I've been playing around with desktop pc builders and from what I've gathered, my upcoming build will only consume about 210 watts of power. Among the power supply units listed in the shops in my city I've found models providing as low as 200, 250, 300 and 350 watts. As far as I remember, the golden rule is to always have a spare 20% headroom from the target power consumption, so I would probably be fine with a 250w PSU. But in case I decide to add some extra ram sticks and hard drives, let's make that 300w.

Until now I've mostly dealt with much older (6+ years) desktops, and the power draw there often required PSUs with >500w rating. And seeing numbers so low I'm not entirely comfortable, even though there is probably no mistake in the total wattage calculations. Is there something I should know about these modern low consumption builds? Will it really be okay if I get a 250w PSU for a system which draws 210w or should I go higher to 300/400w instead?

closed as primarily opinion-based by JakeGould, sawdust, n8te, bertieb, music2myear May 18 '18 at 15:52

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Your "system which draws 210w" is probably only a guess/calculation, and if you measured the actual power consumption, you would probably will not use that much power. – sawdust May 14 '18 at 1:28
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    @user1306322 As a general opinion, I would say 250w is the very least most systems can get away with and there is at least some chance you may regret this choice it in future. A PC being unable to fully power its components at boot is a real thing. From anecdotal experience, I would not be comfortable with less than 350w on average, especially if you ever intend to upgrade any of those components. – Anaksunaman May 14 '18 at 3:57
  • Higher output PS operate more efficiently when powering lower-draw computers. It's very similar to putting a big powerful engine in a small car: the engine doesn't have to work hard to move the car around, and so it can be tuned for efficiency and will usually run very reliably for a very long time. Research available online supports this. – music2myear May 18 '18 at 15:52

The biggest potential problem is lack of room for expansion.

  1. USB ports each requires power 2.0 500ma@5v, 3.0 900ma@5v, charging ports anywhere from 1 to 2.4 amps @ 5v. Finally USB 3.1 can provide 100w or 19v @ 5a under certain limited conditions.

  2. You ever going to get a good video card, those will need way more power.

  3. If the ambient temperature goes up the power provide can be reduced. This mainly affect cheap power supplies as expensive power supplies have temperature threshold higher than you would expect to find under any normal condition.

The downside for a power supply that is bigger than necessary is generally efficiency. If you run a power supply to far above your power demands it will generally be less efficient. This varies on a case by case basis so you have to do research on individual models to determine the highest efficiency.

The downside to a small power supply is if you do have to upgrade now you have a small power supply just sitting around that you can't get back the money you spent on it.

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    General info I've found says PSUs have ~80% efficiency and run most efficiently when their output is ~200% of the system's consumption. If that is true, would you say that for a 210-250w system a 500w psu would be optimal? – user1306322 May 14 '18 at 0:54
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    As a general guideline it is ok. If you are trying to maximize efficiency then you will need to consult the power efficiency graphs of each candidate power supply. For me I would rather risk spending a few extra dollars a month in electricity than have to buy another bigger power supply. – cybernard May 14 '18 at 0:57
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    "PSUs ... run most efficiently when their output is ~200% of the system's consumption." -- This is wrong, or a statement that makes no sense; it's missing some key words. The power output of a switch-mode PSU will match the system load/"consumption". Switch-mode PSUs typically operate at peak efficiency at about 60-80% of the maximum capacity, meaning the system load needs to consistently consume about 75% of the PSU (max) power rating. – sawdust May 14 '18 at 1:47

If you are confident that your consumption will max at 210 watts, then a 250 watt power supply should be plenty. It gives you plenty of headroom if your estimation is a little low.

However, PSUs are so cheap, there is no reason not to go higher. A quick Google search (in the US), shows Walmart selling a 300w PSU for $23. NewEgg has a 650w PSU for $27. Are these the best quality PSUs? Probably not, however they should be more than good enough for a low power build.

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