# Do transformers lose energy?

EDIT: The title should rather be how/why transformers lose energy

My idea of a transformer is that it is composed of two separate wire windings around some metal core. The purpose is to increase/decrease AC voltage. The transfer of energy from the primary to the secondary winding is due to magnetic coupling or mutual inductance.

My question: how much energy is lost in this system due to the metal core AND why? What determines the efficiency of a transformer? For example, it must be based on some property of the metal core. I suppose this question is directed at electrical engineers in particular.

For instance, I've read in textbooks that transformers are fairly efficient, with an "efficiency rating" of around 98% or so.

The equation that governs this is

$$\text{Efficiency(%)} = 100\times\frac{P_{\text{out}}}{P_{\text{in}}}$$

where $P_{\text{in}}$ is the primary power (i.e. $V_p\times I_p$) in Watts and $P_{\text{out}}$ is the secondary power (i.e. $V_s\times I_s$) in Watts.

• In addition to the main loss by the currents in the core, there is EM radiation loss . This thesis explores experimentally ( measuring) the radiation from transformers staff.najah.edu/sites/default/files/… . Jun 10 '15 at 3:17