The efficiency of a transformer can be calculated from
% efficiency = [ (Output in Watts) / (output in watts + total losses in watts) ] x 100
or
% efficiency = [ (input in Watts - total losses in watts) / (input in watts) ] x 100
The total losses include the losses in the electric circuit, magnetic circuit, and dielectric circuit. Transformer has its highest efficiency at load at which the iron loss and copper loss are equal. Therefore the load at which the efficiency is highest can be found from,
%load = [ iron loss/copper loss)^0.5 ] x 100
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