So, when using the kelvin temperature scale, it's lower case when using "kelvin", but uppercase when using the abbreviation "K"? That annoys me as kelvin is a name and even my phone wants to capitalise it.
The Kelvin(uppercase) scale is named after Lord Kelvin(uppercase), the unit is the kelvin(lowercase), and the symbol is K(uppercase).
This agrees with all SI units named after a person. The unit's name is lowercase (unless at the beginning of a sentence), and the symbol has its first letter in uppercase. Examples: newton (N), tesla (T), hertz (Hz), pascal (Pa), joule (J), watt (W), volt (V), ampere (A).
Note that "degrees Celsius" is capitalized in a special way.
See also: "I bought one bitcoin (1 BTC) on the Bitcoin network."
I've always assumed "giga" is capitalised because it one might confuse a lower case g for "grams". Technically, 1gm is one gram-meter (grams * metres), whereas 1Gm is a thousand kilometres. You don't often encounter weird combined units like gram-metres outside of physics, except maybe for kWh (kW * 1 hour), but I don't see why you wouldn't be nice to physicists and give them the extra clarity.
Other order of magnitude indicators are capitalised to distinguish them from their smaller counterparts (millimetre/Megametre).
Wikipedia states that the distinction is because units named after a person always start with a capital letter. I've always assumed Hz has to be capitalised to prevent confusion with the hecto prefix (hHz being 100Hz). I don't think there's a unit or order of magnitude that's abbreviated to `z`, so in theory "Hz" could just be "H",
I think Hz is more readable, but that's probably because I'm used to it.
I had structural engineering and electronics in school and especially when you get compound units like Nm (newton meters) not wondering if the N is a newton or the SI-prefix nano is not nothing.
In the end the reason for the weird capitalization is just to avoid situations where that ambiguity will occur.
I remember me and a friend asking our physics teacher if it wasn't a problem that tera (T) and tesla (T) were the same symbol. He looked at us, paused a bit, then explained how absolutely enormous a terra tesla would be, and thus why that isn't actually an issue :P
I disagree with that rule. I get that people spell liter, gram, meter in lower case, but units based on names of historic persons like Volt, Ampere, Coulomb, Pascal and Kelvin deserve to be capitalized.
The unit symbols for those units (A, C, P and K) are capitalized, but the units when spelled out (ampere, coulomb, pascal and kelvin) are not, just like the spelled out forms of liter, gram and meter.
Capitalisation isn't universal. Languages like German capitalise every noun, so from a German perspective, Gram, Metre, and Litre should all be capitalised and it's the English that are wrong for using lowercase units.
Meanwhile, English capitalises stuff that other languages don't, like the names of languages, times (days and such), and for some reason "I". By the time the metric system was conceived, English still capitalised most nouns (just look at the US constitution for example) after very recently coming into contact with the concept of capital letters in the first place, something copied from German printing presses.
And then there are the many languages that don't even have capitals, which will probably question why there are two ways to write every letter when they first learn languages with a Latin alphabet.
Nothing "deserves" to be capitalised, every language just decided to stick to some arbitrary rules. There's no good reason why `Monday` is more important than `website`, or why `I` is more important than `you`.
