Transformers are rated using VA because Volts determines the insulation / isolation requirements and Amps determines the conductor size. Watts is more or less meaningless when designing a transformer.
When using a transformer, knowing only the Watts rating of the load does not provide enough information. For instance, say we have a 10VA rated transformer with 10V, 1A conductor size for secondary winding. Connect a 10VA load and everything is fine. Connect a 10W load at 0.5PF, then the load is 20VA (2 amps @ 10V) and the transformer would overheat and probably be damaged.
ive never really understood when you have 2 winding on the output,
say 2 12v if you parrallel these do you get double the current?
what current do you get when you get 2 12v windings in series you get 24v, same current?
All you need to know is that
VA Primary = VA Secondary
Say the above transformer is rated for 24VA. Therefore, VA secondary is 24. For two 12V secondary windings;
When connected in parallel, the voltage is still 12V. The current available is 24VA/12V = 2 amps.
When connected in series, the voltage becomes 24V. The current available is 24VA/24V = 1 amp.
If you double the voltage, you have half the current available. If you double the current, you have half the voltage available.