When a unit of measurement is named after someone, convention has it that the word is written entirely in lower case when talking about that unit, but the single letter abbreviation is written in upper case.
Some examples should clarify this.
The unit of voltage is the volt, named after Volta, and abbreviated to V.
The unit of current is the ampere, named after Ampère, and abbreviated to A.
The unit of resistance is the ohm, named after Ohm, abbreviated in this case with the Greek letter Ω, pronounced omega, not omeega.
When a unit of measurement is not named after someone then the single letter abbreviation is not written in upper case.
Examples are the second, abbreviated to the lower case s, and the hour, abbreviated to h.
Furthermore there is no justification for the capitalisation of other frequently used abbreviations - a common mistake.
Examples are Va.c. not VAC, a.m. not AM.
Other conventions apply to prefixes which are used to denote magnitude or multipliers. A useful list can be found here, but a few common ones follow.
The lower case k for kilo, one thousand times, as in kg = one thousand grammes.
Lower case m for milli, the thousandth part, as in mA = the one thousandth part of an amp.
Upper case M, for mega, one million times, as in MHz = one million hertz.
These conventions would occasionally be explained in the monthly magazines, at least in the U.K., and some of them were explained to us in college. The absence of such explanations has lead to obvious confusion and the proliferation of every imaginable corruption of the correct way of writing them.
No one teaches them correctly, therefore no one learns them correctly.
We were taught in English Language, at school, that abbreviations, unless they were of Proper Names, or Proper Nouns, were not written in upper case and that each letter of the abbreviation was followed by a full stop. e.g. c.m.o.s., not CMOS. Yes, I know what the OED, as they themselves abbreviate it (no stops), say CMOS, but the dictionary bows to common misuse instead of sticking to what they were taught at school. In other words they are throwing in the towel. It makes me wonder, sometimes, how the "education system" justifies employing English teachers. Basic grammar and other linguistic rules which were taught us at school are casually and constantly brushed aside in deference to the latest illiterate herd.