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How to Understand Electricity: Watts, Amps, Volts, and Ohms

Author:

Rik is an engineer who has held a range of marketing, technical support, and management roles. He is also a qualified teacher.

Welcome to your guide to the basics of electricity.

The four most basic physical quanities in electricity are:

  • Voltage (V)
  • Current (I)
  • Resistance (R)
  • Power (P)

Each of these quantities are measured using different units:

  • Voltage is measured in volts (V)
  • Current is measured in amps (A)
  • Resistance is measured in ohms (Ω)
  • Power is measured in watts (W)

Electrical power, or the wattage of an electrical system, is always equal to the voltage multiplied by the current.

A system of water pipes is often used as an analogy to help people understand how these units of electricity work together. In this analogy, voltage is equivalent to water pressure, current is equivalent to flow rate and resistance is equivalent to pipe size.

In electrical engineering, there is a basic equation that explains how voltage, current and resistance relate. This equation, written below, is known as Ohm's law.

Ohm's Law

V = I x R

Ohm's law states that voltage is equal to the current flowing in a circuit times the resistance of the circuit.

One way of understanding Ohm's law is to apply it to the imaginary plumbing system we've employed as a representation of an electrical system.

Let's say we have a tank of water attached to a hose. If we increase the pressure in the tank, more water will come out of the hose. Thus, if we increase the voltage in an electrical system, we will also increase the current.

If we make the diameter of the hose smaller, resistance will increase, causing less water to come out of the hose. Thus, if we increase the resistance in an electrical system, we will decrease the current.

With this brief introduction of the workings of an electrical system, let's jump into each of the units of electricity separately and learn about them in more detail.

The image above depicts a simple electrical circuit with a bulb, some wire, and a battery.

The image above depicts a simple electrical circuit with a bulb, some wire, and a battery.

What Are Volts?

Volts are the base unit used to measure Voltage. One volt is defined as the "difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points." The volt is named after the Italian physicist Alessandro Volta.

In our battery diagram above, the battery provides what is known as a potential difference in an electric circuit, or voltage. If we go back to our water analogy, the battery is like a water pump that propels water through a pipe. The pump increases the pressure in the pipe, causing the water to flow.

In electrical engineering, we call this electrical pressure voltage and measure it in volts. A voltage of three volts can be written as 3V.

As the number of volts increases, the current increases too. But in order for the current to flow, the electrical conductor or wire must loop back to the battery. If we break the circuit, with a switch for example, then no current will flow.

There are standard voltage outputs for everyday objects like batteries and household outlets. In the United States, the standard voltage output for a household outlet is 120V. In Europe, the standard voltage output for a household outlet is 230V. Other standard voltage outputs are listed in the table below.

Common Voltages

ObjectVoltage

Single-cell, rechargeable battery

1.2V

Single-cell, non-rechargeable battery

1.5V–1.56V

USB

5V

Automobile battery

2.1V per cell

Electric vehicle battery

400V

Household outlet (Japan)

100V

Household outlet (North America)

120V

Household outlet (Europe, Asia, Africa, Australia)

230V

Rapid transit third rail

600V–750V

High-voltage electric power lines

110,000V

Lightning

100,000,000V

What Are Amps?

The ampere, often shortened to "amp" or A, is the base unit of electric current in the International System of Units. It is named after the French mathematician and physicist André-Marie Ampère, who is considered the father of electrodynamics.

Electricity consists of the flow of electrons through a conductor, for example, an electric wire or cable. We measure the rate of flow of electricity as an electric current (just as we think of the rate of flow of water in a river as the river current). The letter used to represent current in an equation is I.

Electric current is measured in Amperes, shortened to Amps or simply the letter A.

A current of 2 Amps can be written as 2A. The bigger the current the more electricity is flowing.

The International System of Units (SI) defines amps as follows:

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length."

Electric Current Demonstration (Video)

What Are Ohms?

Ohms are the base unit of resistance in an electrical system. The ohm is defined as "an electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the conductor not being the seat of any electromotive force." The ohm is named after the German physicist Georg Simon Ohm.

