The efficiency of the transformer: At the rated power,the ratio of the output power and input power of the transformer is called the efficiency of the transformer,that is η= x100%. The η in the formula is the efficiency of the transformer; P1 is the input power and the P2 is the output power.When the output power of the transformer（P2） equals the input power（P1）, the efficiency of the transformer is equal to 100% and the transformer will not produce any loss. But in fact this transformer does not exist. When the transformer transmits electrical energy, it always produces loss and this loss mainly includes copper loss and iron loss. Copper loss refers to the loss caused by the resistance of the transformer coil. When the current passes through the coil, it will cause the resistance heating. And part of the electrical energy is changed into heat energy and this part of energy is lost. Because the coil is usually twisted by the insulated copper wire, it is called the copper loss. The iron loss of the transformer consists of two aspects. One is the hysteresis loss. When the alternating current passes through the transformer, the direction and size of the magnetic line of the silicon steel sheet will change along with the transformer, so that the internal molecules of the silicon steel sheet will rub each other and give off heat energy, which will consume part of the electric energy. This is the hysteresis loss. The other is eddy current loss. When the transformer is working, a magnetic line of force will pass through the core, and an induction current will be generated on a plane perpendicular to the magnetic line of force. Because the current itself forms a closed loop and forms a circulation and presents the shape of the whirlpool, it is called a vortex. The existence of the eddy causes the core heating and consume energy and this loss called the eddy current loss. The efficiency of the transformer is closely related to the power level of the transformer. In general, the power is greater, the loss and output power are smaller,and the efficiency is higher. On the other hand, the power is smaller, the efficiency is lower.