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Generator Sizing Calculation Guide

Revised: 8/26/19
Grainger Editorial Staff

It's important to understand generator sizing in order to account for the load it will need to handle. Electric motors are particularly difficult for a generator because starting an electric motor requires 2 to 3* times its nameplate amperage or wattage.

A current surge of short duration can be supplied by a generator, but a current demand of longer duration, such as a heavily loaded motor starting a high inertia system can overload a generator, possibly damaging both the generator and motor.

For this reason, when determining the power your generator is to provide, it is important to calculate electric motor requirements at 3* times the running watts to compensate for the surge needed to start the motor. When sizing by KVA use the following calculation to determine watts: KW= KVA x Power Factor. With this in mind, the following guidelines can be helpful in selecting the right size generator for your application.

How do you determine what size generator you need?

Total the wattages of all small appliances, tools, and light bulbs to be operated at the same time. Most appliances have labels showing wattage (if volts and amps are given: volts x amps = wattage (VA) times power factor).

For example you may wish to operate the following equipment:

Electric heater   1000W
Eight 100w light bulbs   800
TOTAL   1800W

The next item to look at for generator sizing is the volt-amperes (wattage) requirement of electric motor to be operated (e.g., furnace blower motor 1/3 HP), remembering that the starting requires 2 to 3* times the nameplate or running (rated) watts. Thus, if running watts of the motor is 600, multiply the number by 3 to determine approximate max. VA needed.

Total watts and VA in Steps 1 and 2 to get total requirements:

Running = 1800W
Starting = 600 x 3 = 1800 Max. VA

To allow for anticipated future needs or use of extra equipment, add 25% to total in Step 3 when considering generator sizing.

Step 1   Running
Step 2   +600   +1800
Step 3   2400W   3600VA
+25%   +600   +900
Step 4   3000W   4500VA

See performance data charts for each generator listing for the unit that meets your total load criteria.

In summary: For example, a generator that provides at least 4500 max. VA and 4000 watts meets the load requirement for the example above. A 3000-watt generator should not be selected because it does not have enough max. VA to start the electric motor load.

Note: The starting (max.) VA can also be determined by referring to the motor code listing that indicates starting KVA per horsepower (see chart below).

(*) Some motors can require as much as 5 to 6 times their full load amps at start. Check the motor code listing to be sure.

Motor Code Listing
Code KVA Per Horsepower Code KVA Per Horsepower Code KVA Per Horsepower
A 0 to 3.15 H 6.3 to 7.1 R 14.0 to 16.0
B 3.15 to 3.55 J 7.1 to 8.0 S 16.0 to 18.0
C 3.55 to 4.0 K 8.0 to 9.0 T 18.0 to 20.0
D 4.0 to 4.5 L 9.0 to 10.0 U 20.0 to 22.4
E 4.5 to 5.0 M 10.0 to 11.2 V 22.4 & up
F 5.0 to 5.6 N 11.2 to 12.5    
G 5.6 to 6.3 P 12.5 to 14.0    
Example Electric Tool and Appliance Wattages
Equipment Running Watts Maximum VA (Surge Watts)
Light bulb (100W) 100 100
Radio 150 150
1/3 HP Furnace w/Blower 600 1800
1/3 HP Sump Pump 700 2100
Refrigerator/Freezer 800 2400
6" Circular Saw 800 2400
1/2" Drill 1000 3000
Water heater (storage-type) 5000 5000

Note: This chart is provided to suggest typical values. Actual wattages may be significantly higher or lower depending on age, size make and condition of appliance.

Recommended Extension Cords for Use with Generators

Wire Gauge @ Cord Length
Ampere Rating 50 ft. 100 ft. 150 ft
2 18 18 18
3 18 18 18
4 16 16 16
5 16 16 16
6 16 16 14
8 16 14 12
10 16 14 12
12 14 14 12
14 14 12 10
16 12 12 10

It is important to use extension cords of adequate current carrying capacity when utilizing a generator to operate portable electric tools. 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.

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.


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