Transformer Loading Conditions
Take this typical small power transformer datasheet as an example. 


The rated voltage is the nominal input voltage. For example, 115VAC (50/60Hz) for the primaries in parallel (or 230VAC for the primaries in series). This is something you apply to the transformer. 

The nameplate output voltage is the nominal RMS output voltage under full load (and the load is assumed to be a pure resistive load). 



The no-load secondary voltage will be higher. In this case, the manufacturer says it is "typically" 25% higher. 

If you are specifying components to be connected to the transformer, note that the 25% does not include tolerance of the transformer windings (probably small) nor does it include the fact that the applied mains voltage could be higher than nominal. If you allow an additional 10-12% for those factors you need to allow for about 40% higher output voltage. 

So if your nominal output voltage is 12.6V, the peak voltage (which determines your capacitor rating) could be as high as 25V (about double). Different transformers will have different numbers for the 'regulation', generally larger transformers will have less difference than small transformers.