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. How much power it is pulling from the power grid we will measure with a digital power meter from Brand Electronics (model 4-1850), shown on Figure 7.

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Figure 7: Digital power meter from Brand Electronics.

Like we said, efficiency is giving by this simple formula:

Efficiency = Output power / Input power