Just like qualifying and selecting a standby power generator system contractor, the proper generator size selection is important. Spending thousands of dollars on a standby power generator system that won’t perform when called upon can be a very disappointing and expensive experience. Most local municipalities require that licensed electricians, plumbers and pipe fitters must be used to install, work on and/or maintain electrical, fuel and mechanical power generator systems. It is very important to check with your local building department for further information regarding these requirements.

Generator Size

What size generator do I need?

Emergency power generator systems are available in a variety of different sizes from 5KW-50KW and larger. Your choice will depend on the wattage requirements of the critical loads protecting your home or business.

Selecting the proper size unit for the electrical loads to be served is the most important step in providing a reliable standby power generator system.

Power generator under-sizing is one of the most fundamental and common mistakes made by those unfamiliar with sizing techniques and formulas. For some, the philosophy seems to be, the smaller the power generator, the smaller the associated equipment and installation costs will be, thereby reducing the overall project expense and guaranteeing the salesman a sale.

For example, when calculating the generator load wattage for an average home with a 100-200 ampere, 120/240 or a 120/208-volt service, you may have an application where one or both of the following two conditions might apply:

Condition #1 – No motor loads will be included:

In this case, the total nameplate wattage of the appliances and lights can be used to calculate the home generator size. To demonstrate, 10 100-watt light bulbs, a 1,200-watt coffee pot, a 1,250-watt toaster and a 1,500-watt hair dryer would require a 5KW power generator.

Condition #2 – Motor loads will be included:

When motor loads need to be included, the motor starting wattage must be taken into account so the generator can provide sufficient starting current. Generally, motors normally require five to six times the listed watts or full load current to start. A 25% factor is recommended to support electrical appliances, which are equipped with induction motors.

In this example, if the nameplate on your furnace (not air conditioner) indicates it is equipped with a 1/2 horsepower blower motor and the nameplate amperage is 9 amps at 115 volts, the running load of this motor is 1,035 watts. When the motor is started, the power generator will be required to provide 4,000 to 6,000 watts or 4-6KW, just for this single motor. See the appliance chart on our Common Appliances page.

Generator under-sizing is the reason many emergency power generator systems fail. It has been known to cause:

  • The inability to pick up individual loads as they come online
  • A complete system failure or shutdown as the loads are added
  • Unreliable performance
  • Premature and/or excessive maintenance problems
  • Shortened generator system life
  • Risk to personal safety or property

Selecting the wrong power generator is like a compact car trying to pull a semitrailer; it may move it but it will eventually fail. A standby power generator system should be considered as a long-term investment in your family’s safety and the security of your property.

Other Power Generator Requirements and Options

After determining the proper size generator, it will also be necessary to determine the proper component configuration and specification. You should be sure the electrical contractor is familiar with and identifies all expenses attributable to applicable electrical codes. These cost items could be very expensive to resolve if not done in the correct way.


Every standby power generator installation must be provided with a sufficient supply of fresh air by natural (outdoors) or mechanical means (fans). Outdoor locations need to be placed so that exhaust fumes from the generator do not enter habitable spaces.

Power Generator Fuel

A decision must be made about which type of generator fuel is best for you. For home generators and small commercial applications, natural gas is recommended. Other generator fuel factors to consider are: safety, adequate supply, fuel sourcing and cost.

Power Generator Installation

Romitti Electric, a licensed and insured electrical contractor with many years of experience, can either assist in making the above decisions or provide a turnkey project where you do nothing but have the power generator ready for the next power loss. The time spent working with power generator and appliance manufacturers have provided our specifiers and designers the skill to apply computer-aided diversity factors or generator revisions, which, at times, result in the selection of a smaller unit. Our practice does not allow the use of standard “small, medium or large” generator sizing techniques or “dart board” cost estimating frequently employed by others. Our design methods reflect actual job conditions, result in lower final expense and allow for potential cost reduction opportunities by “doing our homework.”