How to Calculate Generator Sizing


Determining the right size for a standby generator for your home is important. Your generator should supply sufficient wattage to run essentials such as water heaters, boilers, HVAC, lighting, household appliances and any power tools you may use during a power outage. On one hand, over-sizing your generator is a waste of money; on the other, too few watts will not only overwork the generator but will also damage anything drawing power from the unit.

Things You'll Need

  • Appliance and device owner's manuals
  • Make a list of start-up and running wattages needed for all the household lighting and electrical devices you will use during a power outage. Most tools, appliances and other electrical devices have the wattage requirements listed in the user manual or printed on a specification plate attached to the unit. Don't forget to add sound systems, computers and TV power requirements to your list.

  • Write down the start-up wattage and ignore running wattage on devices with a motor or compressor, such as refrigerators, vacuum cleaners and air conditioners. Motors require as much as three times more power during start-up.

  • Total all the wattages listed. Add 20 percent to this figure to avoid accidentally overloading your generator and to allow for any other small appliances you may add in the future.

  • Select a standby generator with a rated output as close to your final figure as possible.

Tips & Warnings

  • If for some reason you can't find the start-up wattage on any devices, contact the manufacturer or consult an electrician.

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