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Pardon me, I'm a total newb to electronics. My question is, when a device is measured in watts, such as a 60-watt light bulb, is this ALWAYS supposed to be assumed to be watt-hours, i.e. 60 watts per hour?

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5 Answers 5

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Energy is an amount, while power is a rate at which energy is used.

  • Energy is measured in watt-hours (W·h) or joules (J).
  • Power is measured in watts (W) or joules per second (J/s).

Watt-hours are like buckets, and watts are like buckets per hour. If you have 5 buckets of energy and you pour one bucket per hour, you'll be able to pour for 5 hours before you run out.

If you turn on a 60-watt light bulb for 1 hour, you have used 60 watt-hours of energy. If you use it for 2 hours, you have used 120 watt-hours of energy. If you turn it on for only 1 minute, you have used 1 watt-hour.

It's a little confusing since the "per hour" is inside the term "watt", so to make the rate into an amount, you need to multiply by a time unit to cancel it out.

It would be a lot more intuitive if we worked in kilojoules and kilojoules per hour. :)

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+1. The media gets this wrong all the time. –  Jason S Sep 30 '10 at 12:24
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The media is a great place to learn how to do things correctly ;-) –  Kellenjb Sep 30 '10 at 12:36
    
When the media does a report on something like terahertz, it kills me inside. –  Kortuk Sep 30 '10 at 14:47
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In fact, in most engineering and science, we DO use kilojoules instead of watt-hours. Especially since watts and joules are SI units and hours are not. (Hours are in the "units outside the SI that are accepted for use with the SI" category.) –  wjl Aug 2 '11 at 17:19
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@stimpy77 SI is the International System that standardizes metrics. It's a globally (sorry USA :)) accepted system to scientifically represents all kinds of measurements. –  clabacchio Dec 4 '13 at 10:16
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One point not yet mentioned: a 60 watt bulb will use 60 watt-hours per hour, or 60 watt-seconds per second, or 60 watt-microseconds per microsecond, or 60 watt-centuries per century. In other words, the "watts" part of the bulb's power usage has nothing to do with hours or any other unit of time.

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+1 for decoupling watts and hours. (However watts do have a reference time unit as part of their definition: W = J/s.) –  wjl Aug 2 '11 at 17:15
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@wjl: Remember, though, a joule is the amount of energy required to move an object against a static force of one newton, by the distance that light would travel in 1/299,792,458 second. Consequently, a watt is the amount of power required to push an object against a static force of one newton, at a constant velocity of 1/299,792,458 the vacuum speed of light. No time reference required. –  supercat Aug 2 '11 at 17:49
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Stimpy, the power rating tells you the rate at which the device consumes energy. So yes, a 60-Watt bulb will consume 60W*h or 0.06kWh of electricity in one hour. Watt-hours measure energy consumption. There is a simple little page here that shows some calculations.

I would also strongly recommend reading the HyperPhysics box on Work, Energy, and Power, and especially the links on the Power Concepts page.

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Thanks, "Pingswept". Also, what is different about our two answers? Is this just a case of the topmost effect? –  Mark C Oct 5 '10 at 6:15
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The concept of 'Watt-hours' as Watt x Hours will be confusing to someone who cannot conceptualize Watt - being 'enegergy used per amount of time'.

I sometimes try to explain this using more familiar concepts: If we use the term 'Keem' in stead of 'km/hour', one could use 'Keem-Hour' to describe distance travelled - going 60 Keem for half an hour means you've travelled 30km as 60 x 0.5=30

Just like a rental company that's interested in the distance your travelled in their car, the energy company is interested in the energy used - they will charge you per Watt-hour. If a Watt-hour costs 1c, it will cost you 60c if you leave a 60 Watt lamp on for one hour.

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You are correct in assuming a 60 watt device will consume 60 watt-hours in one hour, but the former (power) is a rate, the latter is an amount (energy).

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You mean that it will consume 60 watt-hours in one hour –  Ian Sep 30 '10 at 10:16
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protected by clabacchio Dec 4 '13 at 10:48

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