The words watt, what, wot sound the same but have different meanings and spellings. Why do watt, what, wot sound the same even though they are completely different words?
The answer is simple: watt, what, wot are homophones of the English language.
Electricity An International System unit of power equal to one joule per second. See Table at measurement.
Which thing or which particular one of many: What are you having for dinner? What did she say?
Which kind, character, or designation: What are these objects?
One of how much value or significance: What are possessions to a dying man?
That which; the thing that: Listen to what I tell you.
First and third person singular present tense of wit2.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").