Why are computer power supplies seemingly inefficient

powerpower supply

Due to a recent question I asked here on SuperUser, I'd like to ask another question. Computer power supplies now have ratings for their efficiency, scaling between 80% utilization and higher. My question comes from a hopeful misinformed place: Why is the efficiency so bad? Where are we losing the wattage?

Best Answer

Considering that the very most efficient gasoline-powered cars are about 30% efficient, I don't think it's nearly as bad as you think it is. But the answer is primarily resistive losses. When you try to make electrons move and stop, they rub against each other and against the materials they pass through. This rubbing wastes some of the energy you used to move them, and that comes out as heat.

In addition, the power supply has inductive losses. When you make electrons move, this creates a magnetic field. When you stop them, you tear down this magnetic field and get most of the energy back. In fact, this is how a switching power supply works to convert electricity from one voltage to another.

As you build up and tear down the magnetic field, nearby materials line up with the field and then resist your attempt to tear it down. As these magnetic domains flip to line up, they again rub against each other, stealing energy from the magnetic field. When you tear it down, you never get back as much energy as you used to build it up. This also comes out as heat.