The second step in our tests is to measure the amplitude of pulsation of the power supply’s output voltages. This pulsation may occur at two frequencies: at the frequency of the PWM-regulator of the PSU (from 70 to 130kHz, but usually not higher than 75kHz) and at the double frequency of the electricity network (100kHz in our area). This much simpler test produces a few numbers that should be within the limits described in the industry standard.

The third step is to measure the speed of the PSU fan depending on the load. The load on the tested power supply is being increased in a step-like manner from the minimum point of 50W to the maximum permissible load. The power supply works for a while under each load until its temperature stabilizes; the fan speed is measured after that. Besides, we also check if the PSU can work normally under different loads, including the maximum load it is rated for – the PSU must be able to work for 30-40 minutes under it. I know from my experience that power supplies with overstated wattage either fail in the first 10-15 minutes or show unmistakable signs of being about to fail, e.g. begin to smell of burnt insulation; that’s why I don’t think it’s necessary to test a power supply under its maximum specified load for too many hours. But again, each PSU is tested for at least half an hour in our labs for its ability to output the full power declared by the manufacturer.

The last item in our test procedure is about the efficiency and power factors. You should not confuse these two parameters – they have nothing in common at all. The efficiency is the ratio of output power to input power. The difference between the input and output power transforms into heat you have to pay for in your electricity bill. This heat also makes it harder to cool the PSU and makes a higher fan speed necessary.

The power factor has nothing to do with the processes taking place inside the PSU or on its output. It is a ratio of active input power to full input power. The difference between these two powers is called reactive power and it presents an interest for power engineering specialists rather than for computer users: reactive power does not do any work in the load (and it is not dissipated in it in any way, heat or anything), but is just flowing forwards and backwards in the circuit, purposelessly loading the wires, sockets, substations and other equipment, even including the generators at the power station. That’s why it is mostly the electricity suppliers’ rather than the consumers’ trouble to try to reduce the amount of reactive power, i.e. to increase the power factor (well, this indirectly affects the consumers, too, but it would be too great a digression to dwell more upon this topic).

I also check the power supply visually for any assembly defects and for extra functionality, count up its cables and connectors, and give my opinion about it from an aesthetic point of view. After all, the exterior of the power supply matters for quite a lot of users.