Well, SI uses V. Wikipedia page has no mention of "U", either.
U usually represents potential energy (unit J), and voltage the potential energy per unit charge (V = J/C), so you might have them confused.
Watts is a unit of power (rate of energy transfer, J/s).
Energy is measured in joules.
All these are SI (Système International) units. AFAIK, they are used by everyone EXCEPT pretty much the US (they still use imperial units - pounds, slugs, miles, yards...).
http://en.wikipedia.org/wiki/SI
You are right that most quantities have different abbreviations than their standard units, but voltage is an exception. Maybe at whereever you live, they changed it to U to disambiguate it? AFAIK, that's not a standard practice, though. Understandable, however, just like how electrical engineers use j for the imaginary unit (instead of i) since i is usually used for current in that context.
