Well if it is 5A RMS (AC) you need then this is easy, 24V * 5A = 120VA, but I suspect it is actually not as you are probably trying for DC.
This then gets difficult as it depends on the conduction angle, but for a reasonable sized cap bank I would probably go for a 225VA.
In fact I would go for a 225VA anyway (if it will fit) as having some spare capacity is never a bad thing and the 225VA will run cooler then the 120Va.
Regards, Dan.