Summary of advantages (blue) and disadvantages (red) of the three most common fuels, natural gas, propane (gaseous) and diesel for standby generators*:


Natural gas:

  1. Is less expensive than diesel or propane and is forecasted to become relatively even less expensive in the future.
  2. Hasn't been turned off since it has been turned on (aside from the San Bruno incident). In effect, a limitless fuel supply.
  3. Can be easily and fully automatically backed up 100% by propane, a nearly perfect backup fuel.
  4. Generators under 150kW are usually a fraction of the cost of an equivalent diesel generator.
  5. Generators have the lowest maintenance costs, as there is no wetstacking (requiring annual load banks), fuel filters, tanks, etc.
  6. Storing fuel on site requires an ANG or CNG tank, which are much larger than similar propane or diesel fuel tanks.
  7. If not stored locally, availability cannot be guaranteed.
  8. May be expensive to bring to the generator site if a long pipe run is required.
  9. May take several seconds to start.

Propane:

  1. Is considered the perfect fuel for storage, as its tank may require replacement after several decades but the fuel itself is simply moved to the new tank.
  2. Generators under 150kW are usually a fraction of the cost of an equivalent diesel generator.
  3. Generators have low maintenance cost, as there is no wetstacking (requiring annual load banks), fuel filters, etc.
  4. Is much more expensive than natural gas and may be more (or less) expensive than diesel.
  5. Is historically less available than natural gas in an emergency (but may be more easily available than diesel, depending on your area and fuel supplier). Fuel supply is limited to size of tank.
  6. Storage tanks, in larger sizes, may require a large 'setback' area and, if space is limited, may not be allowed.
  7. May be expensive to bring to the generator site if a long pipe run is required.
  8. May take several seconds to start.

Diesel:

  1. Engines are considered premium engines because they can usually start a couple seconds faster, run longer between engine rebuilds (not at all useful in standby applications, where the gen usually runs less than 20hrs/yr) and older engines were very simple and had fewer things that could break this hasn't been a clear advantage since turbos and electronic controls).
  2. Fuel is the most easily stored of the three, with pressurized tanks not required. Still, on site fuel supply is limited to size of tank.
  3. Is much more expensive than natural gas currently, may be more (or less) expensive than propane and is forecasted to become relatively even more expensive than natural gas in the future.
  4. Is historically less available than natural gas in an emergency(but may be more easily available than propane, depending on your area and fuel supplier)**.

  5. Rots. Over time, diesel fuel will itself require maintenance and/or replacement.
  6. Diesel tanks eventually rust, both inside and out.
  7. Diesel generators, up to about 150kW, are much more expensive to purchase than similar natural gas or propane systems
  8. Diesel tanks are usually more expensive than propane.
  9. Storage tanks, in larger sizes, may require a large 'setback' area and, if space is limited, may not be allowed.
  10. Generator maintenance costs are higher. Maintenance can even cost triple or more when you factor in load banks required to minimize wetstacking, a problem unique to diesels***.
  11. Produces emissions that are more harmful to the environment and people.
  12. Generators are harder and more expensive to permit with California air boards and usually receive a fraction of the 'discretionary usage' allotment per year compared to gaseous fuelled generators.
  13. Diesel in the past few years has become relatively unstable thanks to the biofuels being added. These biofuels act like catalysts, breaking down diesel so that, instead of lasting years, diesel often now only lasts a few months. While this is fine for vehicles that are driven often, it is deadly for fuel that is stored for years at a time. This problem cannot be understated and is the reason why many cities that wanted to be first to switch to "environmentally friendly" fuels ended up destroying (and having to replace) their generators, fuel tanks and fuel.
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*As gasoline is almost never used anymore for standby (for good reason), we do not consider it as a viable standby option.

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**Always, with natural gas, concern arises because the fuel is not stored on site. However, every recent large-scale natural disaster has been characterized by people and animals suffering and even dying because generators ran out of diesel but the natural gas was always right at the pipe. Examples include Hurricane Katrina and Hurricane Sandy but it is worthy of note that natural gas was not interrupted even during California's Loma Prieta earthquake.

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***Full diesel maintenance costs are often so great that full maintenance is often not implemented, eventually leading to deterioration and an unreliable and/or non-working generator. This is quite common and lends to diesel generator reliability figures that are sometimes lower than reliability of their gaseous-fueled counterparts.


We hear a lot of discussion about efficiency of diesel versus natural gas and propane but there are several reasons why this comparison often favors gaseous fuels in standby applications. With standby generators, where between 10hrs/yr and 20hrs/yr of total usage per year is most common, the amount of fuel used is not usually a factor. However, because the question frequently arises, here are the numbers:

In average installations, more fuel is spent during the weekly 15min exercise cycle of a standby generator than during outages (averaged over a generator's lifespan). Diesel generators should be exercised weekly under at least 30% load to avoid wetstacking or otherwise be load banked annually. For diesel gens, often a load bank is purchased and permanently installed, an expensive proposition. The other option is to pay annually for rental and connection of a load bank by a generator service company, an even more expensive proposition. Gaseous generators don't require being run under load at all (ever) as they don't experience wetstacking. In fact, gaseous generators from many major manufacturers will, by default, exercise at a low-speed with no load, consuming a fraction of the fuel that a diesel generator consumes when being run under 30% load. For example, a 70kW diesel will likely consume around 2 gallons per hour running under 30% load while exercising weekly (~1 gallon per hour under no load). A 70kW propane generator, from the same manufacturer, will consume around 1.2 gallons of propane per hour while exercising. And if an annual two hour 75% load bank is chosen (instead of weekly loading) for the diesel generator in order to burn off wetstacking, that's 9-10 gallons of fuel, 2.25 hours of runtime and significant expense annually to the generator service company (all of this is never required for gaseous generators). Total fuel used for maintaining a 70kW generator annually: around 13.6 gallons of propane for the gaseous and either 26 gallons or 22 gallons for the diesel, depending on whether the load is applied during its exercise (weekly at 30%) or annually (75% for two hours but no load during weekly runs), respectively.

Conclusion: If you must have fuel on site and can have diesel but not propane -or- if you absolutely have to have the fastest start time, diesel may be the best choice. Most often, however, the fuel type we recommend will simply depend on whether natural gas is available or not. When natural gas is available, we almost always will recommend it with a propane backup, if on site fuel is desired. When diesel is absolutely required, we will usually recommend a bi-fuel arrangement, where the generator runs on up to 70% natural gas with 30% diesel, in order to extend the runtime, though this is an expensive option that still leaves the site without power once diesel runs out. But if you just want a box that makes cheap, reliable electricity when the power is out, let us help you choose which generator is best for your application.