How is zakat paid on money generated from variable-yield investment certificates from an Islamic bank, knowing that the principal amount exceeds the nisab and the yield is used to cover the family's monthly living expenses?

If the return on investment certificates is spent on essential needs, then there is no Zakat due on it. However, if it is added to the principal, then Zakat is calculated on the principal plus the return, at a rate of 2.5%, when one lunar year has passed on the principal amount.