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Power Supply / UPS Guide

There are numerous and frequent posts asking about Power Supply Units (PSU) and Uninterruptable Power Supplies (UPS) compatibility and selection. Answering these posts has been getting really complicated since the PSU standards and hardware power requirements are getting updated every few months.

I did some research and threw this guide together. I will update it as the PSU standards change (we are at ATX12 V2.2 this week).

Wattage rating is marketing and tells you absolutely nothing about whether your PSU can stably power your computer! What types of voltages (V) and how much current (A) a computer uses has changed multiple times in the last 4 years.

The old days - ATX PSU standard. PCs used a reasonable amount of all voltages (3.3V, 5V, +12V) and a little bit of some other voltages (-5V, -12V). Any 250-350W PSU was fine. The biggest concern was if they used a junk fan in the PSU which would fail and fry the PSU. There are still many of these PSUs out there in older systems. These PSUs will not support any major upgrade to your computer. They are just too wimpy and do not the ATX12 (extra 4 pin connector or 24 pin MB connector used to supply additional 12V to the MB).

The not so old days - PCs started using massive amounts of 3.3V and 5V power for RAM, faster processors and even disk drives. During this period power supply wattage ratings were increased to 300-500W and the extra capacity added to the 3.3V and 5.5V outputs pushing them up into the 30-40A range. Some of these supplies have a 17A 12V capacity and if you get a molex to MB 4 pin adapter you might be able to do an upgrade with these. Check the MB, many have both a 24 pin connector and the original ATX12 4 pin connector.

Recently - There was just too much ultra clean 3.3V and 5V power required on the MB so first Intel and then AMD MBs started using the 12V (through an extra 4 pin connector) and converting it on the MB to make clean power for the processor and RAM. The ATX12 PSU standard added either a separate 4 pin MB connector or uses a 24 pin MB connector instead of the old 20 pin connector. The 24 pin connector usually has the last 4 pin section detachable leaving a 20 pin connector so most newer PSUs can support either 20, 24 or 20 + 4 pin connector type of MBs.

Now - This created our present problem. Most older power supplies do not have enough 12V capacity to supply the processor, RAM, add-in cards (especially video), fans, disk drives, etc. Upgrading from an older 2 gig system with a 9800 Pro/5xxx/4xxx video card and a 350W-400W power supply to a 64 Bit 3-4 gig processor and a X1800/68xx/78xx video will probably not work. So plan on a new PSU when upgrading.

ATX12 V2.2 Standard - Two separate 12V power rails are required AND the 4 pin MB connector (usually detachable) must have it's own overload protection. The outputs are called 12V1 and 12V2. 12V2 supplies cleaner power to the MB connector and is usually slightly higher rated than 12V1 to support the processor and the PCI-e slot demands.

The Wattage Myth
Think you are safe because you bought a 500W power supply a year or 2 ago? Probably not. Current top end systems require 30-39A of 12V power. Last year's 500W power supplies only have 17-25A of 12V. In addition, Intel has been requiring that the stability and noise level on the 12V power to the MB be improved. Separate overload protection is now required to prevent fires (this is driven by EU and explains why there are different PSU brands between North America and Europe).

Nvidia Specials
Nvidia and ATI now require additional 12V connections to their high end video cards in spite of the extra power available to the PCI-e connector on the MB. To make it even worse Nvidia is requiring special testing to certify that the 12V1 power for their extra power supply connectors is clean enough to support dual card SLI operation.

What to Look for in a new PSU
1) Ignore the wattage rating.
2) Make sure it says ATX12 V2.2 and at least ATX12 V2.01
3) 12V1 of 17A or more, 12V2 of 18A or more.
4) If you plan on SLI look for "SLI Certified" (meets Nvidia 12V1 standard). Do not fall for the "supports SLI" scam (it just has special connectors).
5) Make sure it has SATA power connectors.
6) Make sure it has enough power connectors. Using splitters and adapters to connect many devices to just a few molex plugs makes for dirty power.
7) Modular connector systems are optional. They make for a cleaner layout and better cooling flow but add nothing to power quality and can actual make the power dirtier due to the extra plug connections.
8) Make sure the PSU has a decent amount of hold up time, at least 10 milliseconds (see the UPS/BBS section). The longer the hold up the more spike/crash resistant your system will be. (Speaking of which while I was typing this my monitor and desk lamp flickered badly but my system kept on running. Ya gotta love Antec.)

Power Backup
True Uninterruptible Power Supply (UPS)
Runs continuously and converts whatever garbage comes out of the wall socket into clean 115V AC power for your Monitor, PC, Modem, Router, Printer, etc. A true UPS also protect against voltage spikes from lightning, power company problems or misbehaved appliances in your home. A UPS contains a battery that it uses to provide power if the wall socket dies completely.

The disadvantages of a UPS is cost, they are generally more expensive than BBS, and since the UPS runs continuously converting power it consumes electricity continuously.

