You are right about resistor losses, it is an inefficient way to decrease power.

In AC an efficient approach to decrease power to load is to use a triac chopper. It cuts part of sine so effectively decreases a voltage suppling the load.

A newer option is some of SMPS topology.

The older way is to use a transformer with multi-tapped secondary. If decreasing power is needed the switch connects the tap with less turns.

I would say decreasing power is easier in Ac than in Dc btw. In Dc it cannot be done without Smps, aka making an Ac or Dc pulses at least.

You are right about resistor losses, it is an inefficient way to decrease power.

In AC an efficient approach to decrease power to load is to use a triac chopper. It cuts part of sine so effectively decreases a voltage suppling the load.

A newer option is some of SMPS topology.

The older way is to use a transformer with multi-tapped secondary. If decreasing power is needed the switch connects the tap with less turns.

I would say decreasing power is easier in Ac than in Dc btw. In Dc it cannot be done without Smps, aka making an Ac or Dc pulses at least by switching the DC on & off.