How to Calculate Wattage

There is a very simple calculation equation for computing watt usage for an electronic device. All you need to know is the number of amperes (amps) and the number of volts for the device, which is easily located. See step 1 to get started calculating the wattage for your device.

Steps

1. Learn the formula. There is a very simple formula for calculating watts. All you need to know are the number of amperes (amps) and the number of volts. The formula is Watts = Amps x Volts.^[1]
   
   
   
   
   
   
   


2. Find the number of amps and volts. For a electronic device or appliance there should be an information sticker located somewhere on the back of the bottom of the item. This will contain the number of amps and volts in that device, which you'll use to calculate the wattage.
   
   
   
   
   
   
   
   
   
   • Volts, can also be called input voltage for your device and are often designated by a capital "V."
   
   
   • The amperage number will usually be smaller than the voltage number, and may include decimal numbers. Usually the number will be followed by a capital "A" or the abbreviation "amps."
   
   
   • These numbers are the highest that the device goes, not necessarily the wattage its using typically, so your wattage estimates based on these calculations will be higher than they probably actually are.
   
   
   
   


3. Use the numbers you found to calculate the device's wattage. Take the formula Watts = Amps x Volts and plug the numbers you found for the Volts and the Amps into their appropriate spots.
   
   
   
   
   
   
   
   
   
   • For example if the Amps = 12.52 and the Volts = 20, then the wattage of the device would equal 250.40 (that is 250.40 = 12.52 x 20).
   
   
   • If you are planning on using more than one device on the circuit, be sure to add them all together to get your total usage.
   
   
   
   


4. Check your inverter or circuit. If the watt value of the circuit/inverter is less or equal to the number of watts your device uses, you should not use it. Also, when a device starts, especially one with a starting capacitor like a motor, it may draw more power than it is rated at.^[2]
   
   
   
   
   
   
   
   
   
   • For this reason, it is recommended by manufacturers that you use a circuit or inverter that is the next size rating up from what the label requirement is.
   
   
   
   

Tips

• Be sure to allow for a small amount of "phantom" wattage for each device plugged into the circuit/inverter. Many electrical items continue to use power when they are turned off. This particularly true of anything that shows an LED light when it is off.

Warnings

• Putting too many devices on an inverter may result in low power to the devices. This can cause damage to them, or cause them to shut down.


• The numbers are only approximate, so if you want specific and accurate wattage readings you'll need to use a wattage meter.^[3]


• If you pull too much power through an inverter, you risk the inverter burning out.

Related wikiHows

• How to Rewire a Chandelier


• How to Calculate Kilowatt Hours


• How to Build a Solar Powered Laptop Charger


• How to Convert Watts to Amps


• How to Use a Voltage Converter Transformer

Sources and Citations

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