The amount of extra power can be between 10-30% of what your PC equipment uses. If the UPS runs 24/7 that means 100-300W. At $0.12 a kWhr that means an extra $150-315 a year in electric bills. Higher in the summer when your air conditioning needs to cool that extra heat load, less in the winter when the waste heat warms your house.

Battery Backup Supply (BBS) Many times what you think is a UPS is really a BBS. The BBS monitors the power line garbage continuously and if the voltage gets really low then turns itself on after a short start up delay to keep your equipment running.

There are disadvantages to the BBS. It is not uninterruptible. It depends on your computer equipment (PC, Monitor, Modem, etc) to have enough local energy storage to keep operating while the BBS starts up. This means you need a PSU with at least 10 milliseconds of hold up capability. Good power supplies will show the hold up time in their technical specifications. If a power supply does not show a hold up number find another power supply to buy or buy a UPS. The second draw back to a BBS is that since it is not cleaning the garbage continuously it is not as effective at protecting your
system from low line voltages, lightning and spike damage as a UPS.

Sizing the UPS/BBS
On the labels for equipment (or in the manual, on the box, etc) the power consumption is listed in watts. Add the monitor, modem, router, printer power consumption and 1.3 times the PSU rating. You add 30% to the PSU because its rating is output power and under the ATX PSU standards the PSU can waste between 30% (V2.2) and 40% (V1.0). This number will be very large and you probably won't want to spend the money for a UPS/BBS with that high a rating.

Volt Amps (VA) vs Watts (W)
Watts are what a lamp, heater or DC uses. Wall powered (115Vac) Electronics use Volt Amps. The difference is harmonic distortion and power factor (don't ask, it's too complicated). The EU requires that equipment be designed so that W and VA are almost the same. The USA and Canada allow crappy design. If the equipment says "Power Factor Correction" VA and W are very close and you can ignore this section. The better power supplies are designed the same for EU and NA so they have power factor correction. Check the spec's. If the equipment does not have power factor correction you need to adjust the Watts upward. This computation can get ugly (a bigger UPS/BBS is required). Fortunately most UPS/BBS I've seen have a table on the box. It will give a Watts rating and a VA rating assuming typical crappy North American designs. If the PSU/Monitor/etc is not power factor corrected as rule of thumb add 25% to the Watt ratings.

At this point a True UPS adequately rated costs as much as a PC and you will want to start taking risk and reduce the power number, decide on a BBS or decide to pay the price if you are rich (or single). If you don't mind the risk assume the printer is not printing (assume 0W), the PC is not folding, gaming, etc. and only using 50-70% (depending on how lucky you feel) of rated power. Switching to and LCD from a CRT display will help also. Or you can just feed the display (modem, router, etc) through a surge protector and just keep the computer alive.

What to Look for in a new UPS/BBS
1) Decide if you want/need a UPS or a BBS. The key will be a statement like "always online (a UPS) or fills in on power loss (a BBS)".
2) Do the math on how much capacity you need, then make compromises if you must.
3) Make sure that everything connected to your PC (phone, modem, router, printer, TV inputs!) is powered by the UPS/BBS OR is on a good quality surge suppressor (APC, etc).
4) Any UPS/BBS or power surge suppressor that is any good will have an equipment replacement warranty for your PC equipment if it is damaged by lightning or a power spike.
5) Battery life. This depends on if you just want to operate through spikes (minutes), have time to save data and to do an orderly shutdown (5-10 minutes), or if you need to operate for an extended period to keep a server up (30 minutes, after that start thinking standby generator).

Found this for Tripp Lite brand. UPS configuration tool. You can use this to determine how large a UPS you need and see Tripp Lite's options:

UPDATE2 the sequel
1) Many power supplies are showing up with 3 independently regulated 12V rails now. They are doing this to support SLI/Crossfire systems so each vid card and the MB will have clean, separate power. Not a bad idea but the most important thing if you are running SLI is the Nvidia Certification.

2) Antec has same neat new PSU's in their Truepower lineup called Truecontrol.
-These power supplies let the user tweak the output voltages, nice if you have a heavily loaded system.
-They have several speed controlled "fan only" outputs so your case fan speeds/noise will be reduced when the heat load is low. Saves on buying speed controlled fans or a quad fan controller.

New Info 2-2007
Not all power supplies labeled a certain brand are made by that company. Check out this artical:
Particularly interesting was the Antec Neo line with power supplies being manufactured by Seasonic, not by Antec. That explains why I've seen a lot of substandard Neo line power supplies compared to the Antec Truepower.

New advice:
Get the UR number off the PSU label and check it at the UL web site to see if it is really the brand you are paying for.

And even better. Check the UR number on any "cheapie" power supplies you are looking at. They may turn out to be a hidden gen made by a good name company and sold as OEM.

