Coppersmith
Senior Member
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- Tampa, FL, USA
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- Electrical Contractor
:? I might be confused but ....
The Watt is a unit of power therefore a quantity. Watts per hour would be a rate.
This is your brain on power
- Thread starter Coppersmith
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:? I might be confused but ....
The Watt is a unit of power therefore a quantity. Watts per hour would be a rate.
kwired
Electron manager
- Location
- NE Nebraska
I was going to say about the same thing.
Ultrafault
Member
Watts per hour makes no sense, becuase, like you guys said, a watt is a rate. Watts per hour would be analogous to an acceleration. We all know Watt-hours is a quantity of energy, As are Joules, one watt-hour is equal to 3600 Joules.
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- Placerville, CA, USA
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- Retired PV System Designer
No. The Joule is a unit of energy and the watt is one Joule per second.
A 100 watt light bulb uses energy at a specific rate rather than containing a specific total amount of energy (like, for example, a battery).
An amount of power per unit of time is not a particularly useful quantity. Example: "His energy use increased by 1kW per hour. He used 1kwh the first hour, 2kWh the second hour, etc."
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Ultrafault
Member
Im reading this stuff to avoid doing my thermo homework and only end up talking about thermo. Go figure.
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
There's a youtube video of this guy, Olympic cyclist I think. You should see the legs on this guy. He was able to do about 700 watts for almost a minute. He was on the floor completely spent after. Its crazy how much energy is in a gallon of gas compared to what us puny humans can do.
JFletcher
Senior Member
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- Williamsburg, VA
Would it make more sense to say the human body generates about the same amount of energy that a hundred watt light bulb uses?
Perhaps I'm misremembering what I read. Heck is bad as my memory is, I might be quoting you something out of The Matrix. LOL
Watts per hour is indeed a rate. 100 watt light bulb uses 100 watts per hour or 2.4 kilowatts per day if were on continuously.
JFletcher
Senior Member
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- Williamsburg, VA
That is very impressive for a human. Almost 1 horsepower for 1 minute.
At some point though, speed is the limitation, and torque comes into play. I would think that somebody on a rowing machine could generate far more torque via using their arms and legs, and not be limited by their body weight.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Here is the link to that
https://youtu.be/S4O5voOCqAQ
Like you said, what would be perhaps more interesting is when weight is a consideration. The sikorsky challenge (human powered helicopter) was just achieved several years ago. The operator wasn't very big, and he could use his arms too.
https://youtu.be/S4O5voOCqAQ
Like you said, what would be perhaps more interesting is when weight is a consideration. The sikorsky challenge (human powered helicopter) was just achieved several years ago. The operator wasn't very big, and he could use his arms too.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
No!
A 100W light bulb uses 100 Watt-hours (Wh) per hour.
Looking at it slightly differently, 100Wh/h = 100W.
There is a fundamental difference between using 100W for an hour, which makes sense, and using 100W per hour which makes no sense.
ggunn
PE (Electrical), NABCEP certified
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- Location
- Placerville, CA, USA
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- Retired PV System Designer
If you did, you'd both be wrong.
:thumbsup:
gadfly56
Senior Member
- Location
- New Jersey
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- Professional Engineer, Fire & Life Safety
You remember correctly. The average human consumes 2,000 calories per day. Those are large calories; it is the same as 2,000,000 thermodynamic calories. That is the same as 2,326 watt-hours. For a 24-hour day, that would be a rate of 2,326/24 = 96.9 watts. Obviously you pump out more during the day when active so 100 watts is a reasonable ballpark. It's the easy way to estimate HVAC loads for a given space. How many people x 100 watts. This also means that 20 watts is not an outrageous estimate for what keeps the ole' noggin ticking away.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
He remembers correctly that the human body dissipates about 100W (while awake and active, or averaged over the whole 24 hour cycle? It also varies with ambient temperature because of the need to maintain body temp in a cold environment.)
But the rest of his remembrance is flat out wrong.
gadfly56
Senior Member
- Location
- New Jersey
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- Professional Engineer, Fire & Life Safety
His line 1 is correct.
His line 3 is not correct. As pointed out, Watts are already a rate, it is incorrect to add "per hour".
I think a major source of confusion is the fact that we use BTU's as a unit of energy and blithely use it interchangeably with Watts and forget that one is a rate, the other is a quantity.
JFletcher
Senior Member
- Location
- Williamsburg, VA
You are correct. A watt is a unit of power just like horsepower, and it would not make any sense to say like your car makes 300 horsepower per hour.
Could one use watts per hour if discussing a rate of change, such as a hundred watt light bulb dimmed down to 10W over the course of a minute?
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
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- Registered Professional Engineer
Yes.
But using multiple different units (seconds and hours) for the same thing in a single expression would be confusing.
Watts per second would be preferred.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Please reconsider. The rate of change of wattage is also a rate.
As in the example of a light dimming from 100W to zero Watts over 10 seconds is *dimming* at 10 Watts per second.
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drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
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- Registered Professional Engineer
Watts per second, or Joules per second-squared, could be correctly used to describe rate of change of power.
Watts per hour, or Joules per second per hour, could also be used, but it would be confusing.
But rate of change of power is something that almost never comes up, and it's a lame attempt to find a gnat's penis worth of redeeming value in an earlier post that was actually just a mistake.
Another place it might be used is to describe how quickly a powerplant can change its output to match a changing load.
Watts per hour, or Joules per second per hour, could also be used, but it would be confusing.
But rate of change of power is something that almost never comes up, and it's a lame attempt to find a gnat's penis worth of redeeming value in an earlier post that was actually just a mistake.
Another place it might be used is to describe how quickly a powerplant can change its output to match a changing load.
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