Understanding backup power system fuel choices

While diesel remains popular as a fuel supply, engineers have several additional fuel options from which to choose.


In the early days of commercial and industrial backup power, fuel choice was not an issue in the selection of a backup generator system because the preferred fuel was consistently diesel. Such is not the case today. Engineers and end users have several fuel options from which to choose, and each offers unique benefits in different applications.

Backup power generators are driven by internal combustion engines, which are in turn powered by fossil fuels. Diesel fuel has been used in backup power systems for decades. Gaseous fuels, such as natural gas or liquid propane, are gaining acceptance. Combining these fuels in unique ways provides additional fuel options. For example, dual-fuel generators run on either natural gas or LP vapor fuel, depending on which fuel is available at the time. Bi-fuel generators run on diesel fuel and natural gas simultaneously and leverage the benefits of each.

Gasoline is noticeably absent from this list because it is generally a poor fuel choice for backup power systems. Not only is it extremely volatile compared to diesel or gaseous fuel, making it problematic to store in quantity, but compared to diesel fuel it has a significantly lower thermal density. Additionally, gasoline cannot be easily used in combination with a gaseous fuel. As such, commercial and industrial backup power systems are rarely—if ever—fueled by gasoline.

Table 1Diesel fuel

As previously mentioned, diesel fuel has been the traditional fuel of choice for commercial and industrial backup power applications (see Table 1). Among the diesel-engine advantages is its high thermal efficiency, which can yield a low capital cost per kW in large-kW applications—typically 150 kW or more. Because diesel fuel must be stored on-site, diesel-fueled generators can also provide backup power in remote areas that do not have the benefit of a natural gas infrastructure. For the same reason, market segments with mission critical applications, such as hospitals and 911 call centers, often choose diesel-fueled generators because on-site fuel helps to ensure reliability. Finally, because diesel fuel has been used for so long in backup power applications, there is a perception within the market that diesel engines are the most reliable prime movers for backup power systems.

Despite its widespread acceptance, diesel fuel does have its drawbacks. For example, the U.S. Environmental Protection Agency requires the use of ultra-low sulfur diesel (ULSD) in all standby generator applications. ULSD goes through additional refining processes, which makes it less stable than traditional diesel fuel. If not maintained, diesel fuel will degrade over time. Within the first year of storage, it will suffer from oxidation, which occurs when the hydrocarbons react with oxygen to form a fine sediment and gum. If pulled into the engine, these contaminants could clog the fuel filter and fuel injectors. Microorganisms can similarly contaminate the fuel. Water, which can enter the fuel system as condensation, promotes bacteria and fungi growth. These microorganisms actually feed on the fuel itself. If allowed to grow, they can form gelatinous colonies that can also clog fuel systems. Additionally, their waste is acidic in nature, which can lead to fuel tank corrosion.

These are significant concerns in backup power applications. A diesel-fueled generator with a tank sized for 72 hr of full-load operation could easily take about 20 years to burn a single tank of fuel, assuming a 60% typical load level, weekly no-load exercising, and average power outages of only 4 hr per year. However, these issues can be mitigated by instituting an ongoing fuel testing and maintenance plan that regularly removes both water and sediment from the fuel tank. For emergency applications, fuel maintenance is required by code within NFPA 110: Standard for Emergency and Standby Power Systems. This type of maintenance program adds to a genset’s total cost of ownership, which also must be considered. Automatic fuel polishers, which consist of a pump and filtration system, add to the upfront cost of a backup power system, but they reduce ongoing fuel maintenance costs. Manual maintenance plans are more costly over the long term.

For some applications, diesel-fueled generators also face the more stringent Tier 4 emissions standards for stationary, non-road diesel engines adopted by the EPA, with initial phase-in within 2011. However, the Tier 4 rule affects “emergency” and “non-emergency” generators differently because the running times—and therefore the emissions—for each tend to be very different. The EPA defines an emergency power generator as “a generator whose sole function is to provide back-up power when electric power from the local utility is interrupted.” Emergency applications require only EPA Tier-2/Tier-3 compliance. By comparison, a non-emergency generator is one that is not being used exclusively for emergency power, such as those used for load management/peak shaving. In non-emergency applications, Tier 4 emission requirements are applicable. Thus, when considering diesel as a fuel choice in a backup power system, the application’s impact on generator emission requirements must be considered.

