There are many resources online that use calculators to determine watts. They will do the formula for you.  Such resources typically ask you to enter the number of volts, and the number of amps. Then you are asked to hit the “calculate” button to get the watts. Keep in mind, though, that online calculators aren’t always accurate because every appliance is going to be slightly different in its power needs. Some online sites will give you the watts needed if you click on the appliance type, such as a television or desktop computer. Sites sometimes have charts that list the watts used by various appliances, from refrigerators to boom boxes. You can figure out how many watts an appliance needs by looking for the data plate.  Find this on the back of your appliance. It’s likely to list how many volts, amps, and watts you need to power your appliance. You might find this information stamped on the back of the appliance. Or you could find the wattage listed on the nameplate.  Wattage meters plug into the appliance, and they tell you the exact amount of power that the appliance needs to run. The wattage of an appliance might vary depending on its setting. For example, a radio is going to use more watts if you turn the volume on high.
Find an online calculator. Check your appliance.