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Comment by atwood22 | Hacker News Reader

> Why can't journalists use units correctly?

> Side remark: it's actually kind of cool that you can literally specify the battery output of these factories in Watts.

The unit is Watt hour storage capacity.

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I think he means the capacity will be 40 GWh / year = 4.56MW.

Sorry I’m not sure I understand. Watt hour is a unit of energy. Energy can be stored. They’re making batteries that store energy. A Watt is a unit of power. They’re not making power. So after a year of making batteries, they will have produced exactly 0 Watts. I guess in that sense you can measure their (power) output in Watts: they produce 0 Watts.

I think the confusion is that the unit of their product is not GWh. It’s (GWh of capacity) which is not the simple product of Watt and hour)

How much battery capacity are they producing per day? Impossible to tell from the article. I’m relatively sure that the period of time the specified capacity is produced in is one year.

You can divide the hours in GWh by the year time span and the time units cross out. The unit is Watt, but the physical quantity it’s describing is arguably not power but energy capacity production rate or something similar.

There’s no 1:1 mapping between units and physical quantities, just think about specific impulse in rocket engines being quantified in seconds.

The unit is not GWh. It's "GWh of storage." It's ironic that this thread started with you lamenting that units are used incorrectly.

The unit is GWh, the physical quantity it describes is “energy storage capacity”. Those two are not the same.

Take this example:

Unit: second, physical quantity: could be things like “time”, “specific impulse”, “distance” (light-second), etc. Many physical quantities can be measured in seconds.

Is it really true that you can measure distance in seconds? I think light-seconds are different units from seconds hence why they measure distances, similar to how Watt-hours and Watts are not the same unit. If a second was a measure of distance then speed would be unit-less (second / second) as opposed to the speed of light which is light-second / second ≈ 3e8 m/s.

You are right, light seconds don't fit in well. My point still stands, you can't really know what physical quantity is measured just from looking at the unit alone, that's now what units are made for.

You can still divide GWh of capacity by a year and get GW of capacity, ie the instantaneous production rate of capacity.

No you cannot. Watt hour is a unit for energy derived by applying power over time. That’s why when you divide by a period of time you’re left with power. The unit “GWh of storage” is a complex unit that’s not easily broken up. When you divide by time you get a new unit, “GWh of storage per unit time”

You are conflating two different concepts here. One is "unit" which is a measure of a physical quantity (ie "GWh"). The second is the physical quantity itself ("battery storage capacity"). The relationship between the two is that a unit can be used to quantitatively describe a physical quantity ("5 GWh of battery storage capacity"). Physical quantities like "battery storage capacity" can usually be quantified in different units which can be converted to each other ("5000kWh = 5GWh = 1.8 * 10^13 J"). The unit itself ("GWh") doesn't describe precisely what physical quantity is being measured. It could be mechanical energy stored in a flywheel, the total heat output of a chemical reaction, battery storage capacity, etc.

You’re fundamentally misunderstanding the units. I don’t know what else to say. “Watt hour of storage” is not the same as a Watt hour. You can’t divide by time to get power. That should be obvious.

"Watt hour of storage" is not a unit, it's the physical quantity "storage" quantified by the unit "Watt hour", ie the combination of the two.

That’s what you’re not understanding. “GWh of storage” is a unit. It’s not an SI unit, but it is a unit, i.e. a thing you can use to measure something. The production of this factory is being measured in this storage unit. Watt is also a unit, a unit to measure power. There is no way to convert a unit of power to this unit of storage.

Do you have a reference for the definition of this energy storage unit? I have never heard of it before and my attempts at searching the web seem to contradict you.

Wikipedia writes: “Storage capacity is the amount of energy extracted from an energy storage device or system; usually measured in joules or kilowatt-hours and their multiples”

https://world-nuclear.org/information-library/current-and-fu... writes: “Storage systems for electricity include battery, flywheel, compressed air, and pumped hydro storage. Any systems are limited in the total amount of energy they can store. Their energy capacity is expressed in megawatt-hours (MWh), and the power, or maximum output at a given time, is expressed in megawatts of electric power (MW or MWe).”

Both of which to my layman reading opposes your claim.

You decided all on your own that GWh of storage cannot be divided by a time unit to create another 'new' unit, GW of storage. Repeat: _You_ decided that. I can explain it simply: One GW of storage is equivalent to producing one gigajoule of storage per second.

We really need to rename these things to be based on Joules instead. A lot more intuitive.

This battery stores X Joules. 1 J = 360 kWh

This battery releases Y Joules per second. 1 J / s = 1 Watt

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