This is one of those things that I have always had a really hard time wrapping my head around.

In the horrendously simplified ideal step-up SMPS model below, I have set it up so when at 50% duty cycle, the output voltage is double the input voltage. For the sake of easy math, the current in the inductor is maintained at ~1A.

![schematic](https://i.stack.imgur.com/lcdwk.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2flcdwk.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

As you can see, the power transfer is 1W so it operates at 100% efficiency.

But here is the kicker... only half the current taken from the supply ever reaches the load...

Intuitively one's head is telling one it can only be 50% efficient.

Granted an ideal SMPS would not be 100% efficient, but numbers in the 90 - 98% efficiencies in a real world SMPS are not unusual...

What is going on?