Watts Electronics Mathematics Formulas

Alternate titles

  • How to Figure Out Watts
  • How many watts in amps, volts, and/or ohms.
  • Watts - How to Calculate and Convert Volts / Amps / Ohms to Watts aka Power - A Beginners Guide

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This is one of four pages having to do with Ohm's Law and its derivatives.

  1. How to calculate Power aka Watts (You Are Here)
  2. How to calculate Current aka Amps
  3. How to calculate Electromotive Force aka Volts
  4. How to calculate Resistance aka Ohms

Watts Equals... The Quick, Easy Answer.

Power aka watts equals volts times amps. For examples and other formulas that will give you the answer, or more information in general; continue reading.

Converting Volts and Amps to Watts

  • P = EI Ohm's Law derivative for watts.

Converting Volts and Ohms to Watts

  • P = E2/R Ohm's Law derivative for watts.

Converting Amps and Ohms to Watts

  • P = I2R Ohm's Law derivative for watts.

How to convert amps / ohms / volts to watts.

Source

Watts Is the Unit of Measurement for Power

The amount of power in a circuit, or being used by a device, can be determined by knowing any two of either electromotive-force, current, or resistance.

In other words, the number of watts can be determined by knowing any two of either volts, amps, or ohms.

Each set of two has its own formula. There are three formulas.

The algebraic term for "symbol" is "variable".

"P" is the industry standard to designate power by the unit of measurement, watts. "W" is sometimes used.

"I" is the industry standard to designate current by the unit of measurement, amps.

"R" is the industry standard to designate resistance by the unit of measurement, ohms.

"E" and "V" are both used to designate electromotive-force by the unit of measurement, volts. The industry standard used to be "E", but now both "E" and "V" are being used interchangeably.

How to Convert Volts and Amps to Watts

#1 Calculating Watts Using Volts and Amps

All one does is multiply volts times amps to get watts. The next four statements all say the exact same thing and have the same exact meaning. Pick whichever one that works for you and make it yours.

  • Power equals Electromotive-force times Current
  • Power equals Voltage times Amperage
  • Power equals Volts times Amps
  • Watts equals Volts times Amps

The next four formulas all say the exact same thing and have the exact same meaning. Pick whichever one that works for you and make it yours.

  • P=EI
  • P=VI
  • P = E x I
  • P = V x I

Some everyday examples:

120 volt house current times .8333 amps equals 100 watts (a 100-watt light bulb).

12 volt car battery times .5 amps equals 6 watts (a car instrument cluster).

240 volt house current times 3 amps equals 720 watts (a washer or dryer).

12 volt car battery times 50 amps equals 600 watts (a starter for a small engine).

Some circuit / formula examples (there are a 1000 milliamps in 1 amp, there are 1000 milliwatts in a watt):

3 volts times 100 milliamps (.1 amps) equals 300 milliwatts (.3 watts) .

300 volts times .2 amps (200 milliamps) equals 60 watts of power.

Some math / algebra examples:

300 watts = 200 volts x 1.5 amps

25 watts = 120 volts x .20833333 amps

100 watts = 120 volts x .833333 amps

How to Convert Volts and Resistance (ohms) to Watts

The average solar panel produces 9 to 13 watts per square foot. A number that is expected to change as technology continues to improve.

Source

#2 Calculating Watts Using Volts and Resistance (Ohms)

This one is a little trickier. To calculate power (in watts), one squares the voltage, and then divides by the resistance. The next four statements all say the exact same thing and have the same exact meaning. Pick whichever one that works for you and make it yours.

  • Power equals Electromotive-force squared divided by Resistance
  • Power equals Voltage squared divided by Ohms
  • Power equals Volts squared divided by Resistance
  • Watts equals Volts squared divided by Ohms

The next four formulas all say the exact same thing and have the exact same meaning. Pick whichever one that works for you and make it yours.

  • P=E2/R
  • P=V2/R
  • P = (E x E) / R
  • P = (V x V) / R

Some everyday examples:

Everyday example #1:

The manufacturer claims that your spiffy, new electronics device uses only 200 watts. Is it true? Well, you know you're plugging it into a 120-volt outlet; that's one variable already known. So, you hook up the VOM (see note below) to the device to ascertain it's resistance measurement. The measurement comes back as 72 ohms.

The formula is P=E2/R. 120 volts times 120 volts equals 14,400. So you now have 14400/72. Which does indeed equal 200 watts.

Everyday example #2:

Just how much power does that 4-battery, super-flashlight really use? Well, the four batteries give a 6-volt output. The meter says there is an 18 ohm resistance.

P=E2/R. So you have 36/18, which equals 2 watts.

Some circuit / formula examples (there are a 1000 milliamps in 1 amp, there are 1000 milliwatts in a watt):

There are 60 volts running through a circuit with a measured resistance of 40 ohms. So that would be 3600/40, which gives you 90 watts.

There are 3 volts running through a circuit with a measured resistance of 5 ohms. So that would be 9/5, which gives you 1.8 watts.

Some math / algebra examples:

P=482/500=2304/500=4.608 watts

P=1.52/10=2.25/10=225 milliwatts

A general note about VOM's: A VOM is a multi-range, volt-ohm multi-tester meter. It is a test instrument to measure voltage, current, and resistance of an element or circuit. Do not buy one until you really know what you are doing. Cheaper meters are extremely inaccurate when it comes to measuring certain ranges of resistance. Really research the subject first.

How to Convert Current (amps) and Resistance (ohms) to Watts

Just a Tesla Coil demonstration at Fermilab.

Can click to enlarge.
Can click to enlarge. | Source

#3 Calculating Watts Using Current (amps) and Resistance (ohms)

"Twinkle, twinkle, little star. Power equals I squared R." That little mnemonic trick has been around since the discovery of the science of electronics.

To calculate power (in watts), one squares the current (amps), and then multiplies it by resistance (ohms). The next two statements say the same thing and have the same meaning.

  • Power equals Current squared times Resistance
  • Watts equals Amps squared times Ohms

The formula is:

  • P=I2R

An example:

You have 10 amps flowing through a device or circuit that has a resistance of 25 ohms. How much power is it consuming?

P=I2R

P=102 times 25

P=100 times 25

Power being consumed is 2500 watts.

The Three Formulas for Watts

Summary

P=EI (Watts equals volts times amps. Of the three formulas, this is the one that is used 90% of the time.)

P=E2/R

P=I2R

The how to convert amps / ohms / volts to watts page.

P = EI Ohm's Law derivative for watts.
P = EI Ohm's Law derivative for watts. | Source

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Comments 2 comments

Randy Godwin profile image

Randy Godwin 4 years ago from Southern Georgia

Very helpful and thorough, Para! Should come in handy for many folks.

SSSSS


zampano 4 years ago

Hi PSearch ! How about a limerick on the subject ?

hehehe

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