How To Calculate Transformer Losses

The loss in a transformer compares the input, or primary power, to the output, or secondary power. Most transformer data show their input and output voltages and the current ratings of both sides. A step-up transformer increases voltage, but decreases current. A step-down transformer decreases voltage but increases current. Power in watts (P) equals voltage (E) multiplied by current in amperes (I) or (P=IE). A transformer cannot increase power. To calculate the loss of a transformer you need to know the actual voltage and current in both the primary and secondary.

Step 1

Multiply the voltage in volts by the current in amps of the primary of the transformer. Record the figure.

Step 2

Multiply the voltage in volts by the current in amps of the secondary of the transformer. Record the figure.

Step 3

Subtract the secondary power from the primary power. The answer equals your power loss. Example: the primary voltage of 440 volts with a current of 3 amps equals a power of 1320 watts. A secondary voltage of 220 volts with current at 5.7 amps equals a power of 1210 watts. Subtracting 1254 from 1320 equals 66 watts, which indicates that your transformer loses 66 watts, mostly in the heat it dissipates.