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.

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svpk an hour ago | parent | next [-]

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.

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selcuka an hour ago | parent [-]

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/

	▲	lostlogin 32 minutes ago \| parent [-]
		> 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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B1FF_PSUVM 3 hours ago | parent | prev | next [-]

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 ...

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SigmundA 3 hours ago | parent | prev [-]

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.

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hunter2_ 2 hours ago | parent [-]

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.

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SigmundA an hour ago | parent [-]

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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hunter2_ an hour ago | parent [-]

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 an hour ago \| parent [-]
		Hmm, I thought lbf was to distinguish the force unit from the mass unit (1 lbf = G * 1lb mass)