This is about correct spelling of the symbols for units of measurement. I expect some of you will think this is mere pedantry, but please bear with me, particularly if you'd like your writing to appear more professional, or if you simply want to be respectful to your readers. I'll keep it simple.
Why do I say "respectful to your readers"? Well, for tHoSe oF uS wHo aLrEaDy kNoW tHe cOrReCt sPeLlInG revolution seconds, reading the wrong spelling is kinda like reading that sentence. It's painful.
The title of this topic comes from one of the most commonly misspelled unit symbols on this forum — the symbol for kilowatt hours. I'm not going to deal with the problem of people using kilowatts (instantaneous power) when they really mean kilowatt hours (accumulated energy). That's a battle for another day. I'll stick to the spelling problem for now.
Why do I call them "symbols", and not "abbreviations"? Because an abbreviation is any old shortening of a word. In the case of measurement units, we can't afford to use just any old shortening. There are too many of them and they would be too easily confused. So a specific symbol has been standardised for every unit and every multiplier, like the symbols for the chemical elements.
Using the forum search, I found the following 27 different spellings. Almost all of them are symbols for some metric unit. But only one of them is the symbol for kilowatt hours. I'll give you some simple rules (and the reasons for them) so you can figure out which one is correct.
kwh kwH kWh kWH Kwh KwH KWh KWH kwhr kWhr kWHr Kwhr KwHr kwhs kWhs KWhs KWHs kwh's KWh's kwhrs kWhrs kWHrs Kwhrs KWhrs KWHrs kwhours kWhours
Rule 0. Use the standard symbols, or spell all the words in full. Don't make up your own abbreviations and don't mix words and symbols.
In most cases the symbol consists of a single letter for each sub-unit and a single letter for any multiplier. Because there are so many units and multipliers wanting letters, upper and lower case letters have different meanings.
Rule 1. A unit symbol is uppercase if and only if the unit is named after a person.
Uppercase "K" is the symbol for the kelvin, a unit of temperature named after one Lord Kelvin — sometimes called the "absolute" temperature scale. So we don't want an uppercase "K" here.
The watt is named after James Watt of steam engine fame, so it gets a capital "W". A lowercase "w" could mean "week" (although "wk" is preferred).
The hour isn't named after anyone, so it gets a lowercase "h". An uppercase "H" is the symbol for the henry, a unit of magnetic inductance named after one Joseph Henry.
And because the hour is a very common unit, it gets to have a single character. So "hr" is not a symbol for hour. It is a compound symbol for "hour revolution", because "r" is the symbol for "revolution" as in r/min = revolutions per minute.
If you are wondering why bottles have uppercase "L" for litre, I recommend you do an internet search on "Claude Litre".
Rule 2. A multiplier symbol is uppercase if and only if the multiplier is a whole number (not a fraction), with a nasty exception — lowercase "k" for kilo.
In general, it's big letter= big quantity, but because we already have uppercase "K" for the unit named after Lord Kelvin, the multiplier kilo (= 1000) gets a lowercase "k".
Rule 3. Don't add "s" to a symbol to make it plural.
The same symbol is used for both plural and singular. You must not add "s", because "s" is the symbol for seconds. Apostrophe "s", as in 2 m's, is also unnecessary. "2 m" is simply read as "two metres".
Now you have all the rules you need to find the one correct symbol for kilowatt hours above, and you can work out what the other spellings are really saying, and understand the title of this topic. And when you consider the misspellings that end in "hrs", you should also understand why I threw the seemingly-random words "revolution seconds" into my hard-to-read sentence example near the start of this article.
And by the way, we never use "p" for "per" in the metric system. That way we can tell the difference between miles per hour "mph" and metres per hour "m/h". So, kilometres per hour is km/h, not kmph and not kph. Those would be kilometre picohours and kilopoint hours.
Rules for the full names
I note that the above rules are only for the symbols, not the full unit names. When you spell out a unit name in full, you make a plural in the normal English manner, i.e. by adding an "s", provided the word doesn't already end in "s" or "x" — and henry becomes henries. And, except at the start of a sentence, you do not capitalise any multiplier when spelled in full, and you do not capitalise any unit name when spelled in full, even if it is named after a person. A lowercase "watt" is a rate of energy conversion. A capitalised "Watt" is a person — a member of Mr and Mrs Watt's family.
So that you can see just how bad it can be, when people use the wrong case for unit symbols, I offer the following "A to Ω of unit symbols".
If you're interested in some of the finer points of unit presentation, you might enjoy this sequel.
A to Ω of unit symbols
a atto (10⁻¹⁸), are (100 m², as in hectare), year (annum)
A ampere (alternative full name "amp" unofficial but very common)
b bit, barn
B byte, bel
c centi (10⁻², deprecated)
°C degree Celsius
cu cubic (not metric, use superscript ³ or "^3" for metric symbols)
d deci (10⁻¹, deprecated), day
D deca (10¹, when a single-character multiplier is required, deprecated)
da deca (10¹, deprecated)
e electron (in "eV" electronvolt)
E exa (10¹⁸)
f femto (10⁻¹⁵)
G giga (10⁹)
h hour, hecto (10², deprecated)
i binary (as in "Ki" for kili = 2¹⁰, Mi for megi = 2²⁰, Gi for gigi = 2³⁰, etc., used with bits and bytes)
I (not used, too easily confused with digit 1 or lowercase letter L)
j (not used)
k kilo (10³)
Kg kelvin gram
kn knot (nautical mile per hour)
l (litre, but too easily confused with digit 1 or uppercase letter i)
m milli, metre, mile (in "mph", otherwise "mi", not metric)
M mega (10⁶), molar (deprecated)
mi mile (except in "mph", not metric)
n nano (10⁻⁹)
NM nautical mile (also "nmi")
o octet (8 bits, when "byte" might be ambiguous)
O (not used, as too easily confused with digit 0)
p pico (10⁻¹²), per (not metric, use "/" for per in metric), point (not metric)
P peta (10¹⁵)
pa per annum (not metric)
q (not used)
Q (not used)
R ohm (alternative when Ω is not available)
sq square (not metric, use superscript ² or "^2" in metric symbols)
T tera (10¹²), tesla
u micro (10⁻⁶, alternative when µ is not available), atomic mass unit
U rack unit (not metric)
w week (but "wk" is preferred)
x (not used, as too easily confused with multiply sign)
X (not used)
y yocto (10⁻²⁴), year
Y yotta (10²⁴)
z zepto (10⁻²¹)
Z zetta (10²¹)
µ micro (10⁻⁶) (greek letter lowercase mu)
Ω ohm (greek letter uppercase omega)
The above is not intended to be a complete list of unit symbols — not even a complete list of metric unit symbols. But it does give all single character metric unit and multiplier symbols, and a few double character symbols and non-metric symbols where, if you change the case, you get a completely different but still valid meaning.
* Only joking. See Knuth D, 1957, Mad #33, halfway down the right hand side of page 2.
This series of articles was inspired by this wonderful post by Coulomb, in the My Nissan Leaf forum.
[Edit: Deleted as incorrect: "People have died because drug dosages were described using "mcg" as an abbreviation for micrograms instead of the correct µg." This is not correct. I misinterpreted the problem. The actual problem is with badly-hand-written µg, perhaps looking like ۳ᵍ or ʍg, being read as mg and resulting in 1000-fold overdose. So mcg is actually preferred to µg in handwriting. The recommendation is for doctors to write "micrograms" in full. This is not a problem in typed material.]