How to Calculate Amperage and Wattage Using a Meter

Written by andy pasquesi
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How to Calculate Amperage and Wattage Using a Meter
A 220-volt multimeter (multimeter image by dinostock from

In the study of electricity and magnetism, Ohm's Law outlines the basic relationship between the voltage, current and total electrical resistance in a circuit: current (in amperes) equals voltage (in volts) divided by resistance (in ohms). Using a device known as a multimeter, you can measure the resistance of an appliance and the incoming voltage. Then, with the help of Ohm's Law, you can calculate the current (a.k.a. "amperage") and power (a.k.a. "wattage") consumed by the appliance.

Skill level:
Moderately Easy

Things you need

  • Auto-ranging multimeter

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    Measuring Voltage and Resistance

  1. 1

    Turn off the circuit's power supply.

  2. 2

    Turn on your auto-ranging multimeter.

  3. 3

    Turn the multimeter's knob to the "Resistance" setting, which is signified by the Greek letter omega.

  4. 4

    Place the metal tip of the multimeter's negative probe (black) against one of the appliance's terminals while placing the tip of the positive probe (red) against the opposite terminal. The value that appears on the multimeter's readout is the electrical resistance of the appliance, in ohms.

  5. 5

    Remove the probes and turn the multimeter's knob to a voltage setting. If the power source is a battery, solar cell, dynamo, transformer or solar panel, set the voltage to "Direct Current (DC)." If the power source is an electrical socket or three-phase generator, set the voltage to "Alternating Current (AC)."

  6. 6

    Turn on the circuit's power supply.

  7. 7

    Place the multimeter's red probe tip against the power supply's positive terminal and place the black probe tip against the negative terminal. The value on the multimeter's readout is the voltage, expressed in volts.

    Calculating Amperage and Wattage

  1. 1

    Divide the circuit's voltage (from Section 1, Step 7) by the appliance's resistance (from Section 1, Step 4). The resulting value will be the amperage the appliance draws from the power source, expressed in amperes or "amps."

  2. 2

    Multiply this amperage by the circuit's voltage (from Section 1, Step 7). This new value, expressed in watts, is the power rating or "wattage" for the appliance.

  3. 3

    Record these values.

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