Home / Equipment

Generator Sizing Calculation Guide


Choosing the right size of generator means you need to account not only for the power required to run your lights, appliances and motors, but also the power needed to start those items

For this reason, when determining the power your generator is to provide, you need to determine both the running wattage and the surge wattage. The running wattage represents the power needed to keep items on; the surge wattage accounts for the power needed to start any electric motors. Surge wattage varies among applications, but to account for most residential situations, multiply the running wattage by three to determine surge wattage. Some motors can require five to six times their running wattage to start, however, so check the motor code listing to be sure.

How to Determine Generator Size


Find the total running wattage of all the equipment, lights and appliances that the generator must power. Most appliances have a label showing wattage, such as a 60-watt light bulb or a 700-watt microwave.


Now find the power requirement of any electric motors that generator will run. That will help determine how much surge wattage is needed to start the motor. Using the standard multiplier of three times the running wattage, a 600-watt motor will require 1,800 watts to start.


To determine the surge wattage rating you need, add the highest surge wattage requirement to the total running wattage.

The chart below shows an example for powering eight 60-watt light bulbs, a small furnace and a refrigerator/freezer. These are only examples and the actual wattages may be higher or lower depending on the make and condition of the appliances.

Item Running Wattage Surge Multiplier Surge Wattage

8 60 W light bulbs


1/3 HP furnace w/blower








Total running wattage


+ Highest surge wattage


= Total surge wattage


In this case, look for a generator with a rated wattage of at least 1,880W and a surge wattage of at least 4,280W. See performance data charts for each generator listing for the unit that meets your total load criteria.

Recommended Generator Cords

It is important to use generator cords of adequate current carrying capacity when utilizing a generator to operate portable electric tools. A good rule of thumb is to always use at least a 14-gauge extension cord. As longer cords are needed the thickness should increase, with lower gauge numbers representing more thickness. Undersized cords result in excessive voltage drops and additional generator loading. This also causes excessive heating of the portable tool because voltage drop reduces tool capacity and increases amp draw. It's also important that the cord is long enough to distance the generator from any occupied structures.

Find the Right Generators

Portable & Inverter Generators

Portable & Inverter Generators

Standby Generators

Standby Generators

Generator & Transfer Switches

Generator & Transfer Switches

Generator Accessories

Generator Accessories

The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.