Watts are used to indicate the power availble for work.
VA is used to indicate apparent power. Not all of this may be availble for work.
W=VA x pf (power factor) in single phase.
for 3 phase loads
W=1.73 x E x I x pf
VA=1.73 x E x I
(These formulas were copied from 13th edition of American Electricians hand book)
Typically generators and transformers are rated in VA.
Hope this helps.
