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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2Based on the answers, it seems the last part of my question made no initial sense. Watt as a unit of measure is aligned with hour as its time component, it seems, but a watt-hour is a measurement, not a rate. The rate is the watt itself, and yes, again, one hour is the time component base unit of measure for a watt. So a 60-watt light bulb is measured in watts, not watt-hours, but it burns 60 watt-hours after one hour. – stimpy77 Feb 13 '15 at 19:13
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"Watt as a unit of measure is aligned with hour as its time component" Hours have nothing to do with watts. "a watt-hour is a measurement, not a rate" Yes, a measurement of total energy. "rate is the watt itself" Watts are a measurement of rate, yes. "it burns 60 watt-hours after one hour" Yes. – endolith Jun 21 '17 at 13:54
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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. :)
endolith
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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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3In 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 02 '11 at 17:19
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1@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 04 '13 at 10:16
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"watts are like buckets per hour"
You confused me even more. Watts are Joules/sec so if energy are buckets then a watts should be buckets / sec.
'the "per hour" is inside the term "watt"' I thought "per second" is inside watt.
– Mr. Roland Jun 21 '17 at 10:08 -
@Mr.Roland A "bucket" in this case holds 3600 joules of energy, then. Don't worry about hours vs seconds or joules vs calories, I'm trying to describe the concept. Joules are units of energy. Watts are units of energy/time. kWh are units of energy. – endolith Jun 21 '17 at 13:52
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Can someone please explain then how to calculate amperage based on watts-hour? If a watt equals a joule per second, shouldn't I use 60/3600 watts to calculate amperage using formula Watts / Volts = Amps? I would like to calculate amperage of an air conditioning unit that is rated 2250W under 220V. – demian85 Jun 16 '18 at 16:06
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@demian85 Yes, amperage would be 2250 W / 220 V. Keep in mind 1. It might not actually draw that much on average, and 2. it's RMS current. – endolith Jun 17 '18 at 17:13
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@JasonS This congressman got it wrong, too. Says "4 terawatts" when the actual number is "4,000 terawatt-hours" or "4 petawatt-hours": https://www.youtube.com/watch?v=Uo5Bi_cs2AU – endolith Aug 30 '23 at 14:48
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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.
Toby Jaffey
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supercat
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1+1 for decoupling watts and hours. (However watts do have a reference time unit as part of their definition: W = J/s.) – wjl Aug 02 '11 at 17:15
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1@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 02 '11 at 17:49
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The concept of 'Watt-hours' as Watt x Hours will be confusing to someone who cannot conceptualize Watt - being 'energy used per amount of time'.
I sometimes try to explain this using more familiar concepts: If we use the term 'Keem' instead of 'km/hour', one could use 'Keem-Hour' to describe distance travelled—going 60 Keem for half an hour means you've travelled 30km i.e., 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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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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1Thanks, "Pingswept". Also, what is different about our two answers? Is this just a case of the topmost effect? – Mark C Oct 05 '10 at 06:15
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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).
eruditass
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