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How to Calculate What Size Generator I Need

Marie Jones

A generator is designed to provide power to a home or building when the regular electricity is not working. Home generators are available for purchase with capacities of 5 to 50 kilowatts. Most homeowners require a generator with 12 to 15 kilowatts to efficiently power their homes.

  1. Determine which items and how many of each you want to power. Each additional appliance increases the size of the generator you will need. Include items such as sump pumps, lights, air conditioners, refrigerators and freezers in your calculations. You should also calculate for smaller household items that you may need to run during a power outage, such as a coffeemaker or a hair dryer.

  2. Calculate the starting and running wattage of the items you want to power. This information is usually available in the owner's manual or buyer's guide that came with the product. If the wattage information is not included, contact the company that makes the appliance to request the information. You can also find the wattage of many common items on the Diesel Service and Supply Inc website. Add the wattage of each item together. This is the minimum size generator you will require.

  3. 360Generators.com suggests adding 20% to the total wattage you calculated. This will prevent the generator from becoming overloaded and will give you leeway if you need to use more power than you originally estimated.

  4. Warning

    The Consumer Product Safety Commission warns that a generator must always be operated outside of your home or any enclosure for safety purposes, regardless of weather conditions.