Transformers: how much wattage consumed?

Question

If a step down transformer from 120 to 12 v. has a rating of 300 watts max, and I only connect 50 watts at 12 v.; am I using 300 or 50 watts of 120 v current?

Answer 1

50 watts plus a small amount to excite the transformer approx. 1 percent of rating or 3 watts.

Answer 2

Power In = Power Out

Where Power Out includes Power Loss from the Xfmr conversion, internal resistance of the Primary coil.

Pin max = 300W = 120V x 2.5A

Pout max = 300W = 12V x 25A

The rating of a Xfmr includes the power loss, therefore the 300W can be considered the usable power on the output.

Since you are only using 50W = 12V x 4.16A on the output then the input power Pin = 50W = 120V x .41A.

Answer 3

A typical transformer has a minimum magnetising current at the 120Volt primary.  When its doing nothing it consumes a few Watts.

The bigger the transformer usually the more Watts it consumes to do nothing.  Also the more badly designed (cheaper?) it is the more Watts it consumes to do nothing.

Say you need 5 Watts minimum call it Wmin

When the transformer is working current flows in the primary and secondary and you get losses (called I squared R losses) due to the copper wire resistance.

The formula for the transformer efficiency is something like:-

Efficiency percent = 100 x Watts from secondary /Watts into primary

Typically lets say your transformer is 90 percent efficient when you get 50 Watts out of the secondary

So Watts into primary = 100 x Watts from Secondary / 90

                                 = 100 x 50 /90

Watts into primary      = 55.555555

The 300 Watts is the maximum power into the primary before the transformer starts to overheat and work outside its spec.

Look up on net there are more formulas and more maths about transformers, magnetising current etc etc