Resistance is measured in ohms, or Ω (omega), for short. So, five ohms can be written 5Ω.

In our battery diagram above, if we remove the bulb and reconnected the wire so the battery was short circuited, the wire and battery would get very hot and the battery would soon be flat because there would be virtually no resistance in the circuit. Without any resistance, a huge electrical current would flow until the battery was empty.

Once we add a bulb to the circuit, resistance is created. There is now a local "blockage" (or narrowing of the pipe, per our water pipe analogy) where the current experiences some resistance. This greatly reduces the current flowing in the circuit, so the energy in the battery is released more slowly.

As the battery forces the current through the bulb, the battery's energy is released in the bulb in the form of light and heat. In other words, the current carries stored energy from the battery to the bulb, where it is turned into light and heat energy.

The image above shows a light bulb as the main cause of electrical resistance.

The image above shows a light bulb as the main cause of electrical resistance.

What Are Watts?

A watt is the base unit of power in electrical systems. It can also be used in mechanical systems. It measures how much energy is released per second in a system. In our battery diagram, the size of both the voltage and the current in the bulb determine how much energy is released.

In the diagram above, the light bulb would get brighter as the power, measured in watts, increases.

We can calculate the power released in the bulb, and of the electrical system as a whole, by multiplying the voltage by the current. So, to calculate watts, the following formula is used.

How to Calculate Watts

W = V * I

For example, a current of 2A flowing through a bulb with a voltage of 12V across it generates 24W of power.

How to Calculate with Watts, Amps, Volts, and Ohms

If you want to do an electrical calculation involving voltage, current, resistance, or power, reference the formulae circle below. For example, we can calculate the power in watts by referencing the yellow area in the circle.

This formulae circle is very useful for many electrical engineering tasks. Keep it handy the next time you are dealing with an electrical system.

Below are some example equations that are solved using the formulae.

Example Equations

1. What is the current in an electrical circuit with a voltage of 120V and 12Ω of resistance?

I = V/R

I = 120/12

I = 10A

The current in an electrical circuit with a voltage of 120V and 12Ω of resistance is 10A.

2. What is the voltage across an electrical circuit with a current of 10A and 200Ω of resistance?

V = I x R

V = 10 x 200

V = 2000V

The current in an electrical system with 10A and 200Ω of resistance is 2000V.

3. What is the resistance in an electrical system with a voltage of 230V and a current of 5A?

R = V/I

R = 230/5

R = 46Ω

The resistance in an electrical system with 230V and 50A is 46Ω.

Formulae circle for solving electrical unit equations.

Formulae circle for solving electrical unit equations.

In Conclusion

After reading this article, you hopefully have a better understanding of the difference between electric current, voltage, resistance, and electrical power. Remember that if you know any two of the physical values in the formulae circle then you can calculate each of the other two unknown values.

Basic Electricity Tutorial (Video)

Electricity Quiz

For each question, choose the best answer. The answer key is below.

  1. If I connect a 120V supply to a 60W bulb, what current would flow in the circuit?
    • 1A
    • 2A
    • 0.5A
    • 5A
  2. If a 3V battery is connected to a bulb and a current of 1.5A flows through it then what is the rating of the bulb?
    • 3W
    • 2W
    • 4.5W
    • 0.5W

Answer Key

  1. 0.5A
  2. 4.5W

Interpreting Your Score

If you got 0 correct answers: Perhaps you need to reread this article?

If you got 1 correct answer: Do you see where you went wrong?

If you got 2 correct answers: Well done. You certainly know Watt is Watt!

Questions & Answers

Question: What is the resistance of the heating element of an electric iron if the ampere draw is 8 amperes when 115 volts are applied?

Answer: R= V/I = 115/8 = 14.4 Amps

Question: Can I run two appliances at the same time when the max amps available is 5A? One requires 3 amp, and the other requires 4.15 amp.

Answer: The answer is no. The total current drawn is 7.15 Amps. This would overload a 5A socket, and result in a 5A fuse blowing or a 5A circuit breaker being triggered.

