What is the formula for efficiency in a transformer?

• A. Eff = Pout + Pin
• B. Eff = Pin / Pout
• C. Eff = Pout / Pin
• D. Eff = Pout - Pin

Answer: (C)

The main idea is that efficiency compares how much power you get out to how much power you put in. In a transformer, the useful output power is delivered to the load on the secondary winding, while the input power comes from the source on the primary winding. So efficiency is the ratio of Pout to Pin: η = Pout / Pin. It’s often shown as a percentage, η = (Pout / Pin) × 100%. In an ideal transformer, Pout equals Pin and the efficiency is 100%, but real transformers have losses that reduce this value.

The other expressions don’t measure how much of the input is converted to useful output: inverting the ratio, or adding or subtracting the powers, doesn’t reflect the actual transfer efficiency.