Determining Your Stand-by Power Requirements

Make a list of the appliances and motors that must run during an electrical power failure. It is suggested for stand-by service that you include only essential equipment.

When you have completed the list, enter the number of watts required to operate each item. The watts required for operation are usually stamped on the nameplate of the product. (If only the ampere rating appears on the product, multiply the amps by the voltage to obtain the wattage.) Also, 1000 watts equals 1KW.

Remember that electric motors require more power when starting than when running. Starting one motor at a time, beginning with the largest down to the smallest, will reduce the size of the alternator you need.

Some motors require more starting current than others, and all require more starting current than running current. Split-phase motors are more difficult to start and require 5 to 10 times as much current compared to running current, dependent on the NEMA code letter rating. Motors with capacitor start require 2 to 4 times as much current to start compared to running current. Repulsion Induction Motors require 1 ½ to 2 ½ times as much current to start compared to running current. Determine your largest motor starting requirement by the HP rating and the NEMA code letter on the nameplate. Refer to the table below for the approximate wattage per HP by NEMA code letter. For the total starting wattage required, multiply this figure by the HP size of you motor.

NEMA CODE A B C D E F G H J K
Wattage/HP 3100 3500 4000 4500 5000 5600 6300 7100 8000 900
NEMA CODE L M N P R S T U V
Wattage/HP 10000 11200 12500 14000 16000 18000 20000 22400 26000+
Approximate wattage consumption of running and starting electric motors.
Motor Approx. Running
Wattage
All Motors
Approx. Starting Wattages
Split Phase
Motors
Repulsion
Induction Motors
Capacitor
Start Motors
HP Full Load Average
Load
Full Load Average
Load
Full Load Average
Load
Full Load Average
Load
1/6 528 343 5,280 3,432 1,320 858 2,112 1,373
1/4 696 453 6,960 4,524 1,740 1,131 2,784 1,810
1/3 864 552 8,640 5,616 2,160 1,404 3,456 2,246
1/2 1,176 765 11,760 7,644 2,940 1,911 4,704 3,058
3/4 1,656 1,076 4,140 2,691 6,624 4,306
1 1,920 1,248 4,800 3,120 7,680 4,992
1 1/2 2,400 15,600 6,000 3,900 9,600 6,240
2 2,880 1,872 7,200 4,680 11,520 7,488
3 4,080 2,652 10,200 6,630 16,320 10,608
5 6,720 4,368 16,800 10,920 26,880 17,472
7 1/2 9,600 6,240 24,000 15,600 38,400 24,960
10 12,000 7,800 30,000 19,500 48,000 31,200
Total all requirements, and add approximately 25 percent for overlooked items and miscalculations. Select an alternator that is equal to or greater than the rated output requirement of your load. Never pick an alternator smaller than your load requirement, as damage can be done to the alternator by continuously overloading it.
Approx. Wattage Consumption
Sump Pump 400 Glow Heater 600-1000 Battery Charger 750
Refrigerator 400 Electric Fan 300 Milking Machine 1000 & up
Freezer 400-600 Kitchen Ventilator 150 Milk Cooler 1000 & up
Air Conditioner 600-2000 Radio 40-100 Milk Pump 800 & up
Toaster 1200 Television 200-600 Barn Cleaner 2000 & up
Skillet 1500 Washing Machine 400-600 Feed Conveyor 600 & up
Mixer, Portable 150 Dishwasher 300 Silage Unloader 3000 & up
Coffee Maker 1000 Sewing Machine 200-500 Chick Brooder 100 & up
Iron, Steam & Dry 500-1200 Vacuum Cleaner 400 & up 1/4" Electric Drill 250
Range, Electric 1 element 1500 Light Bulbs As Marked 1/2" Electric Drill 750
Water Heater, Electric 1000-5000 Electric Heat System 5000-20000 1" Electric Drill 1000
Clothes Dryer, Electric 4000 Hedge Trimmer 450 9" Disc Sander 1200
Furnace Oil Burner 300 Weed Eater 500 3" Belt Sander 1000
Furnace Motor 1/4 HP 800 Edge Trimmer 1100 Concrete Vibrator 2500
Furnace Motor 1/3 HP 1200 Lawn Mower 1200 Circular Saw 6½" 1000
Circulating Heater 250 & up Chainsaw 1200-2500 Impact, Electric 950
Above figures are approximate. Check nameplates and literature on you equipment for specific consumption and starting requirements.

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