To calculate amperage, you will need to use a multimeter. Simply set the multimeter to the "amps" setting and touch the probes to the positive and negative terminals on your refrigerator. This will give you a reading of how many amps your refrigerator is using. You can also use this method to calculate how many watts your refrigerator is using. Simply multiply the number of amps by the voltage (120 volts for North America). This will give you a wattage reading.