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What are the number of factors and considerations to be taken into account before sizing a prime rated generator ??

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You'll have to decide what you want to run (Refrigerator, furnace, TV, air conditioning, etc.). Then you'll have to determine how much power each item requires.

To determine how much power each item uses, you can use an estimate like this one provided by Energy.gov

TYPICAL WATTAGES OF VARIOUS APPLIANCES

  • Aquarium = 50–1210 Watts
  • Clock radio = 10
  • Coffee maker = 900–1200
  • Clothes washer = 350–500
  • Clothes dryer = 1800–5000
  • Dishwasher = 1200–2400 (using the drying feature greatly increases energy consumption)
  • Dehumidifier = 785
  • Electric blanket (Single/Double) = 60 / 100
  • Fans
  • Ceiling = 65–175
  • Window = 55–250
  • Furnace = 750
  • Whole house = 240–750
  • Hair dryer = 1200–1875
  • Heater (portable) = 750–1500
  • Clothes iron = 1000–1800
  • Microwave oven = 750–1100
  • Personal computer
  • CPU - awake / asleep = 120 / 30 or less
  • Monitor - awake / asleep = 150 / 30 or less
  • Laptop = 50
  • Radio (stereo) = 70–400
  • Refrigerator (frost-free, 16 cubic feet) = 725
  • Televisions (color)
    • 19" = 65–110
    • 27" = 113
    • 36" = 133
    • 53" - 61" Projection = 170
    • Flat screen = 120
  • Toaster = 800–1400
  • Toaster oven = 1225
  • VCR/DVD = 17–21 / 20–25
  • Vacuum cleaner = 1000–1440
  • Water heater (40 gallon) = 4500–5500
  • Water pump (deep well) = 250–1100
  • Water bed (with heater, no cover) = 120–380

You could also check the nameplate of each device, which should either list wattage directly or provide enough information for you to calculate it. If you have to calculate wattage from the nameplate, Ohm's law will be your friend.

Watts (P) = Volts (V or E) x current (A or I)

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  • I am having a situation where i would be having 3 motors of 3.5 KW each and all these motors will be operated simultaneously. In this case i dont think calculating the connected load alone for sizing the generator will do the job as the current drawn would be enormous. What factors should i take into considerations for sizing the generator now ?? – Mustaq Ahmed Jul 15 '14 at 5:10
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    If the current draw is enormous, the generator must also be enormous. The generator must be sized to the load it will supply. – Tester101 Jul 15 '14 at 10:22
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    It must not only be enormous enough to power all three motors simultaneously, it must be enormous enough to START all three motors simultaneously unless they're on manual-start magnetic starters. Starting current can be MANY times running current. – TDHofstetter Aug 13 '14 at 19:18

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