Origin of 'kWh' and mAh' mixed case guidelines?

I’ve noticed that, for example, the, uh, “preferred” display of the abbreviations for “kilowatt-hour” and “milli-ampere-hour” are kWh and mAh, respectively. What is the reference source for indicating the mixed case as opposed to all upper or all lower?

I suspect it’s just the SI abbreviation for the kilo- prefix is ‘k’, the abbreviation for watts is W, and the abbreviation for hours is ‘h’. Smash it all together and you get kWh, etc.

I have only an incomplete answer, and it doesn’t appear to be consistent with your observations.

When in school, I was taught to always use lower-case letters to represent prefixes with negative exponents (e.g., “m” for milli- and “p” for pico-) and upper-case letters for prefixes with positive exponents (e.g., “M” for mega- and “P” for peta-). We always used a lower-case “h” for hours.

This explains mAh (milli-amp-hours), as it distinguishes it from MAh (mega-amp-hours).

As for the lower-case “k” in kWh, I got nothing.

Ampere and Watt are units named after people, so perhaps that’s why they are capitalized.

I think the Caps are proper names - A for Ampere, W for Watt, etc.

Correct. It is customary to capaitalize Watts, Amperes, Coulombs, Farads, Henrys, Ohms and the like.

Basically, you get the correct abbreviation for a derived abbreviation by concatenating the correct abbreviations for prefixes and units - which happen to be k m, W, A and h respectively.

BTW, how would you know if a MAH is a milli- or a megaamperehour?

Small m for milli, capital M for Mega. The h should be lower case.

Uh-huh. Apparently dictionary editors haven’t received that memo.

Well, it isn’t a hard and fast rule

Dictionary editors have been known to have foilbles.

No. According to SI rules, the full names of the units are NOT capitalised: watt, coulomb, volt, farad, ohm, etc. Only the symbol sometimes uses a capital latter: W, C, V, F. But also note Pa for pascal (a two-letter symbol), and capital omega for ohm.

NIST Style Guide, chapter 6:

And you should indicate multiplication by a middle dot:

So, kilowatt-hour is kW·h.


Thanks for then corrections.

You would look four posts up from yours in the thread.

It should work that way, but it doesn’t. The prefixes k (10[sup]3[/sup]), h (10[sup]2[/sup]), and da (10[sup]1[/sup]) are in lower case.

If they were in upper case, we could have K, H, and D (instead of da) to balance out m, c, and d, and we wouldn’t have to deal with the oddity of da for deka.

You’re quite welcome! :slight_smile:

NIST list of prefixes and symbols: http://physics.nist.gov/cuu/Units/prefixes.html

Same source, list of style rules: http://physics.nist.gov/cuu/Units/checklist.html

Look around, it’s a great site for understanding the nature of measurement in science.


The use of lowercase prefixes for larger-than-one multipliers isn’t universally preferred. For example, the excellent unit conversion routine in Hewlett-Packard’s 28S calculator also accepted “K” for “kilo”.

Note that the W isn’t an abbreviation for watt, it’s a symbol for it, and likewise for all SI units. IIRC “h” for “hour” is an abbreviation and can take a period after it. The hour isn’t an SI unit, though its mixing in is practically accepted.

Sorry to nitpick, but I don’t see how this acceptance is an example of the correct case not being preferred. I just see it as good, non-enforcing design. Did it output K for kilo?

The hour is not only practically accepted - it is one of the “Units accepted for use with the International System” and therefore is on the same level as the liter and the tonne. And h is its symbol, no period. Germans, for example, write their Kilometer pro Stunde as km/h, not km/St. or such.

You probably meant German scientists, and not Germans in general. Because they’ll happily use km/st, just as us Norwegians use km/t.