Transformer regulation is defined as the percentage change in output voltage from no-load to full load with respect to full load. It can also measured with respected to no-load voltage where as in the first case it is called as regulation up and in second case it is called as regulation down.
% regulation = No-load Voltage – Full load voltage/ full load voltage * 100;
Under no load condition the induced voltage E2 is presented at the output terminals and when the transformer is loaded due to drop across the reactance it changes to V2. So the expression for transformer regulation is
% regulation = (E2 –V2)/V2 * 100.