This is a new pop-up window on top of your GeneratorJoe browser window.  CLICK TO CLOSE WINDOW

Step by Step Basic Generator Information

Buying a Generator - Considerations

Now that you have decided to purchase a generator, there are several considerations you must keep in mind when choosing which unit to buy, where to install it and how to install it. This guide will help you select a generator using a step by step method. Choosing the right machine is not difficult if you take the time to analyze your requirements carefully.  GeneratorJoe is here to help, if you get confused or have a question, ask us.

You need to know a few terms and have a basic understanding of the different types of generator sets and their operating principles. We will explain in simple terms.

You should have your equipment installed by an expert. A knowledgeable person, who knows the electric code, can do wiring and simple plumbing can do the installation, but you will have to know what your doing. Installation may require expert assistance and must adhere to local codes and regulations, not only to obey the law but to make sure you don't void your insurance by installing equipment illegally or without permits. We recommend that you have a contractor do your installation or, at the very least, have him provide professional advice. It is up to you to make sure the installation is done correctly.

For a list of resources on this website and to skip to specific areas of interest see: Generator Information.

Most homeowners first think of portable generators rather then stationary generators. If you want to haul a generator outside or put one outside in a shed and plug in cords when there is a power outage, it can be done. You will not necessarily save any money doing it but if you have a use for the portable generator for non-emergency times then it might be an alternative. It has been our experience that it is more economical and less hassle to purchase a stationary system and power the whole house or business. Not only do you get more power for the dollar but your family and/or employees don't have to do anything to have emergency power. Do you want your wife, children or employees hauling out equipment and plugging things in and starting up the system, switching the transfer switch and having to refuel? At some point all of this becomes ridiculous and costs you more in staff time and potential liability then its worth.

Before you decide read: Generator Sizes and Types for Your Home or Business

The first determination you will need to make is whether you will require stand-by or prime power. Simply stated, prime power is required when you have no other source of power or you are using the system as you prime means of power. Any generator that is used everyday or on a fixed schedule to provide power is considered a prime power generator. Another word for prime is "continuous". If you need a prime power generator, use the generators prime or continuous rating as a guide.

A standby set is a backup to normal utility power. Standby units are used only when your utility power is not available and will not be used frequently. Many stand by generators run at 3600 RPM and are not designed for constant daily use. Another word for standby is "emergency". If you need a standby power generator, use the generators standby or emergency rating as a guide.

Generator sets produce either single or three phase power. You should use the type of power your panel provides. Residences and small business generally use single phase. Three-phase power is used for medium to large businesses especially where power is used for motor starting and running. Three phase generators are set up to produce 120/208 or 277/480 volts. Single-phase sets are 120 or 120/240. Use the low voltage to run domestic appliances and the high voltage for your motors, heaters, stoves and dryers.  Your service panel is either single phase or three phase, you don't need a three phase generator if your panel is only single phase. Check with your electrician before you begin your search.
Your service panel is a good place to start. Go and look at your service panel and see what the amperage is. If the panel is 100 Amps that tells you that your won't need more then 100 amps of power. As the panel gets bigger so will your generator needs. It is possible to install a generator to power only a small portion of your service panel, if you install appropriate sub-panels to sort out what will and will not be powered.
FUEL: GAS OR DIESEL? See also What Generator Fuel is Best?

We recommend diesels due to their longevity and lower operating costs. Today’s modern diesels are quiet and normally require much less maintenance than comparably sized gas (natural gas or propane) units. Fuel costs per kW produced with diesels is normally thirty to fifty percent less than gas units. 

1800 rpm water cooled diesel units operate on average 12,000 to 30,000 hours before major maintenance is required. 

1800 rpm water cooled gas units normally operate 6,000 to 10,000 hours because they are built on a lighter duty gasoline engine block. Gas units burn hotter (higher BTU of the fuel) so you will see generally see somewhat shorter lives than the diesel units

3600 rpm air-cooled gas units are normally replaced – not overhauled at 500 to 1500 hours. These are "stand-by" generators, not intended to be run long hours or very often.


Electric equipment is designed to use power with a fixed frequency: 60 Hertz (Hz) in the United States and Canada, 50 Hertz in Europe and Australia. The frequency output of a generator depends on a fixed engine speed. To produce 60 Hz electricity, most engines operate at 1800 or 3600 RPM. Each has its advantages and drawbacks. 1800 RPM, four pole sets are the most common and least expensive in large generators. They offer the best balance of noise, efficiency, cost and engine life. 3600 RPM, two pole sets are smaller and lightweight, best suited for portable, light-duty applications. 3600 RPM sets are considered "Standby Generators" and can never be considered for prime power use. 