Compared to gaseous fuels, current diesel fuel (and gasoline) costs are relatively high. The high cost per barrel of crude oil as well as the additional EPA engine emission regulations has increased the total cost of both diesel engines and fuel. As of May 2012, off-highway diesel fuel costs were approximately $3.46/gal (an estimate based on the average cost per gallon of on-highway diesel fuel as reported by the U.S. Energy Information Administration for May 2012, less an estimate of the cost of state and federal excise taxes, which only apply to on-highway diesel fuel). By comparison, commercial natural gas prices for May 2012 were $8.09/thousand cubic ft (as reported by the U.S. Energy Information Administration). A 150 kW diesel generator running for 24 hr on diesel fuel at full load would likely consume 260 gal, or about $900 of diesel fuel. A similar natural gas-fueled unit running at full load for the same amount of time would likely consume about 48,000 cubic ft, or about $388 of natural gas. As such, when considering diesel fuel for an emergency backup power system, consider the average length of power outages that will affect the application to predict fuel costs and determine if they are acceptable.

Table 2Natural gas

In the past, gaseous fuels were avoided in industrial backup power applications based on cost effectiveness, power density, and perceptions of durability and fuel reliability. However, recent technological innovations have changed that. These innovations include hardened valves and seats, and optimized air/fuel mixtures. Engine speed optimization has been a significant improvement. Historically, generators were configured for direct connection to a four-pole alternator that limited engine speed to 1,800 rpm. By implementing a gear-on-gear powertrain, or two-pole alternators as appropriate, generator manufacturers have been able to optimize the power and performance of spark-ignited engines. This has improved transient performance, reduced the stress on engine bearings, and increased power densities. In a nutshell, it means more powerful engines and reduced capital costs.

Particularly, with regard to natural gas as a fuel for backup power systems, a key benefit is long running time (see Table 2). Because natural gas is supplied by a utility rather than stored in a finite quantity on-site, refueling is not an issue—regardless of the length of the power outage. It is this benefit in particular that is a key selling point in residential backup power solutions, as well.

Natural gas is also more environmentally friendly than diesel fuel. Not only do natural gas-fueled engines emit less NOX and particulate matter than comparable diesel-fueled engines, they also avoid the fuel containment issues and environmental concerns associated with storing large quantities of diesel fuel. Additionally, because it is a gas, spillage is not a concern. For these reasons, the local regulations that apply to fuel containment are considerably less stringent than those for diesel-fueled engines, making compliance far less costly.

Automotive-style spark-ignited engines are also more readily available in high volumes, making them more cost-effective components for generator manufacturers. They are also typically more cost effective to source than similarly sized diesel engines. This means that gaseous-fueled backup power systems tend to cost less per kW in single-engine backup power applications 150 kW and below. For larger-kW applications, gaseous-fueled generators can be configured to combine their output in an integrated approach to generator paralleling (see Figure 1). Their general cost effectiveness combined with the reliability and scalability benefits offered by integrated paralleling (compared to one very large diesel-fueled generator) can make them attractive alternatives even in large applications. In applications requiring the generator to assume the emergency load within 10 sec, the system can be configured so that the first generator online is large enough for that load. This first generator can meet the 10 sec requirement, while the remaining generators can pick up the other load categories.

The long running times provided by natural gas unfortunately result in a perceived downside: it is delivered by a utility, and as such its availability is outside of the facility’s control. On-site fuel storage is preferred by many authorities having jurisdiction (AHJ) because there is no question of its availability. It is generally required by NFPA 70: National Electrical Code, Article 700: Emergency Systems for emergency system loads in many municipalities. While natural gas is delivered largely by underground pipelines that are generally unaffected by the kind of severe weather that knocks out power, the natural gas infrastructure is not 100% reliable. Engineers should work with the local gas utility and AHJ to understand the reliability of the natural gas infrastructure compared to on-site diesel fuel. Also work with the system owner to ensure that the facility is not subject to a curtailment policy that would cut off the natural gas fuel supply at the local utility’s discretion. It is not uncommon for the reliability of natural gas to be favorable in many applications when refueling and fuel spoilage concerns are completely understood.

Table 3LP fuel

LP-fueled backup power systems can run in either LP liquid or LP vapor configurations. LP vapor is perhaps the more prevalent in backup power systems (see Table 3). All of the general gaseous fuel benefits described earlier apply to LP fuel as well—including lower cost per kW in single-engine backup power applications 150 kW and below. As a spark-ignited fuel, LP fuel runs in automotive-style engines adapted for its use.

