Formulas: turns ratio a = V_p/V_s (equal to N_p/N_s for an ideal transformer). Secondary power P_s = V_s×I_s; primary power P_p = P_s ÷ efficiency (accounts for core and copper losses); primary current I_p = P_p ÷ V_p. Reflected impedance Z_p = a²×Z_s — how a load on the secondary "appears" from the primary side, the basis of audio output-transformer impedance matching.