What do local fire, building, air-quality and electrical codes require?
- What are the manufacturer’s service intervals under my expected duty?
- What will fuel, maintenance and testing cost over 10 to 20 years?
Frequently Asked Questions
Is diesel more reliable than natural gas for backup power?
Diesel can offer greater fuel independence because the fuel is stored on site. That does not automatically make every diesel system more reliable. Reliability also depends on maintenance, fuel quality, starting batteries, controls, cooling, transfer equipment and correct sizing. A well-designed natural gas standby system can be highly reliable where gas service is dependable.
Will a natural gas generator run indefinitely?
No generator should be described as indefinite. A natural gas set can continue receiving fuel as long as the gas network supplies it, but the engine still has maintenance limits, oil-service requirements and component wear. Utility gas service itself can also be interrupted.
Does diesel produce more power than natural gas?
Fuel type by itself does not tell you generator output. Manufacturers sell both diesel and natural gas generator sets across broad power ranges. What matters is the rated kW/kVA of the specific set and how it performs under your load profile.
Which generator is cheaper to install?
It depends on the site. Diesel may require an expensive tank and containment system. Natural gas may require costly piping or a utility service upgrade. The installed comparison has to include fuel infrastructure, transfer equipment, site work, permits and commissioning.
Is natural gas cleaner than diesel?
At the point of combustion, natural gas generally has lower direct CO2 per unit of fuel energy and lower particulate and sulfur-related emissions. Lifecycle comparisons are more complicated because they also involve engine efficiency and upstream fuel production and transport.
Which is better for whole-house backup?
If your home has a reliable gas service and you want automatic operation without managing stored diesel, natural gas can be convenient. Diesel can make more sense where gas service is unavailable, where you want a defined on-site fuel reserve, or where your load profile favors a diesel set.
Can I convert a diesel generator to natural gas?
I would not treat a diesel-to-gas conversion as a simple fuel swap. Diesel and spark-ignited natural gas engines use different combustion and control systems. Some engineered bi-fuel systems exist, but any conversion has to be specifically designed, approved and compatible with the engine and emissions requirements.
Bottom Line
When I compare a diesel generator vs. natural gas generator for backup power, I do not ask which fuel is universally better. I ask which failure mode you are trying to protect against.
Diesel gives you a visible, measurable fuel reserve on site, strong transient performance and wide acceptance in critical-power applications. Its weaknesses are tank cost, fuel aging, emissions and the need for refueling during long outages.
Natural gas removes the liquid-fuel tank, can reduce operating fuel cost where utility rates are favorable, and generally produces cleaner local exhaust. Its weakness is dependence on a gas network that must continue delivering sufficient pressure and flow during the very event that caused the electrical outage.
If I were choosing between them, I would first define the critical load, outage duration and fuel-security requirement. Then I would compare the actual manufacturer data and local installed cost. That approach gives you a backup system designed around your real risk instead of a generic claim that one fuel is always better.
