Just nicked this from another site,
[FONT=arial, Arial, Helvetica]Transformer load :
transformer load ratings can be indicated for the primary and/or secondary side. The majority of the time, we size transformers by the secondary rating. Typically the transformer will be rated by VA (volt x amperes) or KVA (1000 x volt amperes). Volt amperes is actually the wattage of your load, so if the load of your secondary is 4 amps and the secondary voltage is 120 volts, the minimum VA rating you require is going to be 120V x 4A= 480 VA (watts). Therefore the minimum VA you require is 480VA or the next size up available.[/FONT]