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Power Consumption In Standby Mode, Inrush Current, Energy Efficiency

Soon At Tom's Hardware: Full-Scale Power Supply Testing
By , Patrick Afschar

Power Consumption in Standby Mode

As long as the power supply's main switch isn't turned off or the computer isn’t physically disconnected from the power grid, the PSU will consume some power even when shut down. This is called standby power and is required to turn on the PC.

Some computers even provide sufficient standby power to charge a cell phone through a USB port. A constant load on the 5V standby rail (5Vsb) has to be defined and used in order to compare the standby power of different power supplies. We chose 0.25A, which equals a power consumption of 1.25W. The test result shows what the PSU actually consumes in order to provide the required 1.25W. The higher the number, the lower the quality of the power supply in this respect.

Inrush Current

Turning on a computer creates a very strong temporary peak in its input current due to the charging of capacitors and so on. If the inrush current is too high, this could negatively impact other devices connected to the same power strip or even the local power grid if they are really sensitive.

Tom's Hardware tests each PSU to make sure its inrush current stays within the ATX specification of 50A at 230V and 100A at 115V. Our 150A Tektronix TCP150 probe enables us to gather helpful screenshots.

Energy Efficiency According to the 80 PLUS Specification

Next to its power output, the most important characteristic of a PSU is efficiency. It's no surprise, then, that almost every PSU manufacturer out there today advertises the compliance of its products with the 80 PLUS certification. A PSU must reach a certain efficiency under given load percentages in order to use this label.

The different levels (standard, Bronze, Silver, and Gold) symbolize what efficiency level for which the PSU is suitable. The next certification, Platinum, is already in preparation. 80 PLUS has its main focus set on 50% loads, which are most common in everyday use. Tom's Hardware tests whether PSUs are labeled correctly or if certified models comply with the requirements of the 80 PLUS initiative. Since the certification enjoys worldwide recognition, all tests are carried out at voltages of 115V, as well as 230V.

The table shows the criteria the individual 80 PLUS levels are based on.

Energy Efficiency20% Load
50% Load
100% Load
80 PLUS 80%80%80%
80 PLUS Bronze 82%85%82%
80 PLUS Silver 85%88%85%
80 PLUS Gold 87%90%87%
80 PLUS Platinum 90%94%91%

Energy Efficiency Under a Range of Defined Loads

In addition to the efficiency testing done to check the 80 PLUS compliance, we also test the efficiency of power supplies under a number of defined loads. This allows the user to get a picture of PSU efficiency under a range of typical loads. To depict the most realistic scenarios possible, the following loads are used:

  • 25W: Compact all-in-one Atom-based PC without a dedicated graphics card
  • 50W: Office PC without a dedicated graphics card
  • 85W: Multimedia PC with a dedicated graphics card
  • 300W: Gaming PC with a powerful CPU and a modern graphics card
  • 500W: Gaming PC with a high-end CPU and multiple graphics cards
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