In simple terms it’s like operating your car at 90 mph, versus 45mph – at 45mph your car will last longer, is quieter, less maintenance and longer life.  Most 3600 rpm units are twin cylinder air cooled lawn mower engines, while the water cooled 1800 rpm units are comparable to those found in forklift and tractor engines. The bottom line is the 1800 rpm water cooled units will last longer, offer less maintenance problems and be more fuel efficient. In addition, 1800 RPM generators are designed to be rebuilt, 3600 RPM units are designed to replaced and are much lower in cost (most of the time). Some 3600 RPM stationary units and most RV and commercial power units can be rebuilt at least one or more times but this process is not inexpensive.

Engine block. For long life and quiet operation we recommend four cycle, liquid cooled, industrial duty diesel engines.
Air or liquid cooling. Air-cooled engines require a tremendous amount of air and may require ducting and they are somewhat noisier. Liquid cooling offers quieter operation, more even temperature control and therefore longer engine life. Modern air cooled engines are suitable for many applications, especially short run, portable or standby uses.
Intake Air. All quality generators have intake air filters with replaceable filter elements. Today even small portables have replaceable air cleaners.
Mufflers. Most generator come equipped with an industrial grade muffler. One good investment is a residential or critical muffler that is much quieter and lasts longer. All enclosed generators should be equipped with at least a residential and preferably a critical muffler.
Lubrication. The lubrication system should have a full flow, spin-on oil filter. Larger generators should have a filter bypass. Most generators today have low oil alarms and shutdowns, make sure the generator you select has this valuable feature, its simply a must have protection.
Major brand of engine. We do not know why people would even consider an inferior "scrap metal" unit or "off brand" engine, you will not be able to obtain the necessary parts, service and support. Many engines come with a box of spare parts including pistons, rings and bearings because your going to need them all. Save yourself some grief and buy a major brand of engine. If you buy a junk engine, we wont service it and most other reputable dealers won't either.
Electrical system and circuit breakers. Standard 12 volt system should include at least the following: 1) Quality starter motor and battery. Larger generators should include a charging alternator with a solid state voltage regulator.  2) Larger diesel units should come with a pre-heat switch and all generators should have a start/stop switch. 3) Al generators should have a safety shutdown system to protect the engine in case of oil pressure loss, generator over-speed or over-crank and high water (or operating) temperature. 4) System circuit breaker to protect the generator. On small systems and portables there should be a circuit breaker on each circuit.
(The part that makes the generator "generate" electricity.)
AC generator should have a 4-pole revolving field. An automatic voltage regulator will provide “clean” power.  Normal utility power is +/- 6% voltage regulation; most generators are even better ranging from +/- 5% or .25 % and even better.  Most modern generators offer AVR – Automatic Voltage Regulation or some other proprietary brand of voltage regulation and can be safely used with modern electronics and computers.
Lifetime lubricated bearing. Cheap generators are not supplied with these bearings. They often require complete disassembly every two or three years for bearing replacement.  Most modern power alternators, or generator ends, are provided with industrial quality lifetime lubricated bearings
Upon determining the generator size you will need, make a list of optional and installation equipment you require. For noise abatement, we recommend a residential (not industrial grade) muffler. A good primary fuel filter/water separator is a must to protect your engine's fuel system. Stand-by sets may require a block heater to keep the coolant/water mix at an adequate temperature for easier starting and less smoking on startup.

Sizing is the most important step; nothing is more critical in your choice of a generator. A set that is too small won't last, will smoke and can do damage to your electrical equipment. If it is too large, the engine will carbon up, wet stack or “slobber” and this means excess fuel consumption and early failure.  We recommend that a generator set never run continuously with less than 40% load - 50% to 75% is optimum.

Additional factors, which may affect efficient operation of your generator, are high altitude and high air temperature. These conditions will lower generator output. You must take into account your elevation, normal and extremes of temperature and other factors. Ask your sales engineer for de-rating information.  Allow three (3) percent loss in efficiency for each 1000 feet above sea level minimum. Check the manufactures specifications and use the de-rating factor they specify. Nothing is worse than buying a generator that is too small.

