For each load test we will measure efficiency. Efficiency is how much power the power supply wastes in the process of converting AC power into DC. For example if a given power supply is delivering 500 W on its outputs but it is pulling 650 W from the power grid, this means that the power supply is wasting 150 W. The problem with this wasted power is that you are paying for it but you are not using it!

In this example, this power supply would have an efficiency of 77% (500 W/650 W). Good power supplies have an efficiency of at least 80%. The higher this number, the better, as it will mean lower electricity bills.

Measuring efficiency is easy. For each load test we know how much power the power supply is delivering, since this number is being shown on our load tester display (this value is published in our reviews as "Total"). Then we have how much power the unit being tested is pulling from the power grid, which is published in our reviews as "AC power". Efficiency is calculated dividing the DC power by the AC power.

AC power is measured by a precision digital meter GWInstek GPM-8212. This is a very expensive equipment, mainly because it provides a 0.2% precision, which can't be achieved with cheap units like Kill-a-Watt and Brand Electronics. It is important to note that we bought this equipment in June 2009 and before we used a Brand Electronics 4-1850 power meter, which is not so precise. Power measured with this Brand Electronics unit gives higher readings, making efficiency results to be greater than they should be. In other words, the efficiency numbers published on reviews posted before 06/23/2009 are a little bit higher than they should be, and you should keep this in mind when comparing new reviews to old ones. We are re-testing several units but we won't be able to fix all reviews. 

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Figure 7: GWInstek GPM-8212 precision power meter.

Another advantage of this equipment compared to cheaper solutions is that it allows calibration via PC, so from time to time we can recalibrate this unit to make sure it is accurate.

This instrument also measures the AC voltage and the power factor (PF) and we are adding this information on reviews posted after we bought it. Power factor equals to active power divided by apparent power and is a number between 0 and 1. The closer this number is to 1, the better. This result measures the efficiency of the power factor correction (PFC) circuit from the power supply. Click here to understand more about this subject.