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?
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.
It is possible to design power supplies that have efficiency in excess of 90%. However, these are not necessarily suitable for mass-market consumer power supplies. For example, lossless snubbers require additional hardware to store energy from suppressed EMI sources instead of converting them to heat which means loss.
An another problem with very high efficiency is the load range. No power supply has constant efficiency over the permissible power range. To make things worse, the input voltage is also variable as most supplies are expected to work with 110VAC and 230VAC without issues.
A regulation mandated source of loss is the PFC or power factor correction. Relatively high-power devices are required to be "nice" to the electric grid. The preconditioner PFC circuit adds another level of switching power conversion. So even in the best case you're stacking a flyback supply (supplies) on top of the PFC. So a very nice efficiency of 90% would become 0.9x0.9 = 0.81% !
Truly high-efficiency power supplies are found in industrial "brick" format DC/DC converters which are required to handle very high power levels in a relatively small enclosure. If your output real load is 600W and the PSU is the size of a 2.5" HDD, extra 10% of waste heat makes a big difference on thermal management.
In fact, waste heat is the primary reason an engineer likes to design an efficient supply. Energy efficiency is something of a happy byproduct.