© 2009 Rik Ravado

Comments

Rooprick on November 22, 2019:

I'm trying to attach a cork to my Fork so when I eat I don't hurt myself or others how do I do that

brit on November 21, 2019:

cool

Branson on September 24, 2019:

I am trying to design a battery power led light strip. My question is if my led controller is 12v, what amperage 12v battery supply can I use? 2A? 4A? 6A?

remlalnghaka on September 21, 2019:

very interesting

Rahimullah Sahak on August 02, 2019:

hi dear thinks for your good and a best incarnation I have one question what is the spicefication of wire and his color and his cade

ubaid on July 31, 2019:

how much units will be comsumed by an electronic device of 3 amps per hour

manivannan on July 18, 2019:

thaks to all informations

Ghulab hameed on May 15, 2019:

sir plz tell me how we know a device how many voltage applied and which condination will be prove?

Krystal on May 14, 2019:

Trying to get the answer to this question can anybody help me please need if by the moring for school

1. Question:

I=.5amps,R=50phms,P=

Heidy on April 01, 2019:

thank you for explaining the basics of electricity. It make sense now

Asitya on March 20, 2019:

Nice it's so easy

COOPER IS SIMPLE on February 11, 2019:

Cooper said he's being simple but he's using highly unnecessary "big" words and it is stupidly simply annoying.

robert cannon on January 21, 2019:

I don't care about small electrical quiz I what one question answered what gauge wire is needed to hook up a welder rated at 230 volts 220 amps as far as I got as it is with electricity is this high a amperage will kill you

Aamir on January 17, 2019:

very well explained.

yoyo on November 05, 2018:

thanks !..it was interesting.

Cooper on October 30, 2018:

So we know plugging an extension into another extension (and having load being drawn from said setup) will (possibly) hit the maximum allowed Amperage/Wattage for that extension and burn it out...

But we can have multiple devices like Fridges, Freezers, TV's, Irons, etc, etc... Each on their own "Plug Socket" but aren't the Plug Sockets essentially just 'extensions' of the house Mains? -

Presuming my answer is probably available on the house's Main Fuse Board?... I'm curious to ask how one would calculate the Maximum Potential Amperage & Wattage load of one's entire house!

I know I could add up all my devices and calculate 'My Specific Total Draw' but how much further 'til I hit the 'maximum for my residence'?

The answer is probably stupidly simple, but I'm being stupidly simple at the moment so such an answer would be warranted :D

- Cooper

Sanju pradhan on October 18, 2018:

It's very helpful to me for my studies.

Oh ya daddy on October 10, 2018:

Thanks helpful

Liam on October 05, 2018:

Very Interesting, thankyou!

hi baby on July 03, 2018:

very good and very helpful

Brunsrm on April 02, 2018:

Can I run 2 appliances at the same time when the max electric provided is 5 amps? One requires 4.12 A and the other requires 3 A.

Thanks for your help

Taylor on March 27, 2018:

I had to do a speech on inventions that changed the quality of life. I chose electricity. This website helped.

Prabhjot on March 17, 2018:

Thanks a lot for teaching us in easy ways so we could understand thanks :)

Hendrik on February 25, 2018:

Ω my god, 31 years old and finally understand the basics!

Gabe on February 23, 2018:

This make me understand how does volts and wats work. Before i cant do the difference between volts and watts.

I love Yucheng Jade Shao on January 23, 2018:

This was an amazing presentation and it helped me alot to understand volts, Amps, Ohms, and Watts!

David on January 15, 2018:

This is an excellent video that I highly recommend for all beginners like me. I now have a good basic understanding of how electricity works in our everyday world. Thank you!

ramesh kumar verma on January 01, 2018:

pleas help me how understand electricity ie wattage, volts and amps, how to increase amps & decrease in the circuit

a random guy on December 15, 2017:

a skele-TON of information, thanks.

a random guy on December 15, 2017:

this helped a lot

toby on November 10, 2017:

Examples used in teaching were the best i read so far.Thank you so much :)

Esmaeel on October 30, 2017:

It's really an added value

zulfiqar Ahmad on October 25, 2017:

Very nice

S Kanda on October 21, 2017:

It helped me to understand electricity ie wattage, volts and amps, better than books.

