What Size Generator Do I Need?


Before choosing a generator, you will need to decide what you will be using it for, and what types of devices you are going to be running with the generator.

Step 1.   List all of the devices you plan on using with the generator. 

Step 2.   Identify the wattage requirements for the tools and appliances that you want to power. The power requirement for the tool or appliance can be found on its identification plate or in the Owner’s Manual.  If the power requirement is given in amps, multiply the amps times volts to derive the required watts.  Amps x Volts = Watts

Step 3.   Add up the required watts of all the tools and appliances you expect to operate simultaneously. The total watts calculated in step 3 is the size generator you need. These three simple steps will “size” a generator.

The Additional Guidelines section explains the procedures to calculate and size for motor starting.

This chart lets you immediately add up all of the appliances you will most likely utilize:


GENERATOR WORKSHEET
 
RUNNING
WATTAGE
REQUIREMENTS
ADDITIONAL STARTING 
WATTAGE 
REQUIREMENTS
 TOTALS
HEATING/COOLING
 
 
 
Furnace Fan, gas or fuel oil 
furnace
 
 
 
1/8 horsepower
300
500
 
1/6 horsepower
500
750
 
¼ horsepower
600
1000
 
2/5 horsepower
700
1400
 
3/5 horsepower
875
2350
 
Central Air Conditioner
 
 
 
10,000 BTU
1500
2200
 
20,000 BTU
2500
3300
 
24,000 BTU
3800
4950
 
32,000 BTU
5000
6500
 
40,000 BTU
6000
6700
 
HEATING/COOLING
 
SUB-TOTAL:
 
KITCHEN
   
Refrigerator, Average
600
2200
 
Dish Washer – Cool Dry
700
1400
 
Dish Washer – Hot Dry
1450
1400
 
Clothes Dryer – Gas
700
1800
 
Clothes Dryer – Electric
5750
1800
 
Microwave Oven, 750W
750
800
 
Washing Machine
750
2300
 
Coffee Maker
850
0
 
Toaster – 2 Slice
1100
0
 
Toaster – 4 Slice
1650
0
 
Electric Skillet
1500
0
 
Electric Range 6-in element
1500
0
 
Electric Range 8-in element
2100
0
 
Freezer
2500
2200
 
KITCHEN
 
SUB-TOTAL:
 
BATHROOM
 
 
 
Hair Dryer
800-1700
0
 
Iron
1200
0
 
BATHROOM
 
SUB-TOTAL:
 
APPLIANCES
 
 
 
Lights-Wattage
Actual:
0
 
VCR
50
0
 
Heating Pad
65
0
 
Radio
100
0
 
Television – Black & White
100
0
 
Television – Color
300
0
 
Dehumidifier
400
0
 
Electric Blanket
400
0
 
Garage Door Opener – 1/4hp
550
1100
 
Garage Door Opener – 1/3hp
725
1400
 
Well Pump – 1/3 hp
750
1400
 
Well Pump – 1/2 hp
1000
2100
 
Sump Pump – 1/3 hp
800
1300
 
Sump Pump – 1/2 hp
1050
2150
 
Vacuum Cleaner – Standard
800
0
 
Vacuum Cleaner – Deluxe
1100
0
 
APPLIANCES
 
SUB-TOTAL:
 
COMMERCIAL PRODUCTS
 
 
 
1/4″ Drill
300
300
 
Jigsaw
300
300
 
Electric Weed Trimmer
500
500
 
Router
1000
1000
 
Belt Sander
1000
1000
 
Disc Sander
1200
1200
 
Chain Saw
1200
1200
 
Worm Drive Saw
1560
3100
 
12″ Concrete Saw
1800
3600
 
7 1/4″ Circular Saw
1500
3000
 
Disc Grinder
2000
4000
 
Air Compressor, Average
2000
4000
 
COMMERCIAL PRODUCTS
 
SUB-TOTAL:
 
  
GRAND TOTAL:
 



ADDITIONAL GUIDELINES

CONVERTING AMPS OR HORSEPOWER INTO WATTS

If necessary, use these formulas to assist in sizing your generator:
 
Watts = Amps x Volts

Running Watts* = Horsepower x 932**  (for motors)



Remember, this worksheet lists average power requirements — a particular manufacturer’s device may use more or less than the listed wattage.  Add 10% if you are not sure.

If your customer plans to operate devices that use electric motors, list both the starting and running requirements of each.

Starting requirements of some devices maybe significantly higher than their running requirements. This higher demand must be  considered when estimating your power needs. Some small, universal motors — which do not draw a heavy starting load (drills, small  saws, blenders, etc.) — require very little extra current for starting.

When listing items that use motors, take them in the order of highest-to-lowest starting requirements, as shown in the example below.  Motor A, for instance, has a starting requirement of 2,600 watts, so it’s listed first, followed by Motor B at 1,300 watts, and Motor C at 1,000 watts.

MOTOR
/DEVICE
STARTING
WATTS
RUNNING
WATTS
Motor A2,600850
Motor B1,300600
Motor C1,000750




Once you have compiled an accurate list of what you will be operating, you can calculate the maximum power requirements. There are three different calculations you can make, depending upon the kinds of tools and appliances on the list, and their intended use:

  • No electric motors.
  • One motor running at a time.
  • More than one motor running at a time.


NO ELECTRIC MOTORS
If your list does not include any devices that use electric motors, simply add the power (running) requirements of all the items on your list to
obtain the maximum power needed.

  • For example, if you intend to use only an electric skillet, a 100-watt light and a heating pad (as shown below), the maximum power requirement would be 1,655 watts. In this case, a generator like the EG2500 or EN2500, that can produce 2,300 watts rated output is recommended.

 

DEVICE
WATTS
Electric Skillet
1,500
Light
100
Heating Pad
65
Total:
1,665

NOTE: The EU2000’s rated output is 1,600 watts, so its output would be too low to handle this load on a continuous basis.

Footnotes:
*Running Watts is the amount of power a motor consumes once it has started to run at normal speed.
**932 is the factor used to convert motor horsepower ratings to needed electrical energy. It takes into account normal losses in utilizing 
that power.