In some documentation I have commented that a field contains "the forward active power, in Wh". Which way should it be written (when I don't want to use the abbreviation)?

  1. the forward active power, in watt hours
  2. the forward active power, in Watt hours
  3. the forward active power, in watt-hours
  4. the forward active power, in Watt-hours

On the wikipedia page for kWh they seem to use both hyphenated and non-hyphenated versions. I don't see "watt" being capitalized there. Is there a standard? This may be more of a question for english.stackexchange.com, not sure.

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    \$\begingroup\$ +1 for caring about the proper way to capitalize and spell things. \$\endgroup\$ – JYelton Apr 1 '14 at 17:01
  • \$\begingroup\$ Looking into a question about one of the answers I found a NIST document that covers most of the conventions defined by US and international standards. \$\endgroup\$ – The Photon Apr 9 '14 at 18:17
  • \$\begingroup\$ According the Power Engineering Society (PES) of the IEEE March 2013 Appendix A, the correct full name is none of the above. kilowatt and kilowatthour are given, for kW, kWh... so my builtin dictionary is outdated... Remember new words become one word after hyphenated becomes old or when common. \$\endgroup\$ – user38637 Apr 9 '14 at 19:31
  • \$\begingroup\$ @ThePhoton, that NIST document never has the Wh unit in it. Probably because it is more of a utility industry unit of convenience (can be simplified to joules). \$\endgroup\$ – Anssssss Apr 9 '14 at 21:09
  • \$\begingroup\$ @Anssssss, on p. 40 there is a general rule for writing out units, "when the name of a derived unit is formed from the names of individual units by multiplication, then either a space or a hyphen is used to separate the names of the individual units." However, hours are not an SI unit, and, as you say, the example of the watt hour is never mentioned. \$\endgroup\$ – The Photon Apr 9 '14 at 21:46

According to the IEEE/ANSI standard, the acceptable spellings for Wh, when it is written out, are

watt hour (preferred)

The names of SI units are not capitalized unless they are used in a situation where any word would be capitalized, such as the beginning of a sentence. Of course, the standard unit of energy would be the joule so the watthour is more of a colloquialism anyway.

EDIT: The standard that defines usage of SI units is IEEE/ASTM SI 10-2010, "American National Standard for Metric Practice".


The standards used by IEEE are;

  • Long form text is always lower case
  • abbreviations of a name are always capitalized, to indicate reverence to names, such as tesla (T), henries (H) and pico-coulombs (pC) as opposed to second(s) which is not named after someone.

  • Greek abbreviations are capitalized as a rule for large positive exponents +6,+9,+12...+24 and lower case for negative exponents or powers of 10, which extends to 10^-24.

Thus Y= yotta down to 10^-24, y=yocta

 - one exception is K was already assigned to Kelvin lower 

e.g. kilowatt (kW), nanosecond (ns), nanosiemen (nS),

  -  the other exception is 10^-6 is the Greek letter "mu", μ.

"mu" is spoken as micro as in uF or μF where often the font is substituted with lower "u", - sometimes ASCII range are imposed (eg 8 bit to 7 bit) and we see Greek font letter Ω, which is ANSI letter W and 100Ω shows by mistake as 100W. Thus the "long form" 100 ohms is "error free" when plain text is sometimes stripped down such as plain text.

e.g. exp.=+6= M = Mega, and exp. = -3= m = milli

The trends of english when society creates new words are;

  • Introduction phase... separate words
  • Frequent usage .. Hyphenate the words e.g. Never-the-less watt-hours
  • common words .. Combine into one new word. e.g. watthours which looks confusing with "th", so this version is not popular for some, but is now "NIST" and IEEE/PES standardized
  • 1
    \$\begingroup\$ I'd be explicit that while named abbreviations are always capitalized (W, K, C, J, S, F, Hz, T, A, V, N, °C, etc.)--that you mention--spelled out they are always lower-case (watt, kelvin, coulomb, joule, siemen, farad, hertz, tesla, ampere, volt, newton, degree Celsius ("degree" is lower-case)). \$\endgroup\$ – Nick T Apr 1 '14 at 16:46
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    \$\begingroup\$ How should one distinguish a 0.001 ohm resistor from a 1,000,000 ohm resistor if not by capitalizing the magnitude prefix, or is "mega" not Greek? \$\endgroup\$ – supercat Apr 1 '14 at 17:25
  • \$\begingroup\$ @Joe TO which IEEE standard are you referring to? I find it extremely strange that IEEE would recommend minuscule letter for Greek prefixes other than mega. What about giga, tera and higher? In fact, here's a reference from IEEE claiming otherwise! All prefixes of mega and higher are majuscule. \$\endgroup\$ – AndrejaKo Apr 9 '14 at 7:55
  • \$\begingroup\$ Your ref. is correct. As in english, there are exceptions. It appears positive exponents (except K is reserved for Kelvin and k has exponent +3 ) are Capitalized and negative exponents are lower case. Only one is a Greek letter, mu for which lower u is often a substitute when the Greek font is not avail. I will correct remark \$\endgroup\$ – user38637 Apr 9 '14 at 16:41
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    \$\begingroup\$ Where did you find that "nano second" is preferred? On the IEEE page linked by @AndrejaKo I find "microsecond", "megawatt", "nanofarad", etc., etc., all written as single words. \$\endgroup\$ – The Photon Apr 9 '14 at 17:16

Capitalization applies to the abbreviation such as V for volt and W for watt. H for henry shouldn't be confused with hours and neither should S be confused with seconds because it stands for siemens, the unit of conductance. All fully spelt units have a small capital letter and let it be known that the unit of frequency is Hz and not hz. It's also worth mentioning that megahertz is MHz and not mhz or mHz. The last one is milli hertz.


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