How to Determine How Many Watts You Are Pushing to a Speaker

Finding the wattage being driven through the speaker driven can be accomplished by using the power equation P = (E*E)/R.



P is power in watts, E is voltage being provided by the amplifier, and R is resistance, or impedance of the speaker.



You will need to measure the voltage from the amplifier by using a volt meter or DMM and find the impedance value marked on the speaker.

Things You'll Need

• Multimeter/Volt meter

Instructions

1. 

   Set the multimeter

   Set the multimeter to the 200 Volt AC range.

2. 

   Turn on the stereo

   Turn on the stereo. Pick a song to play and turn up the volume all the way.

3. 

   Measure the voltage

   Measure voltage by placing the black multimeter probe on the negative speaker terminal and the red multimeter probe lead on the positive speaker terminal.

4. Write down the highest value displayed by the multimeter. This is the "E" value for the equation.

5. Record the speakers impedance value. For an open frame speaker this will be located on the speaker magnet. On a home stereo speaker this will usually be on the back of the speaker enclosure, near the where the wire connects to the speaker.

   Ratings for speakers are in Ohms. The ohmic symbol is the capital Greek letter Omega. It looks sort of like like a horseshoe.

   Typical values for speakers are 2,4,8 or 16 ohms.

6. Square the voltage (E) value (Multiply it by itself)

   For example: If the measured voltage is 60 Volts, multiply 60 times 60 to get 3600.

7. Divide the squared voltage value by the impedance value of the speaker.

   For example if our speakers impedance is 8 ohms, we divide 3600 by 8 to get 450. This is the peak value of power in Watts

   Peak power means that this is a number that can be driven through the speaker at any given instant in time, but not continuously.

   To get a more realistic number, one that tells us what wattage the speaker can potentially put out on a constant basis, we need to find the RMS value.

8. Multiply the peak wattage by .707 to get the RMS wattage. RMS stands for "Root Mean Square." This value is the true constant power that can potentially be pushed through the speaker.

   For the example of 450 Watts, we multiply 450 by .707 to get and RMS value of 318.15 Watts.

   This is the true constant power that can be driven through the speaker.

Tips & Warnings

• When measuring the voltage, using an old analog style multimeter may be better than the new digital style as you can see the needle respond in real time to the music. With digital Multimeters (DMMs) there is a slight time lag in measurement.

• Since the voltage coming from the amplifier is an AC voltage, it is not absolutely necessary to obey polarity when measuring the voltage.

• For really high wattage amplifiers I like to convert the power rating from watts to horsepower. One horsepower equals 746 watts, so just divide the wattage by 746 to get an equivalent horsepower rating.

• A one thousand watt audio amplifier can also be classified as a 1.3 horsepower amplifier.

• When measuring voltage at the speaker terminals, be careful not to short the speaker leads together as this could cause damage to the amplifier.

• Do not measure the amplifier's output without a speaker or equivalent load hooked up to that channel. An unloaded amplifier channel will have a higher voltage output than one loaded by a speaker and using this measurement will cause an erroneously high wattage calculation.

Resources

• Electric current, electricity and electric charge