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How to Choose Computer Power Supply?

Buy a Power Supply by the Pound

- A simple and easy way to estimate the quality of a PC power supply.

By Dr. Michael, Directron.com

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The following article is based on years of experience. It is provided as a free service to our customers and visitors. However, Directron.com is not responsible for any damage as a result of following any of this advice.

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A PC power supply is one of the most important components in a computer, yet it is often the least appreciated due to it’s "low-tech nature". When a power supply is dead, your entire system is dead. A bad computer power supply could also cause other parts of your system to fail. As personal computers become ever more powerful, the importance of a reliable power supply is more than ever before.

It’s easy to tell if a PC power supply is working or not. However, until now it has been hard for a PC end-user to measure the quality and reliability of a power supply. Herein we attempt to document a very simple way to estimate the quality of a computer power supply by the weight. We measured the weights of 38 standard ATX power supplies of different wattages, models and manufacturers. The weight data were then correlated with the current price and wattage on the label. The findings are in agreement with the return rates and general experience we accumulated from many years of selling and servicing power supplies.

Equipment

The weight is measured by a digital scale with the accuracy of +/- 0.01 lb.

Results

Table 1 shows the models, wattage, net weights, and current prices (on 12-20-00) of 38 power supplies from 23 manufacturers. The weights were measured with a digital scale. The error margin was within +/- 0.1lb. The market prices of the products (as they were, not changed or adjusted for this study) were plotted as a function of the weight in Figure 1. The wattage of the power supplies was plotted as a function of the weight in Figure 2. The wattage data were what were shown on the power supply stickers.

Observations

Several observations can be made from the data:

There is a large variation in the weight among the same wattage. For instance, among all the 300W power supplies, the weight ranged from 2.15 to 3.8lb, a 1.65lb difference. The lightest one is only 57% of the heaviest in weight. Among the 250W, the weight ranged from 1.7 to 3.1lb, a 1.4lb gap. The heaviest one is 82% heavier than the lightest one. Judging by these data, some of the good-quality 250W power supplies are clearly better than the poor-quality 300W’s. Therefore, don’t be fooled by the wattage on the sticker.
Generally, the Athlon-approved power supplies weigh more than the non-Athlon approved of the same wattage, confirming that Athlon-approved ones are of better quality. It's possible that manufacturers only submit better models for the AMD approval process.
The name-brand power supplies typically weigh more than the less-known, generic brands of the same wattage.
FCC approved power supplies often weigh more than those without FCC labels.
Figure 1 clearly shows that the market price of the power supply is, to a degree, proportional to the weight, especially for items heavier than 2.25lb. The higher the weight, the higher the price tends to be. You pay for the pound! For power supplies weighing lower than 2.25lb (the blue dots), the correlation does not work well, suggesting either the existence of a minimum market value or some of these power supplies should be priced even lower (due to low quality).
Figure 2 shows that the wattage of a good-quality power supply is, to a degree, proportional to the weight for the good-quality power supplies such as Athlon approved 250W's and 300W's. This diagram also shows that some of the low-weight power supplies (blue dots) most likely do not have the true wattage as labeled.

Why Weight Matters?

The more appropriate question is why size matters.

The weight of a power supply is directly related to the quantity, quality, and size of the material (thus cost) used to build the power supply. We opened a light and a heavy 300W-labeled power supplies. The heavy one has larger capacitors, thicker wires, larger transformer, larger heat sinks, more connectors, and more capacitors than the light one, all of which are important factors for the overall cost and quality of a power supply. The difference is illustrated below:

Conclusions

This brief study clearly confirms an empirical knowledge: the quality of a power supply can be estimated by its weight. While this is not a true scientific or thorough measurement of the power supply reliability, it is nevertheless a very simple and easy way for ordinary PC users to estimate and compare the quality of a power supply. Directron.com has pioneered this measurement and has now published the weights of all power supplies sold on Directron.com’s web site.

It’s important for the end-users to know the quality of a power supply. The specs of power supplies are simply printed on stickers. Don’t be fooled by them! The stickers can be easily removed and replaced. Even the manufacturers might place a 300W sticker on an otherwise 250W power supply. Once products are shipped it is quiet rare for any private or public entity to go back and verify the specs on the sticker. However, it’s difficult, if not impossible, for a dishonest vendor to alter the weight of a power supply.

By no means is the weight the best way to measure the quality of a power supply. There are other factors one should consider in purchasing a power supply such as wattage, output, brand, price, noise level, and long-term stability. However, to most end-users, measuring the weight may be the only viable method that's widely available.

