power supply

when using a capacitor input as smoothing after the rectifier the transformer will see a large charging pulse with a long period of zero current during each cycle. The power lost is based on Isquared * R and is worse in this non constant current situation.
Much literature recommends that you draw about 70% of the transformer rating to account for this.
But many designers use the full rating and because transformers can meet quite large overloads and rarely run at full power this works for most.
Answer;- use somewhere between 36VA and 60VA by 12v depending on your use.
12Vac + regulation (15%) + suppler overvoltage(6%) times sqrt2 less low current diode drop (0.7) = 20Vdc max on no output current.