Comment by Waterluvian
3 days ago
If 1 watt is 1 joule per second then, honestly, what are we doing with watt-hours?
Why can’t battery capacity be described in joules? And then charge and discharge being a function of voltage and current, could be represented in joules per unit time. Instead its watt-hours for capacity, watts for flow rate.
Watt-hours… that’s joules / seconds * hours? This is cursed.
I believe it's just a matter of intuitively useful units. There's simply too many seconds in a day for people to have an immediate grasp on the quantity. If you're using a space heater or thinking about how much power your fridge uses kilowatt hours is an easy unit to intuit. If you know you have a battery backup with 5 kilowatt hours of capacity and your fridge averages 500 watts then you've got 10 hours. If you convert it all to watt seconds the mental math is harder. And realistically in day to day life most of what we're measuring for sake of our power bill, etc. is stuff that's operating on a timetable of hours or days.
There are two types of jobs, the ones which require you to know that a day is about 8.5x10^5 seconds, and those which don't.
I use the conversion factor so often that I know it by heart: 1 day = 86400 seconds. I punch that 5-digit integer into a calculator, not an approximation like 8.5e5 (which is the same length, haha).
Is this sarcasm?
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True. Otherwise we would be using square meters for measuring gas mileage instead of miles-per-gallon (or litres-per-km) [1].
[1] https://what-if.xkcd.com/11/
Well, if you want to be pedantic, it's litres-of-fuel per km-driven. That doesn't cancel as nicely, if you don't drop the annotations.
Arguably, we should probably use kg-of-fuel (or mol) instead of litres-of-fuel anyway.
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> miles-per-gallon (or litres-per-km) [1].
The UK is metric except for distance and beer.
So the disgusting ‘miles-per-litre’ is presumably needed too.
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Plenty of people use Joules or rather kilojoules or megajoules or even gigajoules for various purposes.
Watt hours is saying, how long will my personal battery pack last me that powers my 60 W laptop? Which is also fine in that context.
1 Wh = 3600 Ws = 3600 J
It is not more cursed than km/h (1 m/s = 3600 m/h = 3.6 km/h)
Both those units are more convenient than their SI equivalent and their "cursedness" come from the hour/minute/second time division.
If we had decimal time, as it was initially proposed with the metric system, we wouldn't have this problem, but we weren't ready to let go of hours/minute/second.
Yeah. I get this is all kind of silly. I think what trips me up is that a watt doesn’t represent a timeless amount of something the way a meter does. A watt involves a unit amount of time.
Imagine if the distance between you and I was 438 kiloflerp-hours. And to get to you in one hour I have to drive at a speed of 438 kiloflerps. It works, it kinda makes sense. It just feels inconsistent with all the other units I work with.
You're right. If you really want to mess with speed and distance, just rename "nautical mile" to "knot-hour". In fact, that might be a great idea for trolling – it is fewer syllables (4 vs. 2), and aviation pilots definitely use knots for speed, so why not simplify the vocabulary and ditch the unique term "nautical mile" in favor of pairing two existing words?
Another place where the cursed unit of hour crops up is describing the amount of electric charge that you can pull out of a battery (especially rechargeable ones) in terms of millamp-hours (mA⋅h). Note that in actual SI, 1 mA⋅h = 3.6 C (coulombs). Even more cursed is high-capacity lithium-ion USB power banks that are advertised like 10,000 mAh (or even "10K mAh"), which should at least be simplified to 10 A⋅h (ampere-hours). But mA⋅h isn't a good way to describe batteries because you also need to multiply by voltage (3.7 V for Li-ion, I think 1.2 V for NiMH) to figure out the energy (usually expressed in W⋅h).
One more fun fact - photographic flash units are advertised in watt-seconds (W⋅s) for the maximum amount of energy delivered in a flash pulse of light. But that just simplifies to joules, which is a shorter and less confusing unit name. People really need to stop multiplying watts with time and use joules as designed in the SI.
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For me, one of the most cursed unit, but not because it is ill-conceived is the Nm (the unit of torque).
