Electrical watts measure the flow of electrical energy in a circuit. Electrical energy is measured in Joules. Watts measures how many Joules are flowing through the electrical circuit over a given amount of time. To determine watts, you must multiply the volts by the amperes. To determine the number of volts and amperes, use a multimeter that measures these quantities. Insert the numbers in your formula and you can calculate watts of electrical use.

Write down the formula for calculating watts. This will help you when it is time to calculate the wattage. Watts equals volts multiplied by amperes.

Obtain a multimeter that measures both voltage and amperes in a circuit. These tools are available at most hardware stores; you can also find them at Internet retailers.

Insert the black probe of the multimeter into the "COM" or "-" slot on your multimeter. Insert the red probe into the "V" or "+" slot. Black indicates the negative probe of the multimeter; red is the positive side.

Turn the multimeter on and set it to measure volts (by turning a dial or scrolling through a menu; each model will vary slightly). Take the free probes. Touch the black probe to one metal point in the circuit; next touch the positive probe. You do not connect the probes; you simply touch the probes to the circuits. When both probes are touching the circuits, the multimeter will display a reading. Write down the reading. Remove the probes.

Switch the multimeter to measure amperes. Take the free probes. Touch the black probe to one metal point in the circuit; next touch the positive probe. You do not connect the probes; you simply touch the probes to the circuits. When both probes are touching the circuits, the multimeter will display a reading. Write down the reading. Remove the probes.

Insert the numbers in the wattage formula to calculate watts. For instance, if you measured 10 volts and 5 amps, then the wattage in this circuit would be 50 (10 volts multiplied by 5 amps).