5 ms·

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 disch

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.

B1FF_PSUVM 10mo ago
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 ...
SigmundA 10mo ago
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.
- hunter2_ 10mo ago
  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.
  - SigmundA 10mo ago
    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.
    - hunter2_ 10mo ago
      Ah! I guess that explains the "f" for "force" in the imperial abbreviation "ft-lbf", to distinguish it from work. I wonder if there's ever been an analogous variant for metric such as "Nmf"...
      - gpm 10mo ago
        Hmm, I thought lbf was to distinguish the force unit from the mass unit (1 lbf = G * 1lb mass)
        hunter2_ 10mo ago
        It seems the common thread is that the f means to introduce G, but not exactly. In my own research, the AI summaries are about as sloppy as I've ever seen, due to the vague and often regional differences (with the difference between ft-lb and lb-ft sometimes being described as relevant, as well).
svpk 10mo ago
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.
- selcuka 10mo ago
  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/ https://what-if.xkcd.com/11/
  - lostlogin 10mo ago
    > 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.
    - skissane 10mo ago
      Also the UK gallon is different from the US gallon. And the same applies to all the other non-metric fluid measurements such as pints and fluid ounces. Historically the UK gallon was used throughout the former British Empire (Australia, Canada, India, Ireland, Malaysia, New Zealand, South Africa, etc). By contrast, almost nobody ever officially used the US gallon except for the US (and a small handful of highly US-influenced countries such as Liberia).
      - throwawaymobule 10mo ago
        Each standardised on a different gallon. Prior to that, gallons depended on that you were measuring. One, a beer gallon, the other a wine gallon. The US still also has 'dry gallons' for things like pints of blueberries.
acyou 10mo ago
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.
tirant 10mo ago
Don’t stay there: EVs are even reporting consumption in terms of kWh/100km or kWh/100miles instead of just average kW.
- ekr 10mo ago
  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.
  - SyzygyRhythm 10mo ago
    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.
    - stavros 10mo ago
      I understand all this but the most important question for me is definitely still "how much distance can I cover on a charge"? That's why I prefer kWh/100km.
    - tor825gl 10mo ago
      The problem with this is that destinations are a fixed distance away, whereas their time distance is not fixed. In most journeys people want to reach a specific place rather than drive for a given amount of time.
  - catlikesshrimp 10mo ago
    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)
fulafel 10mo ago
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...
- vanviegen 10mo ago
  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! :-)
  - hgomersall 10mo ago
    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.
hamdingers 10mo ago
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.
GuB-42 10mo ago
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.
- Waterluvian 10mo ago
  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.
  - nayuki 10mo ago
    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.

B1FF_PSUVM 10mo ago

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 ...

SigmundA 10mo ago

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.

hunter2_ 10mo ago
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.
- SigmundA 10mo ago
  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.
  - hunter2_ 10mo ago
    Ah! I guess that explains the "f" for "force" in the imperial abbreviation "ft-lbf", to distinguish it from work. I wonder if there's ever been an analogous variant for metric such as "Nmf"...
    - gpm 10mo ago
      Hmm, I thought lbf was to distinguish the force unit from the mass unit (1 lbf = G * 1lb mass)
      - hunter2_ 10mo ago
        It seems the common thread is that the f means to introduce G, but not exactly. In my own research, the AI summaries are about as sloppy as I've ever seen, due to the vague and often regional differences (with the difference between ft-lb and lb-ft sometimes being described as relevant, as well).

svpk 10mo ago

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.

selcuka 10mo ago
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/ https://what-if.xkcd.com/11/
- lostlogin 10mo ago
  > 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.
  - skissane 10mo ago
    Also the UK gallon is different from the US gallon. And the same applies to all the other non-metric fluid measurements such as pints and fluid ounces. Historically the UK gallon was used throughout the former British Empire (Australia, Canada, India, Ireland, Malaysia, New Zealand, South Africa, etc). By contrast, almost nobody ever officially used the US gallon except for the US (and a small handful of highly US-influenced countries such as Liberia).
    - throwawaymobule 10mo ago
      Each standardised on a different gallon. Prior to that, gallons depended on that you were measuring. One, a beer gallon, the other a wine gallon. The US still also has 'dry gallons' for things like pints of blueberries.

acyou 10mo ago

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.

tirant 10mo ago

Don’t stay there: EVs are even reporting consumption in terms of kWh/100km or kWh/100miles instead of just average kW.

ekr 10mo ago
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.
- SyzygyRhythm 10mo ago
  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.
  - stavros 10mo ago
    I understand all this but the most important question for me is definitely still "how much distance can I cover on a charge"? That's why I prefer kWh/100km.
  - tor825gl 10mo ago
    The problem with this is that destinations are a fixed distance away, whereas their time distance is not fixed. In most journeys people want to reach a specific place rather than drive for a given amount of time.
- catlikesshrimp 10mo ago
  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)

fulafel 10mo ago

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...

vanviegen 10mo ago
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! :-)
- hgomersall 10mo ago
  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.

hamdingers 10mo ago

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.

GuB-42 10mo ago

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.

Waterluvian 10mo ago
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.
- nayuki 10mo ago
  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.