When an ideal transformer is considered with no losses, the Power of the transformer will be constant, as the product when voltage V multiplied by current I is constant.
We can say that the power in the primary equals the power in the secondary as the transformer takes care of that. If the transformer, steps-up the voltage then the current is reduced and if the voltage is stepped-down, the current is increased so as to maintain the output power constant.
Hence the primary power equals the secondary power.
PPrimary=PSecondaryPPrimary=PSecondary
VPIPcosϕP=VSIScosϕSVPIPcosϕP=VSIScosϕS
Where ∅P = Primary phase angle and ∅S = Secondary phase angle.