South FL & New England
Welcome to
? Contact | Home

Generator FAQs


Skip to:  


What size generator do I need?

This depends on the number and size of loads you want to run at the same time. If a house uses natural gas (or propane) for heat and hot water, a 4,000 to 6,000 watt generator can maintain a high level of comfort (assuming natural gas or propane is available). A house using an electric furnace and electric water heater may need a generator in the 20,000 to 35,000 watt range.

Generator size can be estimated by determining the major loads to be supplied and allowing some overhead for convenience appliances (television, computer, etc.). Some of the most common major loads are listed below (these are average numbers and can vary):

Furnace blower motor (propane/natural gas heat)

1,500 - 2,000 watts


1,500 - 2,000 watts

Well pump (1 horsepower)

3,500 - 5,000 watts

Water heater (electric)

4,000 - 5,000 watts

Electric furnace

11,000 - ???

Heat pump

10,000 - ???

By manually switching loads off and on, the owner can ensure that only one major load is running at a time. This can allow a smaller generator to be used.

(Back to top)

What is a manual transfer switch?

A manual transfer switch is a panel wired into the house electrical distribution system that allows a generator to be used to supply power. It prevents the generator from back feeding the utility and reduces the potential for lineman injury.

(Back to top)

How do I connect a generator to my house?

The simplest and lowest cost method is to run extension cords from the generator to the loads to be supplied. If the generator is to be connected to household wiring, an approved method of isolating the house electrical system from utility must be used. A manual or automatic transfer switch can be wired into the distribution system to allow the use of existing household wiring and receptacles. A three position, whole service disconnect (knife switch) can also be used to isolate the house electrical system from the utility.

(Back to top)

Is a manual transfer switch necessary?

No. However, if the generator is to be connected to the house electrical system, an approved means of isolating the house from utility power is required. This is to prevent the generator from back feeding the utility and injuring the line workers that are trying to restore power. Significant damage can result from the generator being on line when utility power is restored.

(Back to top)

How much does it cost to install a manual transfer switch?

This can vary depending on many factors. Typical installations can start at $800 to $1500 for a surface mounted switch located near the main distribution panel. Most transfer switches come with clear documentation and can be installed by the owner in about two hours for the cost of a permit. Some factors affecting cost include:

  • Distance from the contractor: Travel time and mileage costs money. The farther you are from the contractor, the higher the cost.
  • Type of transfer switch: Transfer switches come in a variety of makes, models, and sizes. They can be surface mounted, flush mounted, or designed for outdoor use.
  • Type of installation: Surface mounted switches take less time to install than flush mounted switches, resulting in a lower cost of installation.
  • Distance from the main panel: Most manual transfer switches are designed to be installed within two feet of the main panel. Additional materials and labor would be necessary to install the switch away from the main panel and would raise the installation cost.
  • Remote generator receptacles: Most manual transfer switches have a receptacle that the generator plugs in to. In some cases, it is more convenient to have the receptacle located closer to where the generator will be located when it is running.
  • Load identification: In most cases, the manual transfer switch will not be supplying all of the circuits on the main panel. The more time the electrical contractor spends identifying the circuits to be connected, the higher the installation cost.

(Back to top)

Can I open my main disconnect breakers and connect my generator to a receptacle?

No. In concept this may seem like a good idea, but is not advisable for the following reasons:

  • The national electric code and most city, state, and county ordinances require approved means of isolating generator power from utility power. These requirements are designed to protect the lives of both you and the utility workers, and to protect your equipment.
  • The main disconnect breakers are not designed to isolate power from two sources. Typically these breakers are rated for 240 volts. If the generator and utility are out of phase, the voltage differences between these sources can reach 480 volts.
  • The main disconnect breakers can fail without visible indication. The breaker may feel like it is open but may be electrically connected internally due to a spring or other failure. This can go unnoticed until utility power returns.
  • When both sources of power are available, it is possible to connect both sources together. Accidentally operating the wrong breaker can have severe consequences. Approved means of isolation usually requires three position switches, or breakers, that prevent connecting both sources of power.

(Back to top)

Should I notify my utility company that I have a generator?

Yes. Most utilities maintain this type of information and make it available to the line workers that will be working in the area. Power restoration can be delayed while line workers check to see if a generator is being properly used. Having this information beforehand can reduce these delays.

(Back to top)

What type of fuel is best?

This depends on many factors - shelf life, cost, storage location, availability, etc. See What are the advantages and disadvantages of the different fuel types? for more information on the different fuels.

(Back to top)

What are the advantages and disadvantages of the different fuel types?