D.bell on October 04, 2017:

This helped a lot thanks

mehul lavri on September 26, 2017:

good presentation

asif javed on September 12, 2017:

brilliant description about electricity

J. R. on September 08, 2017:

Very good presentation

andrea Sherman on September 04, 2017:

I never understood how that worked. I still am sort of confused . isn't electricity measured in both particles and linear?. I took a class called physics for poets about 40 years ago. thank you✌

@Jeet on August 29, 2017:

Very useful thanks.

Meseret Yirdaw on May 25, 2017:

That is very nice.I would like to thank more and more.Keep it up.

Ashfaq on May 24, 2017:

Very helpful page regarding understand of simple electricity

Harpal Singh on May 15, 2017:

Amazing.....Honestly speaking this is the first time i fully understood it...

thks....

kira on May 03, 2017:

i really find it interesting while reading this,and I learnt alot,thanks everyone for contributing to this.

Chethan shetty on April 13, 2017:

Helps to understand even for mechanical students

Sandip Deb gupta on March 23, 2017:

It's very good article wonderful experience

Nope on March 08, 2017:

Thanks

Rizwan on January 22, 2017:

Nice explanation

DILLI SAHU on January 22, 2017:

Thank you Sir,

Nice n simple explanation of current, volt and watt.

lavysay on January 05, 2017:

cool

Nancy Marine on October 03, 2016:

I got 2/2 of the question, I guess I know watt is watt.

ade on October 03, 2016:

amazing, never been able to get this stuff in my Head and you've managed it so succinctly mate, thanks ever so much, Cheers

jeo on September 16, 2016:

hi there, i am just getting to know about electricity. and has always been confused to hell, what is Amps, Volts etc. now i understood it all.

thanks for the article. God Bless

SAQIB from HYDERABAD PAKISTAN on January 29, 2015:

Being engineer myself I enjoyed reading the concepts.

sanjay on December 19, 2014:

Very good

MARJ WATSON on July 26, 2014:

Please advise this old lady if I can use Gu10 replacement bulbs with 110/220 in the uk, as I bought them in error, thanking you in anticipation,

Marj

Someone on March 08, 2014:

That was useful thanks

santosh kumar on January 20, 2014:

Thanks a lot for your wonderful detail explainatin. I hope it would be very helpful to all beginers. Best wishes for your blog.

Alpo on January 05, 2014:

Thanks for the tutorial...it has been awhile and was looking for the basic equations. According to your article, "A" is short for amperes or amps. Later, "I" is introduced without relating it to amps. For clarity, you might add that.

vikas pal on September 10, 2013:

great blog

Alex on May 22, 2013:

Can someone help to find out 12 volt ??? w required in the given satiation?

I required 220 volt 1000 w to run my application. i am planning to install solar panel to have full fill my requirement. it involve solar panel ,12 volt battery, and power inverter, how many watts of 12 volt panel do i required to have 220 volt 1000w power to run my application

Rusty on April 09, 2013:

I found it interesting that elkangorito said no way 120 is safer than 230 volts. Then why in England on ALL commercial work sites the max voltage used is 120? Everything is stepped down from 230 to 120.

Thanks

Kiran Chandran on October 04, 2012:

I had cleared many doubts by using this........

amped on July 17, 2012:

Very useful explanation.

saratchandra on May 25, 2012:

Good explanation with examples

ajay on April 16, 2012:

thanks for the answer. sir please give one more answer about transformers. I read somewhere that transformers can increase and decrease the voltage of a said line, so in that case is it possible to decrease the incomming 220v main line of my house to say 24v and then again increase it 220v for normal usage. will the performance be the same and will it decrease the electric bill.

Rik Ravado (author) from England on April 14, 2012:

ajay - the maximum current would depend on the fuse or breaker in the circuit. In theory, if you short circuited the 120v you could get an infinite current but domestic supplies normally have a breaker that trips if you draw too much current. In the UK, our domestic circuits typically allow you to draw about 13A from a 3 pin plug but it my be higher elsewhere, for example the US.

ajay on April 14, 2012:

how do we determine the maximum limit of ampereage that can be drawn out of a 120 volts line

Ak on December 28, 2011:

Thanks for the heads-up Rik. I want to implement cheap renewable energy, could you outline few options? Pv or solar panels are not the options where i live. I heard a lot about fuel cells, are they expensive?

