The voltage regulation of the transformer is the quantity by which the output voltage of the transformer at nod load is compared with the output voltage at full load.
Voltage Regulation Formula
Where
Vs (nl) = Secondary voltage of transformer at no load
Vs (fl) = Secondary voltage of transformer at full load
VR = Voltage regulation
Importance of Small Voltage Regulation
A small voltage regulation there is a small change in output with the load. Generally, we want a small voltage regulation in transformers. And for an ideal transformer VR = 0.
However, this is the general case and some exceptional cases are there.