  • Long shelf life
  • Clean burning
  • Easily stored in both large tanks or in smaller 5 - 10 gallon cylinders
  • Obtainable during power outages - gas stations may be unable to pump fuel during an area wide outage
  • Home delivery available for larger tanks
  • Pressurized cylinder of flammable gas
  • Fuel system is more complicated (increased possibility of failure)
  • Larger tanks are not aesthetically pleasing (unsightly)
  • Fuel system plumbing results in higher installation cost



  • Unlimited fuel source - refueling not necessary
  • Clean burning
  • Available during power outages
  • May be unavailable during natural disasters (earthquakes, etc.)
  • Lower power output (30% less BTU's per unit than gasoline)
  • Fuel system plumbing results in higher installation cost
  • Not available in many areas



  • Least flammable fuel source
  • Easily obtained
  • On site fuel delivery available
  • 18-24 month shelf life
  • Installing large storage tanks raises cost of system
  • May not be available during power outages



  • Common fuel source - easily obtained
  • Increases portability of smaller generators
  • Highly flammable
  • Short shelf life (approximately 12 months)
  • Storing large quantities is hazardous
  • May not be available during power outages

(Back to top)

Can a propane-converted portable be run on gasoline?

No. Converting the Onan portable generators to propane is a one way conversion. The carburetor float and choke assembly are removed, the mixing chamber modified, and the gasoline supply line removed during the conversion. The fuel tank is left on the unit to maintain its appearance and to provide some weather protection during outdoor use.

(Back to top)

What size propane tank do I need to run my portable generator?

That depends on two factors. Larger tanks offer longer run times between refueling, and on site delivery of fuel may be available for tanks larger than 100 gallons. Small tanks like those used on barbecue grills allow several hours of operation and are easy to transport for filling at fuel stations.

(Back to top)

Can a natural gas converted portable generator be run on propane?

Yes, with some minor modifications. The natural gas quick disconnect fitting on the hose will need to be removed and replaced with a propane pressure regulator and tank fitting. The air-fuel mixing valve will need to be adjusted for proper operation (or backfiring will result). Engine speed will need to be adjusted to provide proper output frequency.

(Back to top)

Is a kit available to convert my portable generator to propane?

No. Onan does not produce a propane or natural gas generator kit. Instead, the generators are converted at a Cummins, Northwest facility. This is a program researched and developed by Cummins Northwest. The original warranty will still be honored, and failures resulting from the conversion will be backed by Cummins Northwest as if they were covered by the original warranty. Controlling the quality of the conversions allows us to ensure that all conversions are performed the same way to simplify future repairs and reduce the possibility of failure.

(Back to top)

How long will a generator run on a tank of fuel?

This depends on the size of the tank and amount of load on the generator. Higher loads require more fuel. A 6000E portable will run for 9 hours at rated load. A 5000E portable will run for 11 hours at rated load, and a 4000E will run for 13 hours at rated load. Many users average 10 to 15 hours on a 6000E under normal usage. A conservative rule of thumb is one gallon of fuel per hour for a 5,000 watt generator.

Propane fueled portables use a slightly larger amount of fuel than their gasoline counterparts.

(Back to top)

How long can the generator be continuously operated?

Unfortunately there is no set answer for this question. The Onan portables are designed for long life, heavy duty operation, and long run times. Gasoline units should be shut down during refueling to reduce the potential for starting a fire by spilling gasoline on hot exhaust components. The oil level should be checked and the unit inspected for signs of fatigue or abnormalities (cracked receptacles, etc.) before restarting.

One of the advantages of natural gas and large propane tanks is the longer run time between refueling. However, the generator should still be periodically shut down, the oil level checked, and the unit physically inspected.

The Onan portables have low oil pressure protection, but frequently checking the oil level will help spot excessive oil consumption problems early.

(Back to top)

How often should I shut the generator down?

The answer to this question is similar to How long will a generator run on a tank of fuel?. The generator should be shut down and inspected daily or when refueling. If long periods of no load or light load operation are expected (such as during the night when everyone is sleeping), the generator could be shut down to extend the useful life of the generator and reduce fuel costs.

(Back to top)

Does Onan offer a diesel fueled portable generator?

No. Some of the most important features of a portable generator are size and weight. Diesel engines typically weigh more and are physically larger than their gasoline counterparts. Another important factor is cost. Small air cooled gasoline engines are used in many more applications than small air cooled diesel engines. This helps to reduce the cost of the engine, lowering the cost of the portable generator.

(Back to top)

Does Onan have portable generators larger than 6,000 watts?

No. A 10,000 watt generator requires a larger generator and engine than a 6,000 watt generator. In order to keep the size and weight of a 10,000 watt generator at a manageable level, it is necessary to use the smallest engine and lightest components. This can reduce the useable lifetime and reliability of the product.

