Why Reviewers Test PSU Hold-Up Time and Inrush Current

This article explains why hardware reviewers rigorously test a Power Supply Unit's (PSU) hold-up time and inrush current during benchmarks. While often overlooked by average consumers, these two electrical metrics are critical indicators of a power supply's internal build quality, safety, and real-world reliability. Understanding these tests reveals whether a PSU can survive brief power grid fluctuations without crashing your computer, and whether turning your system on will trip your home's circuit breakers or prematurely wear out the power supply's internal components.

What is Hold-Up Time?

Hold-up time measures how long a power supply can continue to deliver stable power to your computer's components after the main AC power from the wall is cut off. It is measured in milliseconds (ms).

Why Reviewers Test Hold-Up Time

Reviewers test hold-up time to verify two main things: system stability during power flickers and compatibility with Uninterruptible Power Supplies (UPS).

  1. Surviving Brownouts: The electrical grid is not perfect. Brief dips in voltage (brownouts) or millisecond-long outages happen frequently. A PSU with a sufficient hold-up time will bridge the gap during these micro-outages, keeping your PC running smoothly instead of instantly rebooting and causing potential data loss.
  2. UPS Compatibility: If you use a battery backup (UPS), it takes time for the UPS to detect a power outage and switch from wall power to battery power. This transfer usually takes between 10ms and 12ms. The official ATX specification generally requires a PSU to have a hold-up time of around 16ms to 17ms. If a PSU fails this test and only holds power for 8ms, your PC will shut off before the UPS battery can kick in, rendering the UPS useless.
  3. Component Quality Check: Hold-up time is directly tied to the size and quality of the PSU's primary (bulk) capacitor. Because these large capacitors are expensive, budget manufacturers often use smaller, cheaper ones to cut costs. A failed hold-up time test immediately exposes this corner-cutting to the reviewer.

What is Inrush Current?

Inrush current is the maximum, instantaneous surge of electrical current drawn by the PSU from the wall outlet the exact moment you flip the power switch on. Because the internal capacitors are completely empty, they briefly act like a short circuit as they rapidly fill with energy, drawing a massive spike of current that is many times higher than the PSU's normal operating current.

Why Reviewers Test Inrush Current

Reviewers measure inrush current—usually in Amperes (A) at both 115V and 230V—to evaluate safety and component longevity.

  1. Preventing Tripped Breakers: If a PSU has an excessively high inrush current, simply turning on the power strip or the PC can trip the circuit breaker in your home. This is especially problematic in older homes or rooms where multiple high-draw devices share the same circuit.
  2. Protecting Internal Components: Massive electrical surges put severe stress on the PSU's internal components, including the bridge整流器 (rectifiers), fuses, and the mechanical power switch itself. Over time, unchecked inrush current can weld relay contacts together or blow the PSU's internal fuse.
  3. Verifying Protection Circuits: To limit inrush current, quality PSUs use a component called an NTC thermistor, paired with a bypass relay. Reviewers test inrush current to ensure the manufacturer actually included these protective components and that they are scaled correctly for the wattage of the unit.

The Engineering Balancing Act

Ultimately, reviewers test both metrics because they directly conflict with one another. To get a better hold-up time, a PSU needs a larger bulk capacitor. However, a larger capacitor creates a much higher inrush current when charging up. Testing both hold-up time and inrush current allows reviewers to see if the PSU's engineers successfully balanced this relationship using high-quality components, rather than sacrificing one metric to make the other look good on paper.