What is the correct spelling/capitalization of watt hour?
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