PWM output vs Digital output
For example to act like an on-off signal by varying the duty cycle between 0 and 100%.

Sure, a PWM that's on 100% of the time is just a &quot;high&quot; output. Same with 0% and &quot;low&quot;.
However, this sounds like something you ask in context of a microcontroller. I've yet to meet a microcontroller where you have a pin that you can connect to a PWM unit, but not directly use it as GPIO. So, I bet there's an easier solution to what you want.
We couldn't possibly tell you anything about power consumption, because that fully depends on the device you're dealing with. However, for the above reason, I don't think this is relevant. You really might want to ask a new question where you describe the actual problem you're trying to solve with this approach.