If you are looking to purchase a home generator for emergency situations, you are making a wise investment. Keeping the home lit and warm during a blackout are only two advantages. Keeping the security system running to deter intruders, operating the sump pump to prevent basement flooding, protecting food in the fridge and phones charged for communication are all important reasons to keep power on during an emergency.

The question becomes, how big of a generator is needed? Generators come in many sizes, from small portable gasoline models to larger units attached to the home that run off either propane or natural gas.

Take an Inventory of Electrical Use in the Home

Performing an inventory of electric appliances is the first step in calculating energy needs. To determine how big of a generator is needed, a basic electrical formula is helpful. Energy use is measured in watts. A watt is a combination of amps and volts. The formula is as follows:

Watt = Volts x Amps

Volts are pretty standard for most appliances. Except for electric ranges, electric clothes dryers and large window AC units, all appliances use 110-120 volts. The larger appliances use 220 volts. If an appliance uses 12 amps, multiply that figure by 120 volts to determine that the watts consumed is 1440. Look on the label of the appliance to find the number of amps used.

Use a Guide and Avoid the Math

For some appliances, like refrigerators and air conditioners, look for the yellow EnergyGuide label. It will not only tell the estimated yearly cost or running the appliance, but the number of kilowatt hours is will consume in a year. A kilowatt equals 1000 watts.

In addition, the Department of Energy has a general guide for electrical use of common household items. Other websites have energy calculators that might come in handy.

Appliances that Heat and Cool Use the Most Energy

Central Air and window AC units, electric ranges, electric clothes dryers and hot water heaters use by far that largest amount of electricity. A clothes dryer can use 10 times the electricity of a washing machine. A central air unit or electric range can use up to 5000 watts each. A fridge is an exception. Though it uses roughly 600 watts, it is far less than an AC unit. This is because the fridge cools a small volume of area and is super insulated.

In the movie Apollo 13, NASA engineers knew the battery power of the Lunar Excursion Module was limited, so they had to make choices and determine the items to power down. Though your situation isn’t as drastic, you will need to determine what needs to be run in an emergency. If the power goes out, the outage will usually not last beyond a few days. If laundry can be postponed a few days, you eliminate the need for running the dryer. If meals can be prepared in a toaster oven or microwave, the electric range can be avoided.

On the other hand, a furnace is probably needed in the middle of winter and AC may be necessary during a summer storm. With a little planning and a little math, determining your electric needs in an emergency will determine the size of generator you’ll need.