How to Calculate What Type of Standby Generator Is Needed

Decide which appliances, such as a refrigerator, microwave and oven, that you want to power in your home during an outage. Carry a notebook and list all the items you believe are necessary in case of a power outage. You may want to consider what you would use in case of a long-term outage because of a tornado or other natural disaster if you live in an area prone to such disturbances.

Write down the wattage that each item or appliance would use. It typically is listed on the silver tag on the electrical cord or a nameplate on the appliance.

Determine which motors you will use to survive the power outage. Motors take quite a bit of power, particularly when you are starting them up, so you should document only those that are necessary, such as the heating or air conditioning unit or the sump pump.

Total the wattage necessary to run lamps, the refrigerator, microwave and all motors to run air conditioners, sump pumps and other machinery.

Allow for a 10 percent safety margin. For instance, if your total wattage equals 4,290, you will need to add an extra 429 in watts to prevent your circuit breaker from tripping, according to the My Emergency Generator website. (See reference 2.) You will need to purchase a standby generator that has a continuous running power of 4,719 watts.