Four years ago I built a desktop. A few days ago this desktop died (for non-PSU related reasons) and I've decided to build a new one. I would like to save $100 and reuse my old PSU, but I'm not sure of the state of it after 4 years.

The PSU never had any problems in its life, so it could be just fine. However, I never ran any software-based diagnosis on it when my desktop was alive, and I don't want to buy a PSU tester just for this.

The PSU was originally rated for 700W, but my new desktop will only need approximately 600W. What are the odds that it is still running at a sufficient rate? And what are the risks of reusing it?

In case it's relevant, my PSU is a ModXSteam-pro.

  • Blow compressed air through it and see how much dust comes out. If you have never done this, and there's a lot of dust, then the remaining life of the PSU is lower than if you had cleaned it regularly. Under good conditions, I have 8-year old PSUs. – sawdust May 12 '15 at 2:04
  • @sawdust I've cleaned it a few times. Last time was probably 8ish months ago. – David says Reinstate Monica May 12 '15 at 2:11
  • David, it might also help if you can specify what parts will be in the new machine. A 50W dual-core using onboard graphics is a lot different to a gaming machine with a 300W+ graphics card. In particular, if you are intending to run dual cards, I would be very wary. – Ash May 12 '15 at 9:25

HardOCP tested the Mod XStream Pro 700W and weren't too impressed, failing it because they couldn't draw 700W from it. The capacitors were of an unknown vintage to them too.

If it was a better quality unit you could reuse, but I'd be inclined to not risk a whole lot of new parts with it. It might work just fine, but given it's known to be of questionable quality and it's already a few years old, I don't think it's worth it.

Depending on your parts in your new machine, a good quality 500-600W PSU is probably all you need, and you could keep the old one as a spare.

  • Can you say anything about the risks of reusing it? What would happen if I use it and its underpowered? Or maybe it suddenly dies? – David says Reinstate Monica May 12 '15 at 4:05
  • 1
    I imagine it would be pretty rare to actually go bang or anything at normal loads, but as the capacitors age I believe the quality of the power they can deliver (regulation, noise/ripple) as well as the total maximum power will be reduced. Poor quality power may subtly damage components over time or cause instability in the system. (At least, that's my understanding. I'm no expert on PSU internals or elec eng). – Ash May 12 '15 at 4:37
  • Here's a good article at Hardware Secrets that touches on some of the issues: hardwaresecrets.com/printpage/… – Ash May 12 '15 at 4:54
  • I kind of agree with what you saying, but 4 years is not much for capacitors. – ilkhd May 12 '15 at 9:17
  • @ilkhd: no, not usually. But without knowing the quality do you assume the best or the worst? If the new machine needs 700W as David suggests, will the old one handle it? – Ash May 12 '15 at 9:27

There are two aspects to this question on risk. One is the probability of failure, the other is the potential harm that could result from failure or degraded performance. I'll focus on the first.

Wikipedia provides some interesting statistics on computer power supplies.

Life span is usually specified in mean time between failures (MTBF), where higher MTBF ratings indicate longer device life and better reliability. Using higher quality electrical components at less than their maximum ratings or providing better cooling can contribute to a higher MTBF rating because lower stress and lower operating temperatures decrease component failure rates.

An estimated MTBF value of 100,000 hours (roughly, 140 months) at 25 °C and under full load is fairly common.

Understanding the number

This number, by itself, is a little misleading because it doesn't mean what most people think it means. Ignore, for a minute, differences in manufacturing quality, usage patterns, etc. This is a big pool of various models, tested under good operating conditions (so your mileage may vary).

It is an average time, but the failure times are not a bell curve. Some units last an extremely long time, which distorts the average. It is not that half of the units last 100,000 hours; rather, 63% of units fail by then, and 23% of units are expected to fail within three years of operation.

So if your PSU has already survived four years, is "average quality" and has been used under good conditions, then statistically speaking, it has roughly an even chance of lasting another eight years, and in the neighborhood of a 75% chance of lasting another four years.

Is it realistic?

Realistically speaking, I seriously doubt those statistics. But even much worse statistics yield surprising results.

For example, suppose we accept the 23% failure rate within three years, but assume that only 30% last more than eight years (just a number that sounds reasonable based on experience). Intuitively, you might think that the average life under those assumptions is 6 years; and since your existing unit is already 4 years old, you would expect only another 2 years from it. But that's not the case. There would be an even chance that it would last another 3 1/2 years, or roughly what some people would consider the "technology" life of the next computer.

Take that a step further. Under these assumptions, we have a 50% chance that your old one will die within 3 1/2 years. However, if you buy a new one, there is roughly a 27% chance that it would die during that same time. So the increased risk of failure for using the old unit is only 1 chance in 4, even under these assumptions.

Just some numbers to give you some perspective.

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