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From: Selecting A Generator

A generator must be properly sized for the load it will be used to handle. Electric motors are particularly difficult for a generator because starting an electric motor requires two to three times it's nameplate amperage or wattage. While electricity supplied by an electric power company has essentially infinite surge capacity (limited only by the circuit protection provided), a generator is limited by the engine horsepower and inertia of rotating parts. 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. A 3450 RPM air compressor motor is a prime example of this type of load. For this reason, when determining the power your generator is to provide, it is important to calculate electric motor requirements at three times the running watts to compensate for the surge needed to start the motor. With this in mind, the following guidelines will be helpful in selecting the right size generator for your needs.

  1. 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).
  2. Next determine volt-amperes (wattage) requirement of electric motors to be operated, remembering that the starting requires two or three times the nameplate or running (rated) watts. Thus, if running watts of the motor is 600, multiply the number by 3 to determine maximum V.A. needed. The starting (max.) V.A. can also be determined by referring to the motor code listing which indicates starting KVA per horsepower.
  3. Total watts and V.A. is Steps 1 and 2 to get total requirements.
  4. To allow for anticipated future needs or use of extra equipment, add 25% to total load in Step 3.
  5. See performance data charts at bottom of each generator listing for the unit that meets your total load criteria.
EquipmentRunning WattsMax VA
Light Bult (100 Watts)100100
Refrigerator (conventional)4001,200
Furnace Fan (1/3 HP with Blower)6001,800
Vacumn Cleaner6001,800
Sump Pump (1/3 HP)7002,100
Refrigerator/Freezer Combination8002,400
6" Circular Saw 8002,400
1/2" Drill1,0003,000
14" Chain Saw 1,2003,600
Water Well Pump (1/2 HP)1,4004,200
Hot Plate/Range (per burner)1,5001,500
10" Circular Saw 2,0006,000
Water Heater (storage type)5,0005,000
Electric Oven 10,00010,000