Calculating Appliance Energy Use

If someone wants a general estimate of how much electricity an machine or appliance consumes, please refer to the kWh Consumption Levels Table provided at the bottom of this page. This table gives the energy consumption of typical appliances. If someone has appliances that are not listed in the table, or wants a more exact figure based on actual energy consumption, use the formula below to estimate the amount of energy each machine or appliance consumes:

First, determine the watts of power consumption as described below. Next, multiply this by the hours per day and number of days one uses the machine or appliance during the year for the annual consumption. Then calculate the annual cost to run an appliance by multiplying the kWh per year by the local utility's rate per kWh consumed.

For example:

One can usually find the wattage of most machines and appliances stamped on the bottom (or back) of the appliance, or on its "nameplate." The wattage listed is the maximum power drawn by the appliance. Since many appliances have a range of settings (for example, high and low power for a hair dryer), the actual amount of power consumed depends on the "setting" used at any one time.

Refrigerators, although turned on all the time, actually cycle on and off at a rate that depends on a number of factors. These factors include: how well it is insulated, room temperature, freezer temperature, how often the door is opened, if the coils are clean, if it is defrosted regularly, and the condition of the door seals. To get an approximate figure for the number of hours that a refrigerator actually operates at its maximum wattage, divide the total time the refrigerator is plugged in by "3."

AmmeterIf the wattage is not listed on the appliance, it can still be estimated by finding the current draw (in amps) and multiplying that by the voltage used by the machine or appliance. (Most appliances in the United States use 120 volts. Some machines and larger appliances, like clothes dryers and electric cook tops, use 240 volts.) The amps might be stamped on the unit in place of the wattage. If not, find a "Clamp-on Ammeter" (an electrician's tool that clamps around one of the two wires on the appliance) to measure the current flowing through it. This type of ammeter can be obtained in stores that sell electrical and electronic equipment. Take a reading while the device is running; this is the actual amount of current being used at that instant.

Note: When measuring the current drawn by a motor, in the first second that the motor "starts," the meter will show about three times the current than when it is running smoothly.

Typical Office Equipment Annual kWh Consumption Levels
Fax Machines8
Mainframe Terminals27
Task Lights88