Sunday, March 17, 2013

[Article] Is 80 plus rating of a power supply worth it?

I love low power computing. One of the things you can do to lower the power consumption of your PC is to buy a power supply that operates at a high efficiency. The power supply manufacturers can get a certification called “80Plus” to prove that their efficiency claims are, in fact, correct. 80Plus means that the efficiency of the PSU is 80% or more. That means, for 100W of input from the mains to the PSU, the DC output of the PSU is anywhere between 80W and 100W. But the caveat to that is, how would you differentiate between a PSU that can operate at an efficiency of 80% and a PSU that can operate at an efficiency of 90%. Obviously, the latter has good quality components and design, thus wastes less power, hence saves money from your electricity bill in the long run. If both were the same price, you should definitely go with the 2nd option.

But how does the user know which 80Plus certified PSU is actually more efficient? There are 4 more sub-ranks on top of 80Plus certification, namely, 80Plus Bronze, 80Plus Silver, 80Plus Gold, 80Plus Platinum and 80Plus Titanium. Check this Wikipedia article to know which requirements need to be satisfied to obtain each rating.

80plusbsg

Is it worth it?

But there is the one problem that comes up when you are paying more up front to save money in the long run. Do you actually save more money by buying a 80Plus Gold rated PSU over a 80Plus Bronze rated PSU? What do you think? Of course, if the prices are similar, then there is no doubt that you would save some money in the long run. Not to mention, the higher quality components inside the more efficient PSU will give your PSU (and perhaps the components running on top of it) a long lifespan.

Let's take one simple calculation and see.

I'm in Japan. The average unit price is around JPY25. Why I call it “average” is because the price of unit differs from level to level. I guess it is same in other countries as well. The more units you use, the average cost of unit goes up. Just for this calculation, let's say the unit price is JPY25.

Assumptions (case 1 or “extreme”)

Let's assume the following because your PC wouldn't running at full load all the time.

  • Your PC is ON for 24/7 (i.e. the PC is ON all the time, even when you are sleeping)
  • Power consumption
    • At idle your PC draws 100W.
    • At normal load it draws 200W.
    • At full load (for example, when gaming) it draws 400W
      Note: These are DC output power of the PSU. We’ll be looking at the real power consumption at the end.
  • Operating times
    • Your PC is idle for 12hrs
    • Your PC is running with a normal load for 6hrs
    • You're gaming for 6hrs

 

Calculation

For this particular PC, every day, it is consuming 4.8kWh.

If you have no clue how I calculated the value, this is how.

(100x12 + 200x6 + 400x6) / 1000

That means, for every month (assuming 30 days), your PC is using up 144 units (or 144 kWh).

But this is the DC output of the PSU. What would be the input from the mains?

  • 82% efficient (~80Plus Bronze): 175.6KWh (175.6 units)
  • 87% efficient (~80Plus Gold): 165.5KWh (165.5 units)

The difference is 10.1KWh per month or 10.1 units. Now if we look at it in monetary terms, you’ll be spending JPY252 more if you had a 80Plus Bronze PSU instead of a 80Plus Gold PSU. For a whole year, you would be saving JPY3,030.

So what's the price difference between a 80Plus Bronze rate PSU and a 80Plus Gold rated PSU?

Let's look at a popular brand: Corsair. These are the lowest prices of 650W Corsair PSU from HX (80Plus Gold model; there was a 80Plus Silver model before) series and TX series (80Plus Bronze) as of today.

  • Corsair TX650 (80Plus Bronze): JPY 9,000
  • Corsair HX650 (80Plus Gold): JPY 12,700

The price difference is just JPY3,700, which is actually not as bad as I thought. The difference gets bigger when you go for larger PSUs though. For example, a 1000W 80Plus Gold rated PSU would be much expensive than a 1000W 80Plus Bronze rated PSU – at least in Japan.

So, you save JPY252 per month. You have spent JPY3700 more by going with the Gold rated PSU. That means, you would be able to recover the extra price you pay for the 80Plus Gold rated PSU in just 15 months. Considering the fact that you won't replace the PSU that often (usually before 3 to 5 years), you are definitely better off going with the 80Plus Gold rated PSU in this “extreme” case.

More realistic figures (case 2)

But the use-case that I have given in case 1 is pretty synthetic, or hardcore, or “extreme”. Most people won't be using their PCs for that long. Most people don't run their PCs 24/7. Even if they did, they won't be able to get 6 hours of gaming time every day. Maybe 1-2hrs average per day is more realistic. People have to work for a living, right? And then sleep and do other stuff as well…

On top of that, very few PCs would be using 400W when gaming either. If you take a rig with a 3770K overclocked to 4.5GHz and a HD7970 or GTX680, you would use about 300W in total. Taking all that into account, an average user would hardly use more than 1KWh (1 unit) per day.

This means, now the advantage is about 1/5 of the original savings in monetary terms (previously 4.8KWh, now  about 1KWh). That means, you need 7.5 years (1.5 x 5 = 7.5) to recover the extra price your pay for the HX650 80Plus Gold rated PSU over TX650 80Plus Bronze rated PSU.

Huh! How about that? I doubt you would use your PSU for that long, especially on a gaming rig. Even the warranty on these enthusiast class PSUs run for “only“ 5 years. So basically you won’t be able to recover that money.

Note: And we didn’t take inflation into account. The money you spend today is more valuable that the money you spend in 5 years.

Conclusion

Do you calculation and estimate how much time your would be using your PC. But I’ll give some examples where paying more to get a more efficient PSU would make sense.

  • If your PC houses multiple graphics cards, that means the power consumption when gaming would be well above 500W-600W, then go with the 80Plus Gold or even Platinum rated PSU. Even if you play games sparingly, the power consumption would be high enough to justify going with the more efficient PSU.
  • If you are a casual gamer, but have a very high-end PC, then stick to 80Plus Bronze. You won't be able to recover the extra price you pay for a higher rated PSU, because you don’t use the PC that much.
  • If you don't have a very high-end PC, then stick to 80Plus Bronze. You won't be able to recover the extra price you pay for a higher rated PSU, even if you play games all the time.
  • If you are running your PC for 24/7 and the power consumption is high (i.e. you are Folding), then go with a 80Plus Gold PSU.
  • If you are running your PC for 24/7 but the power consumption is low (as in a file server), then 80Plus Bronze is all you need.

Also take a look at this post on TechPowerUp forums.

Now, when I say don't fall for the 80Plus rating, I don't mean you should cheap out on a PSU. For example, the TX650 PSU I used in the earlier comparison is pretty high quality. It is still an enthusiast grade PSU. Well, perhaps you can cheap out a little, but for the sake of the components in your PC, don’t cheap out on the PSU too much because you can either spend too much money on a PSU, or you can spend too less and lose all your hardware. Which option would you choose? First option is better and would let you sleep well at night.

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