Edit: Alt Text: Speed limit c arcminutes^2 per steradian.
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For something that doesn’t run continuously, like eg. a refrigerator, then an average daily usage is more useful, no? “This product draws 1.5 kW with a duty cycle of 0.08” doesn’t really help when comparing efficiencies of potential purchases, you’d need to convert it to electricity consumed in a set period anyway.
No, it’s because watts are joules per second, so kWh are (energy / time) * time. Cancelling the units would be expressing the energy directly in joules.
But the XKCD mentions kWh/day specifically, in theory the times can cancel out, leaving you with kW
But instantaneous and average kW are very different, and it would take more time to describe that distinction than to use kWh/day.
Ok that’s fair, I kinda glossed over that part. Both are valid interpretations, I think.
Exactly, it’s a unit of convenience, not a unit of abstract precision.
Even a unit of “gallons/sqft” could be handy in the right context. If you were trying to design a storage solution for discretely packaged product for example, it could be a figure of merit despite literally factoring out to a unit of length.
I could imagine a scenario where gal/ft² is useful. Like with grocery store shelving figuring shelving and product stacking. If liquid storage containers are stackable then you have have more gallons per square footage of shelf space. Or of they’re not stackable, then taller containers would hold more liquid in the same shelf space than shorter containers with the same footprint.
Yeah it seems odd to represent something as a volume/area, but that is the relevant information you’re comparing and it’s intuitive how that number changes based on changes to volume as projected onto an area. Bigger number points toward a more efficient use of shelving.
Yeah a chest freezer is a good example of a situation where both are useful things to put on the tag
I can’t imagine why kWh would be more convenient than MJ though.
Because people have an intuitive feeling about how much 1 kW is, because they use devices with a power rating in Watt and have a feel for how powerful a device is at what rating. People also know exactly what an hour is. So it makes sense to think about a device of 1kW running for 1 hour, people have a good sense of how much energy that is in daily use. Since most energy bills are also in terms of kWh, people also have a good sense about the costs of that energy.
Given the popularity of the unit, I think people like it, otherwise a different unit would have been used already.
Right, so it’s only the popularity of the unit. If everyone would use MJ that’s what people would be used to and there’d be no real difference.
Yeah i haven’t dealt with joules on a regular basis since college. They may as well be coulombs to my instinctive understanding
Because the power draw of appliances is measured in watts, so a 60 watt light bulb when lit draws 60 watts of power over the course of one hour. So if I have roughly 100 lightbulbs at 60 watts hooked up to my house, then I’ll be using 6 kW of power each hour.
It tells us more information about the rate of use of that energy. It’s like the difference between a 2 lb sphere of uranium being exploded in a fraction of a second vs 2 lb lf uranium fuel in a reactor operating for however long that much fuel lasts for. Both contain the same amount of joules of energy at the end of the process, one just uses all of those joules in one go and the other slowly releases that energy over a longer period of time.
kWh is just a measure of energy though. B it says nothing about the time in which it’s expended. It’s possible to use a kWh in a minute.
Glad to see reading comprehension is at an all time high and I definitely didn’t explain how total joules doesn’t actually mean anything for something drawing power in relation to the time its drawing power. And I didn’t make any comparison about how a 2lb lump of uranium contains the same energy whether it’s detonated in a bomb or slowly released in a reactor.
My point is that kWh is the same. It doesn’t say anything about time. 1 kWh is 3.6 MJ. There is no difference except the factor 3.6.
The important thing is that leaving units uncanceled is a valid way to communicate the relevant factors of what a number represents.
Yes technically kWh cancels down to joules, but that doesn’t communicate the relevant info of how a device uses that energy during a period of time. In other words Work (Watts) multiplied by Time (hours).
Uranium has 2x10¹³ joules of energy stored. You can use all that energy at once in a bomb and explode a city in a second, a lot of Work done very quickly, ooooor you could put it into a reactor and power a city and do a lot of Work during a much longer time period.
You could also list some long term average power draw instead of the peak.
There’s nothing wrong with kilowatts, it’s an SI unit. The problem is hour, which is 3600 seconds, and we have ancient Egyptians to blame for this, who divided the day into 24 hours despite having already developed base-10 numerical system.
Kilowatt per kilosecond, which is 1 megajoule, would work better.
Damn, I was convinced it was the Babylonians with their base-60 system.
That actually works great.
60 is cleanly divisible by 1, 2, 3, 4, 5, and 6, 10, 12, 15, 20, and 30.
10 is cleanly divisible by 1, 2, and 5.
I have no problem remembering what 1 is divisible by.
Yeah Babylon was very clever but also looking at their math and writing makes it clear why they had to have a class of people to do their math and writing
I really don’t get the issue with kWh. Things are rated in W and we mostly care about the hours they’re powered on. If I wanna figure out how many kWh a PC that needs 300W used in 4 hours, I multiply 300*4. If I wanna know how many joules it used, I have to do 300*4*3600. Only one of those can be done in your head in 3 seconds.
You gotta use an escape character, specifically a backslash ( \ ), when dealing with *s on lemmy.
Otherwise you end up with “stufflike this!”
When it could have been “stuff*like this*!”
ETA: Damn, you’re good. Fixed it before I even finished this post!
That’s 203.7cm for anyone wondering
Assuming US Gallons. With Imperial Gallons, it’s 2.447 m
My car needs 0.07 square millimeters of fuel on the highway.
Isn’t that crazy efficient? I seem to remember about 0.3mm²?
Way back of you asked Google “38 mpg in mm^-2” it would tell you.
I love that it’s the size of the thread of fuel you would consume as you drive down the road.
Edit: oh no, that’s about right. It’s a diameter of about 0.25 mm. I think that’s what I was thinking of.
Not sure. I asked my cat Jeepity.
I need the alt text!
Sorry! I added it now.
First of all, thanks for posting new threads here, it was getting somewhat dull that I was in recent weeks always the first one, sometimes after several days.
You might want to post comics the same way I did: https://discuss.tchncs.de/post/28278776 i.e. not just with the alt text, but also with a link to explainxkcd. You can easily copy the alt text if you switch to m.xkcd.com.
Going from Watts to BTU’s while researching for a solid state multi-power-state TEC cooling solution. I feel this.
6’8” in Murica units.
If we just used watts life would be so much easier.
The beauty of SI units is you just add or remove zeroes.
But what temperature is that at? And what is the ambient temperature? And what if the power is not at exactly 120V? And what about if I put a fresh dead hooker in it every day?
