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How to Calculate the Power in Watts Dissipated

All electrical and electronic circuits dissipate power. The standard unit for measuring this power is the watt. To calculate power, you first need to know whether the circuit uses alternating current, or AC, or direct current, which is DC. DC calculations are simple: a formula uses numbers representing the circuit's voltage and current, current and resistance or resistance and voltage. The same information applies for AC circuits but you also need to know if the voltage is a peak or root mean square (RMS) value.

Things You'll Need

  • Scientific calculator
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Instructions

  1. DC Voltage and Current

    • 1

      Key the DC voltage value into the calculator. For example, if the circuit voltage is 4 volts, key in "4."

    • 2

      Press the "Multiply" key. Enter the DC current into the calculator. For this example, your circuit has 2 amps of current, so enter "2."

    • 3

      Press the "Equals" key. The result is DC watts. In this example, 4 volts x 2 amps = 8 watts.

    DC Current and Resistance

    • 4

      Enter the current measurement into the calculator. Multiply the number by itself to square it. For example, if your circuit has 2 amps of current, multiply 2 times 2 to get 4.

    • 5

      Press the "Multiply" key. Enter the DC resistance in ohms. In this example, your circuit has a resistance of 2 ohms. Enter "2" into the calculator.

    • 6

      Press the "Equals" key. The result is DC power in watts. For this example, 4 x 2 = 8 W.

    DC Volts and Resistance

    • 7

      Enter the DC voltage into the calculator. Multiply the number by itself to square it. For example, if your circuit has 4 volts, multiply 4 times 4 to get 16.

    • 8

      Press the "Divide" key. Enter the resistance of the circuit in ohms. In this example, your circuit has a resistance of 2 ohms. Enter "2" into the calculator.

    • 9

      Press the "Equals" key. The result is DC watts. For the example, 16 / 2 = 8 W.

    AC Volts and Amps

    • 10

      Key in the AC voltage value into the calculator. For this example, the voltage is 110 volts RMS, so enter "110."

    • 11

      Press the "Multiply" key. Enter the AC current. For this example, your AC current is 2 amps. Enter "2." If the voltage is an RMS value, as in this example, skip Step 3 and proceed to Step 4.

    • 12

      Press the "Multiply" key. Enter the value ".707." Remember to do this only if the voltage value is peak voltage. Otherwise, skip this step.

    • 13

      Press the "Equals" key. The result is AC watts. In our example, using an RMS voltage value of 2 volts, 110 x 2 = 220 W.


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