When you step down the voltage from say 230v to 12v what happens to the current? Transformers can be up to 99% efficient so presumably power out should nearly equal power in. To simplify calculations say we put 100v @5A into a 10:1 stepdown transformer theoretically the output should be 10v @50A if P=VI. This is clearly not the case. So if both voltage and current is reduced power out does not equal power in and efficiency is greatly reduced. How does it work? I relise this is a pretty basic question but please just humour me.