For more information: Understanding Loads & SizingCalculations, Doing it Exactly, Generator Sizing Procedures, Wattage Guide

In addition to load requirements, it is important to consider motor starting load. We use the rule of thumb that starting a motor requires up to three (3) times more wattage than running loads. Selecting a generator, which is inadequate for your motor starting needs, may make it difficult to start motors in air conditioners, compressors or freezers. In addition, the starting load causes voltage dips, which is why the lights dim when a large motor is started. These voltage dips can be more than annoying – voltage dips can ruin delicate electronic equipment such as computers. You must make sure you account for starting loads, if you cant start the load you can't run it. For more information see: Starting Load Examples, Electric Motor Wattage Guide, Motor Formulas.
Read these sections about generators, you may find some useful information. Delta & Wye Connections, Electrical Unit Conversions (formulas), Proper Generator Operation and Generator Safety.


A detailed installation guide will usually be provided with your generator. Here are some important points to consider when you install your generator.
We strongly recommend that the installation be performed by a licensed electrical or mechanical contractor. They have the tools, the know-how and an understanding of regulations and local codes. Their expertise will save you money in the long run. Should you chose to perform the installation yourself, PLEASE do your homework before tackling the job and obtain the proper permits required by your local jurisdiction. While all GenSets have some basic requirements, each brand and model has unique installation requirements. Also, it is extremely important to have all relative codebooks for reference and to adhere to strictly adhere to the laws which were designed for your safety. Most important of all, your system must be inspected before starting to prevent fires and explosions from improper installation.

Ensure the following items are considered, read the manual for the generator.

  • Air inlet for combustion and engine cooling.
  • Outlets for exhaust and hot cooling air.
  • Fuel, battery and AC electrical connections.
  • Remember to monitor for carbon monoxide!
  • Rigid, level mounting platforms (many sets are already mounted on a steel skid base).
  • Open accessibility for easy service.
  • Isolation from living space. Keep noise and exhaust away from occupied areas.
  • Space and equipment to extinguish a fire. Minimize the possibility of fire danger.
  • Remember, GenSets move on their vibration mounts. Allow clearance to compensate and use flex-joints on all lines and connections.
The exhaust system may need to be covered with insulated material to prevent fire resulting from contact with combustible materials.  We recommend a heat blanket over exhaust outlets to reduce the heat radiated from the exhaust and to ensure personal safety. Some insulation materials are best left to professionals with the proper equipment. Keep all piping away from combustible materials including walls. A seamless, stainless steel flexible joint must be used between the generator set and the exhaust system to prevent metal fatigue. Do not use the exhaust manifold to support the exhaust system because the weight will cause manifold failure. Exhaust pipe hangers are readily available and inexpensive.
Extreme care should be taken in designing and installing the fuel system to prevent fire danger. Fuel lines should have as few connections as possible and be routed to prevent damage. Keep lines away from hot engine or exhaust components. The lines should be no smaller than the inlet and outlet on the engine. Support fuel lines with clamps, as needed, to help prevent metal fatigue from vibration. The fuel tank should be level with or below the set to prevent siphoning in the event of a line failure. Remember to check the lift capacity of the engine fuel pump and stay within its limits. If the set is higher than the tank, an auxiliary fuel pump may be required.

To prevent water ingestion, fuel should be drawn out of the top of the tank with the pick-up extending to no more than two inches from the bottom. Fuel storage tanks must have leakage protection and many jurisdictions require spill basins. Above ground tanks are recommended and cheaper but you must check your local codes before installing a tank. The safest tanks are double walled with alarms. These alarms are simple and well worth the investment to avoid a possible fuel spill, and significant clean-up costs. If the tank is mounted above the generator set, use a fuel shut-off valve, so that you can work on the fuel system without the fuel-siphoning out. It also allows you to cut-off fuel flow in the event of line breakage.

A high quality, fuel/water separator filter should be mounted as close to the generator set as possible. Because of its explosive nature, gasoline fuel systems have special requirements; see your tank supplier for complete information.


The generator set requires air for combustion and cooling.  A radiator and a “pusher” engine fan cool the generator engine temperature.  Your car or truck will normally operate with a “puller” fan.   An internal fan cools the generator.

GenSets that are housed in weather-protective enclosures are designed for installation out-of-doors. Typically a cement pad is placed in a suitable location, out of sight but with easy access for maintenance and fueling.  The generator is secured to the pad.  Choose a site close to the electric service and fuel supply lines (natural gas, propane, or diesel). The image below shows a typical natural gas installation. The main distribution panel, transfer switch and sub-panels are inside the building in this example, but more often the distribution panel, sub-panels and transfer switch are outside. You should make sure that 110v power is available at the generator for battery charging.

