try checking both input and output with 5-9V (reliable voltage and current) 100ohm resistor and check the current usage for a more accurate number. if you want to check voltage you might need to check the inductance of the input and output. check the voltage drop across the resistor as well. it might give suggestion to the transformer
Like a transformer designer wouldHow to calculate the power of unknowm transformer?
Like a transformer designer would
Given everything else stays the same (transformer core material, wire current density, etc.), Power , or more precisely VA is directly proportional to the square of core area.
Core area S is A*B in square centimeters.
For average silicon steel laminations (what 95% of makers use) and average current density (about 3A/mm² seems to be a standard these days) , basic power calculation is:
Here is an example using a few datapoints from the VPS series of transformers made by Triad. VA ratings and weights copied straight from Triad's website. Notice the Rsquared (goodness of fit) coefficient.Weigh it. Compare it with other transformers which appear to be of similar quality.