Beyond the general benefits of LP as a gaseous fuel, LP must be stored on-site, just like diesel fuel. Thus, LP fuel might provide an acceptable gaseous-fuel alternative to diesel for applications requiring on-site fuel. Consulting engineers should also explore this with their customer prior to selecting a diesel-fuel solution. LP meets the same on-site requirements, but has the benefit of no fuel spoilage concerns.

The drawbacks of LP fuel are really more challenges in system design. Regardless of whether the system runs in LP liquid or LP vapor configurations, LP fuel is stored under pressure as a liquid. In LP vapor fuel designs, this liquid fuel must be introduced into the engine’s combustion chamber as a vapor. Because it has a boiling point of -44 F, vaporization occurs naturally within the fuel tank at ambient temperature. However, managing that boil-off rate (the rate at which liquid LP fuel boils off into a vapor) is a design consideration. The ambient temperature, the size of the LP fuel tank, and the generator’s fuel consumption rate must be considered when implementing LP vapor backup power systems.

By comparison, backup power systems running on LP liquid don’t rely on the natural vaporization of LP inside the fuel tank to deliver adequate amounts of fuel to the generator. Instead, these systems require a vaporizer to convert the pressurized liquid to a vapor in sufficient quantities before introducing it to the generator’s engine for combustion. Vaporizers allow for the tanks to be sized for run time instead of boil-off rates. Typically, the vaporizer is integrated into an outdoor generator. However, this is not the case when the generator is located inside a building. Because most building codes do not allow liquid LP fuel inside a building—either stored or piped—the vaporizer must be installed external to the facility. The vaporizer requires some form of internally generated or externally supplied heat.

Table 4Dual- and bi-fuel systems

One way to alleviate the reliability issues that invariably crop up when discussing on-site vs. utility-supplied fuel is to specify a system that makes use of both fuels—either one at a time or simultaneously. Dual-fuel and bi-fuel systems fit these criteria (see Table 4).

As mentioned earlier, a dual-fuel system can operate on either LP vapor or natural gas fuel, depending on which is available at the time. The system will typically start and run on natural gas fuel, and if that fuel supply is interrupted then it will switch over to the on-site LP fuel source. This configuration is very popular for generators up to 150 kW.

For larger applications, a bi-fuel system—one that burns both diesel fuel and natural gas at the same time within a single engine—is an attractive option (see Figure 2). Bi-fuel generators start up using 100% diesel fuel, which ignites at 500 to 750 F, and serves as a pilot fuel. After certain criteria are met, such as acceptance of the electrical load, the generator’s controller introduces natural gas to the fuel mixture. The burning of the diesel fuel ignites the natural gas, which has a much higher ignition temperature of 1,150 to 1,200 F. As the generator controller adds natural gas, the engine’s normal speed governing function reduces the amount of diesel fuel entering the engine. The process continues until an optimal fuel mix is reached, typically 75% natural gas to 25% diesel fuel. If the load increases, the transient will initially be addressed with diesel, after which natural gas will be added back into the system to match the new higher load level.

Bi-fuel generators capitalize on the reliability benefits of both diesel and natural gas fuel while minimizing their respective drawbacks. Initial bi-fuel generator costs are typically 15% to 30% higher than those of diesel generators. However, because natural gas—not diesel—is the predominant fuel in a bi-fuel generator, running times are extended while on-site fuel storage requirements (and their associated maintenance costs) are decreased. Additionally, because on-site fuel remains part of the system, reliability is improved. Should the natural gas supply fail—because it was shut off at the utility or was otherwise interrupted—the generator can run on 100% diesel.


The days of all backup power systems being fueled exclusively by diesel fuel are gone. While diesel remains a popular fuel supply, engineers and end users have several additional fuel options from which to choose: natural gas, LP fuel (liquid and vapor), dual fuel (either natural gas or LP vapor), and bi-fuel (natural gas and diesel running simultaneously). Each offers unique benefits. Consulting engineers should take the time to learn how each of these fuel sources can be applied so they can make the best recommendations to their customers. As always, be sure to consult your local AHJ to understand its policies on the use of a particular fuel in a given application. Knowing your options will make those conversations more fruitful.

Kirchner is technical support manager for Generac Power Systems, Waukesha, Wis., where he supports and trains on all industrial products. He received a bachelor’s degree in electrical engineering and MBA from the University of Wisconsin. He has been with Generac Power Systems since 1999.


Seitz, John S., Calculating Potential to Emit (PTE) for Emergency Generators. Memorandum, U.S. Environmental Protection Agency, 1995.

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