Divide the watts of a given electrical item by the total number of volts available from the electric outlet to calculate amperage draw. The amount of current flowing through the wire is measured in amperes, or amps. The equivalent of available electricity at the power source is voltage, or volts. Finally, the power produced by the electricity is measured in watts. All of these measurements are interrelated when calculating electricity usage.
Calculating from Watts and Volts
Find the wattage load of an device that requires electricity. Any device that draws energy is called a load. Examples of loads include a light bulb and a microwave. The wattage is often printed on the device itself, but if you can’t locate the number, you might need to check the owner’s manual.
Determine the voltage of your power source. In the United States, most household outlets run at 120 volts, although some, such as those for electric stoves or dryers, often run at 220 volts. If your power source is a battery, you will need to look up the voltage. Larger batteries are often 9 or 12 volts, while smaller closed cell batteries, such as C, AA or AAA, run between 1 and 3 volts, depending on size and composition.
Divide the wattage rating by the voltage from your power source. For example, if you have a 100-watt light bulb in a lamp that is plugged into a 120-volt outlet, it will draw 0.83 amps.
Calculating from Ohms and Volts
The electricity flowing through the wires in your house is often compared to water running through a hose. You can observe the size of the hose, the amount of water flowing through it, the water pressure and the result of the water spraying out. For electricity, the flow of the current is limited by the resistance to flow, measured in Ohms.
Use Ohm’s law to calculate amps using resistance. Many appliances have a listed resistance. The wire connecting the circuit also has a variable resistance. In the same sense, you can fit less water through a garden hose than a fire hose. You don’t need to include this resistance unless you have a lot of wire or need to be very accurate.
Find the voltage of your power source as you would when calculating from watts and volts.
Ohm’s law states that the voltage equals the amperage times the resistance, so if you divide the voltage of your power source by the resistance of the load, you will find the amps. For example, if you plug a 40-Ohm dryer into a 220-volt outlet, the appliance will draw 5.5 amps.
TL;DR (Too Long; Didn't Read)
The calculations described are for a single load. When calculating amperage over multiple loads you can simply add wattage ratings together, but resistance can change depending on how the circuit is configured.
Use caution when working with electrical energy, and have your calculations double checked by a trained professional if you are calculating amps for a home electrical system.