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.
Multiply the voltage in volts by the current in amps of the primary of the transformer. Record the figure.
Multiply the voltage in volts by the current in amps of the secondary of the transformer. Record the figure.
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.
TL;DR (Too Long; Didn't Read)
To calculate the transformer efficiency, divide the output power by the input power. Example: with an output power of 1254 watts for an input power of 1320 watts, divide 1254 by 1320, which equals .95, or an efficiency of 95 percent.