Unlike other portable generator manufacturers, Onan produces generators from 2,000 watts to several megawatts. If a customer is looking for that level of power output, it may be beneficial to consider switching to a full fledged standby unit. If cost is a driving factor, carefully reviewing the loads to be supplied may show that a 6,000 watt unit is capable of doing the job with some basic load management.

(Back to top)

Why are Onan generators more expensive?

The Onan portables are designed for heavy duty commercial use, where long life and reliability are very important. To meet these objectives, it is necessary to use only the best components, which does raise the cost of the product. This also allows Onan to provide the longest warranty available for portable generators (3 years) and the option to purchase two years of additional warranty at a reasonable retail cost of $75.

(Back to top)

How loud are the portable generators?

Approximately 79 dBA at 7 meters at rated load. Average noise levels are usually in the 75 dBA range.

(Back to top)

Why are the Honda generators quieter?

Most portable generators use an air cooled engine operating at 3600 RPM with little or no noise attenuation features. Engines of the same horsepower will produce roughly the same noise level. Onan portable generators use a larger engine for a given wattage than most other generator manufacturers. This increases the life of the product, but does produce slightly more noise. Normally this noise difference is negligible, and the increased product life is worth the trade-off.

There are portable generators with sound attenuation features that are quieter than their non-attenuated counterparts. These units are usually more expensive and may trade off a lower noise level for other important features.

(Back to top)

Is it possible to make the Onan portable generator quieter?

Installing panels around the generator to reduce noise levels is not advisable. The engine is air cooled, and restricting the access of cool air to the engine will reduce the life of the engine. Placing a barrier between you and the noise can deflect a majority of the high frequency noise and reduce noise levels. A simple sand filled cinder block wall the height of the generator can significantly reduce noise levels (taking care not to reduce cooling airflow).

(Back to top)

Does Onan offer a quieter muffler for its portable generators?

No. The addition of extra exhaust components does not significantly reduce the overall noise level of the unit.

(Back to top)

Is it okay to add an additional muffler to my portable generator?

Possible? Yes. Recommended? No.
Adding additional components to the exhaust system can increase backpressure and does not significantly reduce noise levels.

(Back to top)

Is it okay to extend the exhaust pipe on my portable generator?

Yes. Flexible exhaust extensions are available in many auto parts stores. Care should be taken to limit the length of the extension to minimize exhaust back pressure.

(Back to top)

Is it possible to run my portable generator in the garage?

This is not recommended. Portable generators are designed for outdoor use. Running them indoors presents the following problems:

  • Increased risk of carbon monoxide being admitted to living spaces.
  • Potential for fire. Garages can have combustible material near the generator than can catch fire when in contact with hot exhaust components.
  • Potential equipment damage. Indoor operation of the generator may restrict cooling airflow to the engine.

(Back to top)

Is it okay to build an enclosure around my portable generator?

Yes, as long as the generator is taken out of the enclosure when operating it. Access to cooling air is vital to the proper operation of the engine. Portable generators do not have large cooling fans and are unable to circulate air in confined spaces. They rely on the natural circulation of heat resulting when the hot air rises away from the engine and is replaced by cool air.

(Back to top)

How does the Onan portable generator compare to the lower cost generators in the home improvement stores?

Onan portable generators are designed for continuous heavy duty commercial use where long life and reliable operation is vital. This is accomplished by using proven manufacturing techniques and top quality components. Many corners could be cut to reduce the cost of the unit, but there would be a tradeoff in the life of the product. The Onan product is backed by the largest service and parts network in the area. Other manufactures may require the product be sent back to them for repair which can take several weeks to perform.

(Back to top)

Why is there such a price increase from the portable generators to the home standby generators?

The larger Onan home standby generators use liquid cooled engines that require belts, hoses, pumps, thermostats, etc. They are mounted on steel frames and come with weather protective enclosures. They have a more complex control system and have two-wire auto start capability. The warranty for the home standby generators covers travel time and mileage.

(Back to top)

How large a pump can a portable generator start?

When starting loads on a portable generator, the larger loads should be started first. A 4,000 watt portable can easily start a one horsepower pump. The 5,000 watt and 6,000 watt generators are capable of starting up to a three horsepower pump.

(Back to top)

Can a portable generator be used with an automatic start switch?

No. The Onan portable generators use a manual, three-wire, start/stop system with no provision for automatic starter disconnect.

(Back to top)




More Questions?

call: (413) 218-2015    



(Back to top)


::: Created by : Web Design Software & Website Hosting :::