The GenSet must be a minimum of 3 ft from combustible material (NFPA 37).  Leave at least 3 ft (or more if the housing and instructions for your particular unit) all around the GenSet enclosure for access to the inside (NEC Art. 110-26a, Art. 110-26b).  The GenSet must be at least 5 ft from any opening (window, door, vent, etc.) in a wall, and the exhaust must not be able to accumulate in any occupied area.  See figure below.

We do not recommend locating generator sets indoors in residential applications and small commercial and industrial applications. The primary reason to avoid indoor installation is safety. Carbon monoxide gas is odorless, colorless and can accumulate in closed spaces. You can walk into a room full of carbon monoxide and be overcome. A gas leak in space attached to your home can kill you and your family.

In addition to safety, it costs more to install a GenSet indoors than for the same GenSet provided with the factory weatherproof housing. When a GenSet is installed indoors, the building must be designed carefully to handle ventilation to remove heat and any fumes due to fuel, exhaust, lubrication and starting batteries.  The radiator must be provided with a duct adapter that interfaces properly with louvers on an outside wall of the building.  Adequate air intake flow must be available not only for the radiator fan but also for cooling the alternator.  The engine exhaust piping and muffler must be tight to prevent any leaks that would allow dangerous carbon monoxide to accumulate inside the building.

Generally, the room or space in which the generator operates, should not exceed 100 F. We recommend keeping it under 85 F if possible. Generator installations require an intake of cool, clean air and an outlet vent for hot air. Whenever possible the cool air should be drawn over the alternator (or generator end) to help keep the alternator cool. The size of the space affects the room temperature (the smaller the space the generator runs in, the higher the room temperature is likely to be), smaller spaces may require ducting. The size of the generator and the outside air temperature or climate must be considered. In an indoor installation, increasing these vent sizes may cool the room down to acceptable levels and ensure “positive” airflow. Positive airflow is cool, clean air in and hot air out, as opposed to circulating hot air inside the room. Generator cooling fans move moisture as well as air. Moist air is corrosive to a GenSets copper windings, so make sure air inlets are positioned to minimize moisture intake.

Automatic fire suppression systems may also be required.  Check with your local fire codes.  You should also contact your fire insurance provider to determine if an indoor GenSet is even permitted.

Installation engineered to meet all the above requirements and all safety codes can still become a hazard later.  For an installation to remain safe, it has to be regularly inspected and maintained to ensure that leaks or other dangerous conditions do not develop with age or use.  Sites that do not have a knowledgeable maintenance staff trained to support an indoor generator set should not install a unit inside a building.

Another factor is the initial cost. It is impossible to construct a building to house a GenSet at a cost as low as the factory housing that can be ordered with the GenSet.  And even if the building already exists, the design expense and costs to adapt it for a generator set installation usually will exceed the enclosure cost available from the GenSet manufacturer.  For a small GenSet the cost of an open unit with duct adaptor and exhaust pipe kit is as little as $600 less than the same GenSet with the factory weather housing.  The additional costs for just the exhaust thimble and louvers exceed that savings.

Please read Policy, Warranties and Disclaimers. You as the buyer and user of generators sold by GeneratorJoe assume all risk and liability in connection with all equipment purchased.

Connecting the generator to your electrical distribution system is a job for a qualified, licensed and bonded electrician who is familiar with local building codes. Electricity is dangerous, respect it.

All generator systems require a circuit breaker and a distribution panel.  The circuit breaker protects the generator set from short circuit and unbalanced electrical loads. The distribution panel divides and routes the connected loads and includes circuit breakers to protect these loads. Stand-by systems also require a main circuit breaker between the utility source and the transfer panel. The transfer panel switches power from the utility to the GenSet and back so that both aren't on at the same time. Residential, commercial and industrial generators are equipped with auto-start to allow connection to auto-transfer switches.

Unless you have no other electric service (utility power), you must install a transfer switch. Normally the transfer switch must be the same size as the service panel or a sub-panel off the service pane. The transfer panel switches power from the utility to the GenSet and back so that both aren't on at the same time. Auto-start, auto-transfer systems are available and are relatively inexpensive. We can help you determine what you need. For more information about transfer switches and how they work see: Small Transfer Switch Information and for large switches see: Transfer Switches. . GeneratorJoe can help you determine what you will need, call us.

We hope this information has been helpful. If you have additional questions, contact your GeneratorJoe sales representative.

 Copyright © GeneratorJoe Inc. & GeneratorJoe®. All Rights Reserved.

This is a new pop-up window on top of your GeneratorJoe browser window.  CLICK TO CLOSE WINDOW