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I know the label says watts on the device, but since units of watts and kWh are used interchangeably to solve for amperage and calculating energy usage, it's not clear to me, and I can't find a definitive answer here, or online.

So, are the units in watts/s, or kWh? I know these units are different, and related, but how? Also, it would help to see examples to show the difference.

AL

AlwaysLearning
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    ”specify the units of the watt value” Watt IS the unit. I believe you are confusing units with physical quantities. ”watts per second” That makes no sense unless you are accelerating a generator, withdrawing the control rods from a nuclear power plant or similar. – winny Jun 24 '22 at 08:53
  • Obviously, I am confused because it's not specified, and I don't want to assume. Can you give concrete examples of the difference? – AlwaysLearning Jun 24 '22 at 08:55
  • Please look up what energy is. Look up what power is. Sit down and think about real world examples of what power is and what energy is and how they are and aren’t related. – winny Jun 24 '22 at 08:56
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    Power is Watts. Watts is joules per second. If something says it draws "1kW", it draws "1kW". So if you have it running for 1 hour, it will use 1kW hour – Puffafish Jun 24 '22 at 09:02
  • I can do that, but at least they can specify units on the label. It would help with the confusion. I've looked and read a lot, and it appears there is more confusion than clarity on this subject. – AlwaysLearning Jun 24 '22 at 09:04
  • I get that a watt is J/s. But is 1Kwh just the number of joules use in an hour? ... from the examples I've read here and online it's not one to one, or the same which is not helpful. – AlwaysLearning Jun 24 '22 at 09:10
  • There is only one kind of Watt (note the capital W as it is important). If you are buying or selling energy, then time comes into play eg: W/h or kW/h. We could talk about VA vs W, but you’d want to do the first year in AC circuit theory to grasp the concepts. – Kartman Jun 24 '22 at 09:31
  • As an aside, in my country we have a rating system for whitegoods that give, say, a fridge a rating of 1 to 5 stars and they specify an average yearly power consumption in kW/h. This is to simplify comparison between makes and models. – Kartman Jun 24 '22 at 09:35
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    Which do you have a problem with? Watts? Joules? Seconds? Hours? All of these are precisely defined. – Solar Mike Jun 24 '22 at 09:36
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    @Kartman The unit watt is not capitalized as per SI standard. W is capital. Surname Watt is capitalized. It’s the highest order you can receive as a scientist to have your last name all lowercased to be used as a unit. – winny Jun 24 '22 at 09:45
  • From my circuits textbook Watt (vi) is instantaneous power (absorbed & expended) as J/s, and 1 joule equals one 1 watt, and 1Wh is 3600J. So, what is 100W on a device label, is it 100W per hour or 100W per second? The difference in magnitude is huge. So, without specifying units it become a problem. I've learned not to assume. – AlwaysLearning Jun 24 '22 at 09:59
  • 100 watts is 100 watts of power. There is no time unit, so it is not energy. You get energy by multiplying power with time. If you operate your 100 watt device for one second, it will consume 100 watt seconds (joules) of energy. If you use it continuously for one hour, it will consume 100 watt hours of energy. Relationship between volts, amperes, watts, and joules. – JRE Jun 24 '22 at 10:48
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    @AlwaysLearning, "... and 1 joule equals one 1 watt" is not correct. I think it was a typo on your part but just to be sure, it's 1 J/s = 1 W. – Transistor Jun 24 '22 at 10:58
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    So, what is 100W on a device label, is it 100W per hour or 100W per second? 100 W means 100 joules per second....watts per second or watts per hour doesn't make any sense. It is like asking at what speed per hour you drove a car....you either ask what is the distance per hour or ask what is the speed.... – Mitu Raj Jun 24 '22 at 11:50

2 Answers2

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The watts label is in watts.

If your toaster says you use 1000 watts that is the same as 1 kilowatt. If you toast bread nonstop for one hour, that is 1 kilowatt-hour. If you toast bread nonstop for two hours, that is 2 kilowatt-hours.

Volts times amps equals watts. If the label only says watts and volts, you can divide watts by volts to get amps. If you live in America where the voltage is 120 volts the toaster uses about 8.3 amps.

These are called nominal ratings. These numbers are basically average numbers, and are good enough for most purposes, like estimating your electricity bill, or how many things you can plug into a circuit. If you actually measure the amperage, you might notice that, for example, the toaster might take 15 amps* for a few seconds while it heats up, because heating elements have less resistance when they are cold.

* (I'm not sure if this number is actually realistic)

user253751
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  • My confusion lies in the difference between j/s and kWh units. If possible, can you give a concrete, or mathematical example of the difference? – AlwaysLearning Jun 24 '22 at 09:31
  • @AlwaysLearning J/s and W are the same thing. 1000 W is a kW. 1 kW for 3600 s (1h) is a kWh. If you do the math you can also see that a kWh is 3,600,000 J. – user253751 Jun 24 '22 at 09:41
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    @AlwaysLearning you're asking the difference between "meter per second" and "mile" units – user253751 Jun 24 '22 at 09:41
  • @AlwaysLearning You are confusing energy and power again. Please look it up and think about it. – winny Jun 24 '22 at 09:47
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Your problem lies in the difference between power and energy. It isn't helped by the bassackwards definition of power that many text books use.

Text books like to tell you that power is how many joules divided by time. That is, w= j/s. That is as confusing as it gets. There is no device that can measure energy directly.

Approach it from the other direction. Use the things you can measure to work your way up to energy.

  • There are volts that you can measure with a voltmeter.
  • There are amperes that you can measure with an ammeter.
  • Power is given in watts.
  • One watt is 1 volt times 1 ampere. Calculate power from the measured voltage and current.
  • Energy is in joules.
  • One joule is one watt for one second. That's volts x current x time (in seconds.) Calculate energy from the calculated power and the measured time.

Devices are marked in watts. That is the power they consume. To find out how much energy a device consumes (how much the power company will bill you,) you multiply the power by the length of time you operate it.

Power companies use kWh (kilo watt hours) rather than joules because joules are very small units in comparison to the energy a typical home appliance consumes.

A 1000 watt device used for one hour consumes 1000 * 60 minutes * 60 seconds = 3600000 joules of energy. That amount on your electric bill would be 1kilowatt hour.

Devices are not marked in units of energy. The manufacturer cannot know how long you will use the device. The devices are marked in power only.

JRE
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  • @winny and you nailed it. I looked it up ... Energy (kWh) = Power (kilowatts or watts/1000) x Time (usage in hours). watts is the watts listed on the device. You're right my physics, circuits textbooks, and most online examples didn't help me, but just added to my confusion. The equation is worth a thousands words! Thanks for the help! – AlwaysLearning Jun 24 '22 at 12:57