Rik Ravado (author) from England on December 22, 2011:

Ak - These products are all scams. You don't get electrical energy for nothing. You can convert mechanical energy into electrical energy, a generator does this. But you still have to put the same amount of energy in. Scammers sometimes talk about perpetual motion in connection with free energy but it doesn't exist.

Ak on December 21, 2011:

I have a question.. Can the magnetic energy be converted to electric enegergy somehow by properly placing them on a flywheel? Rotating the flywheel using the magnetic force. There are lot of things going on the web about it. And how come they only sell guides but not the finished product.

Ak on December 21, 2011:

Clear as crystal

Robert on December 18, 2011:

First, thank you does not say enough for this. I am a slow learner and forward to using this to help learn about electricity. I am finding out that it is not that hard, but like Bert K commented, U+P=R. Thank you again so much Mr. Ravado

Rik Ravado (author) from England on November 11, 2011:

Brett

Well spotted - I agree. (Changed the graphic) Thanks!

Brett on November 10, 2011:

Shouldn't your graphic read I = sqrt(P) / R instead of I = sqrt(P) / V?

Rik Ravado (author) from England on March 12, 2011:

Thanks for the feedback Bert - you are so right with U+P=R.

Learning is much more effective when you put what you have learned into practice!

bert k on March 12, 2011:

thanks for the free lesson. you made it very clear and easy to understand. here is a smart formula i'd like to pass on to beginners like myself. U+P=R or utilization plus practice equals retention. i wish there had been internet and google when i was a kid. best regards teach. bk

topquark from UK on February 07, 2011:

Useful and well-presented article! A lot of people seem to get confused about Watts and Amps and Volts - an explanation like this has been long overdue.

bernie on October 12, 2010:

Everyones a cridic. I understood completely and I understood that it is not on the money, and those of us who don't have to act like know it all's realize that there are factors involved that makes the math close, but there is room for error. Maybe thicknees of the wire being used, battery losing power, or whatever the reason. People chill!!! If I understood and realise this then you brain childeren should. Anal Bastards.

Kevin on October 03, 2010:

Thanks for this mate.

Helped a lot with my schoolwork. :P

Rik Ravado (author) from England on June 09, 2010:

Thanks Peter!

Peter Enmore on June 08, 2010:

Yeah, you really can't get the calculation it to an exact. But you can definitely get an approximate.

Rik Ravado (author) from England on February 06, 2010:

Elk

Thanks for your comments - I still maintain 120v is safer than 230v. In other words the lower the voltage, the less current potentially flows through the victim.

I agree with your 'approximately' as obviously cable and other elements in the circuit will further lower the current. Also the rating of the bulb or other appliance will not be precise and there will be a margin of error.

Thanks for stopping by and commenting!

elkangorito from Thailand on February 06, 2010:

Even though your blog is an excellent "guide to the basics of electricity", there are a couple of things that need to be considered:

You said, "In the USA, the domestic supply is typically 110V, much safer than the UK."

This is not true.

Firstly, the voltage in the UK is officially 230v (single phase, domestic).

Secondly, the voltage in the USA is 120v (domestic).

120v is in no way "safer" than 230v. A "safe" voltage is considered to be less than 50v RMS (AC) or 120v DC (ripple free). These voltages are known as "extra low voltages" & relate to "touch voltage limits".

You also said: "Finally, remember that if you know the voltage of your power supply then you can calculate the current through a bulb, fuse or other electrical appliance or component, based on its rating in Watts."

I think that you meant to say, "Finally, remember that if you know the voltage of your power supply then you can APPROXIMATELY calculate the current through a bulb, fuse or other electrical appliance or component, based on its rating in Watts."

Rik Ravado (author) from England on November 04, 2009:

LiamBean - glad you found it helpful!

LiamBean from Los Angeles, Calilfornia on October 23, 2009:

Lovely. Wonderful explanation of Watts, Amps, and Current.