The large variation in the quality of power supplies with identical wattage rating suggests that an industry agency is needed to rate, confirm, and police the labeling of power supplies to protect the consumers.

Table 2. A guideline to classify the quality of a power supply by its weight (lb).

	Low Quality	Average Quality	High Quality
250W	1.7-2.4	2.4-2.8	2.8-3.1
300W	2.1-2.7	2.7-3.2	3.3-3.8

Based on the above criteria and our general experience in return rates, these are consistently high quality brands on the market: EnerMax, Delta, PC Power & Cooling, Enhance, Antec, CWT, and SPI. These brands are consistently associated with low-eight models: L&C, Deer, Skyhawk, Star, KME, KingStar, and LCT. Our experience shows that the return rates of a low-weight power supply could be as high as twice of those of high-quality ones. How does your power supply measure up?

Physical Dimensions

Besides the specs and form factors, the physical dimensions are also important factors in selecting a compatible power supply. Here is an outline of the physical dimensions of most standard power supplies:

ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.

Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.

MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.

Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.

Use the data above to determine if a particular power supply would fit your case.

Notes: (1) Most case manufacturers do not make their own power supplies. They often use power supplies that are re-labeled as their brands. For instance, PowerMan brand from In-Win is made by SPI (same as FPS). One good way to identify the true manufacturer of a power supply is to look at the circuit board on which the manufacture name is often printed. (2) Power supplies are also heavily cross-branded. For instance, Enhance power supplies are branded as FKI and HP. (3) The largest power supply manufacturer is Delta since Delta makes power supplies for those large OEMs; however, Delta does not make power supplies for the distribution channels. (4) It’s possible that some power supply manufacturers produce two grades of the power supply for different market segments. An example is the CWT-300ATX (FCC approved) and ATX-300 (non-FCC), which are 0.6lb apart. (5) Other factors may also contribute to the weight differences, for instance, the number of power connectors and fans may vary from one to the other. However, such factors typically are in agreement with the trend. The heavier the better - assuming other conditions are the same. (6) It'd be nice to do a further study to correlate the weight of power supplies with their output performances. (7) The purpose of this article and study is not to promote or demote any particular brand or model, but to provide a subjective and factual results. (8) One may ask why some manufacturers choose to cut the corners to produce below-average power supplies. The main driving force is the demand for cost-reduction of a complete system in the reselling market. Nowadays, one can purchase an ATX case with power supply for less than $25, which can significantly reduce the price of a system for those consumers who may have never pronounced the phrase "power supply." (9) Most resellers who sell systems would not quote the brand of the power supply included in the system. If you buy computers instead of build your own, ask for the brand of the power supply.

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A Good First Step - Sep 6, 2005
Reviewer: Scott Allen Rauch
This is a good first step toward providing us end users something we can use. It would be nice if it were updated with brands you currently have and higher powers.

- - Jul 23, 2005
Reviewer: -
Informative and useful for the novice - excellent article. Well done!

great page - Feb 24, 2005
Reviewer: Paul
Really helped in understanding which brands were trustworthy and the difference in pricing between similar products!!!!!!

Power Supply - Nov 18, 2004
Reviewer: Tgmac
The report looked out dated and it was based on year 2000 info data. I think Directron needs an updated for this. This year 2004, I brought 3 cheapest 350 watts Atx and 2 expensive 450 watts Atx. The 350 watts had no problems but both 450 watts failed within one months. I found out 350 watts were cheap because of high volume production. I got both 450 watts replacement from later shipment and both are fine now. I cannot see price is an important factor these days and depending the way you shop. New electronic compotents are smaller and lighter nowadays and is not the other factor. The newer the product has the more problems to start with at the beginning. Reputable supplier is the important issue!

insightful - Oct 7, 2004
Reviewer: Barry
i bought a cheap PSU with cool lights and now I'm regretting it. Don't skimp on the price of the PSU.

GREAT INFO - Aug 12, 2004
Reviewer: Atheria
I looked all over the Internet for info that would help me make an educated decision when purchasing a replacement power supply. I'm replacing a low quality model that doesn't provide the correct amount of power that came with my PC. As Dr. Michael said at the end of the article, be aware, or beware, of what you are buying and ask about the power supply that comes with any system. I have had problems for years with my PC and the lack of sufficient power has damaged some of my components but the manufacturer doesn't care once the PC is sold. Thanks for the GREAT Info!