It is analogous to the Joule, but it doesn't mean the same thing. "This car has a 250 million ft.lb battery and 0.1 Wh of torque" passes dimensional analysis.
It's easier to figure out for people that measure power in watts and time in hours ... 1 kW for 1 hour is 1 kWh.
That camel's nose was already in the tent with the mAh thing in phone/etc batteries, now with electric vehicles we're firmly in kWh land.
Not to mention that's what the power utilities used all along ...
A watt of power multiplied by a second of time has an agreed upon name called joule, but a watt second is also a perfectly valid SI name.
A watt is a joule of energy divided by a second of time, this is a rate, joule per second is also a valid name similar to nautical mile per hour and knot being the same unit.
Multiplication vs division, quantity vs rate, see the relationship? Units may have different names but are equivalent, both the proper name and compound name are acceptable.
A watt hour is 3600 joules, it’s more convenient to use and matches more closely with how electrical energy is typically consumed. Kilowatt hour is again more directly relatable than 3.6 megajoules.
Newton meter and Coulomb volt are other names for the joule. In pure base units it is a kilogram-meter squared per second squared.
So when I torque all 20 of my car's lug bolts to 120 n-M, I've exerted 2/3 of a W-h? So if it takes me 4 minutes, I'm averaging 10 watts? That's neat. I wonder what the peak wattage (right as the torque wrench clicks) would be; it must depend on angular velocity.
Newton meter as a unit of energy is not the same as the newton meter unit of force for torque.
The energy unit meter is distance moved, while the force unit meter is the length of the moment arm.
This is confusing even though valid, so the energy unit version is rarely used.
You can exert newton meters of force while using no energy, say by standing on a lug nut wrench allowing gravity to exert the force indefinitely unless the nut breaks loose.
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Of course it can be. Nobody does it in practice because it's inconvenient.
Watts = volts * amps and the people working with batteries are already thinking in terms of voltage and amperage. It'd be painful to introduce a totally new unit and remember 1 watt for an hour is 3.6kj instead of... 1 watt-hour.
Don’t stay there: EVs are even reporting consumption in terms of kWh/100km or kWh/100miles instead of just average kW.
What people care about when talking about EVs and consumption is generally how much distance they can cover. If you take away the distance factor and just report power, it becomes meaningless/almost useless.
Many people think of driving in time rather than distance. I'd say it's actually more common to say a city is 3 hours away rather than 200 miles.
What makes kW less useful is really just that most EVs don't advertise their capacity very prominently. But if you knew you had an 80 kWh battery and the car uses 20 kW at freeway speeds, then it's easy to see that it'll drive for 4 hours.
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Directly reporting required power is still comparable among vehicles: 55kW vs 49kW, eg
Which is definitely less intuitive because it hasn't been introduced to the public, but is interchangeable in the same quirky way we already compare MPG (Distance/Volume) with lt/100KM (Volume/Distance)
Heh. To borrow an idea from xkcd (measuring gas consumption as area): The kWh measures energy, right? And energy is force times distance. So energy divided by distance is force! Let’s all start measuring EV consumption in newtons, folks. It even makes intuitive sense: It correlates well with how hard you need to push the car to get it going at the usual travel speed. But it sucks if you need to figure out how far you can travel on a given charge.
Yep, it's stupid from a units consistency pov. A bit like using calories instead of joules.
But on the other hand we also use hours for measuring time instead of kiloseconds...
Yeah, if only we would define seconds to be 13.4% shorter than that are, we could have 100ks days. Also, ksecs would be a really convenient unit for planning one's day: a ~15 minute resolution is just right for just about any type of appointment.
Oh, and 1Ms weeks, consisting of 7 working days and 3 off days sound nice too.
One can dream! :-)
Much better to make seconds slightly larger than 2 seconds, and move to a dozenal system throughout. One hour is (1000)_12 novoseconds. A semi-day is (10000)_12.
Oh, we should switch our standard counting system to dozenal a well.