The efficiency of an ideal transformer without any losses is 100%. However, in practice, transformers have inherent losses lowering their efficiencies.

Iron or core losses in a transformer are measured by performing an open circuit test where the primary winding is energized but the secondary open. This eliminates copper losses, directly providing iron loss wattage.

Eddy current losses increase sharply with increasing frequency of supply current as they are proportional to the square of frequency. This is a major factor behind higher iron losses at higher frequencies.

Harmonics generate useless higher-order frequency components in the supply current. These increase both copper losses due to higher skin and proximity effects and iron losses due to increased eddy currents within the core.

Increasing temperature elevates the resistance of conductors increasing copper I2R losses. It also exacerbates iron losses by causing the magnetic material to slightly lose some of its prime properties with thermal drifted hysteresis. This lowers overall transformer efficiency at high operating temperatures.