Update - Ahhh, 1 rail vs 2 rail vs 3 rail... The new great debate.
-The standard (ATX 12 V 2.x is still valid I think) calls for 2 rails.
-Antec is pimping 3 rails now.
-PC Power & Cooling is pimping a single rail.
There are pro's and con's to each.

A rail is an individually regulated power output.

The Problemo
For recent PC's the +12V is critical since it now powers the CPU and GPU which are humongous power pigs that draw big surges of current when they switch in and out of high demand operations. PSUs can't respond to those surges instantly so if the CPU and GPU both say "gimme more now" the PSU may not be able to react fast enough and fall out of spec. (BSOD/Reboot time).

The ATX 12V2 standard decided the best approach was to isolate the GPU and the CPU on two seperate +12V output rails (one for MB/CPU other for GPU's, etc). This has worked fine for a while and then SLI/Crossfire same along. Now there are 3 obscene power pigs and only two +12V rails. What is a PSU maker to do?

-Antec said ok, lets just add a 3rd +12V line for the 2nd vid card.
--Provides isolation between each major power user.
-Power PC said, enough is enough. Lets just make one, honking, fat ass +12V rail.
--Provides a brute force approach which does not isolate the surges from big loads because it's so tough it doesn't care.

When building $100 million dollar airplanes we go with the isolated multiple rails approach. It costs a little more because more parts and more testing is involved. But what the hey, we're worth it. And with a single rail, if it dies, everything dies with it. Bad plan in airplanes and not so good for your $400 video cards and CPUs either. Now the protection in the PSU should prevent collateral damage to the loads, but Murphy was an optimist. (Murphy? Google Murphy's Laws and his corollaries. Good reading.)
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Spammer representing.
Political User
Nice post Lee.

I agree completely that people need to know what the difference is between older specs and newer. I have seen a definate trend in cheaper local builts to use cheap cases with older PSU's. And people wonder why the system is noisy!

Most people can affort a $80 UPS for their computer, and it will provide preotection and peace of mind. I can post some screenies of what happens here when the Pepsi machine-Microwave-Coffee maker turns on. The amount of voltage variation and phase altering that occurs. It is kind of scary when you have $20-30K of computers eating dirty power that shortens life and causes problems.


Stranger Than Fiction
Political User
Considering I just posted a question on this topic yesterday I'd say this is a very important guide. Reps to Lee ... Many thanks as well!
Bouncing. Yours and 2 other posts yesterday are what inspired me.

Steevo. Where can you get a 750VA UPS for $80. That price sounds more like a BBS.


OSNN Veteran Original
Nice job on the thread, with my new system on the way. I also need a new powersupply, which I didnt know anything about.

Great information!

Those are Battery Backup Systems.
The spec's require the computer equipment to stay on line for 5-8 milliseconds before they provide fill in power.

If the voltage is between 88-138Vrms they are not providing power. That is the drawback to BBS vs UPS.

Not as good as a true UPS, but the prices have dropped nicely on the 750 and 900 VA units since I last checked. The 900 VA APC is on sale for $107 at Besy Buy.
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Spammer representing.
Political User
1100 VA is the same at Newegg.

Mine has the bulldog protection software, with no time lapse and you can adjust the voltage cut and step up to what ever you want.

This pic is from my personal one at home.

I guess it is actually a 500VA Belkin. It has passed a 5 miunute test though with 100% CPU useage. It has auto save for any open programs, and shuts down when the batterys are low, as well as being able to put your PC into suspention with the software and a easy setup.



Bow Down to the King
Political User
I need a UPS unit that is space saving, black in color, and nice software. I have four items that need to be plugged in (PC, Monitor, Speakers, and Cordless Desktop -- the PC is the only one I really need on backup).

I don't like the APC units because they are rather bulky. Belkin has a few nice units but I wanted to see what others thought of them before I purchased one.

To be honest I still like Tripp Lite's newest (http://www.tripplite.com/products/product.cfm?productID=3082 or http://www.tripplite.com/products/p...071&ciLinkID=HOM_0905_A_FLASHBOX_SMART1000LCD). It's a smaller unit and tower form. Belkin has a unit that is similar in size and they both offer similar features. Does anyone have any input?
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Spammer representing.
Political User
I like my little 500VA from Belkin, I don't card for the newer series of SOHO crap though. They are ugly, and the cords-transformers are easily knocked out.

I recently got a 1200VA from Opti and really liked it. From mow on that is what I will be buying.


Bow Down to the King
Political User
Ahh, good point. I certainly don't want a UPS with the outlets on the top.

What do you think of Tripp Lite?


Political User
My plugins are on the top, but then again, they are flat, and not standing up. They are also in the basement, so not that big of a deal if they are flat and not cool looking.


Bow Down to the King
Political User
It has to fit into a small space on the side of my desk. That's why I need some thing smaller (tower). Plugs in the back so they don't show.
The Tripp-lite looks fine:

$200k connected equipment warantee.
Tripp Lite is a significant player in the power supply industry.
4 millisecond power transfer.
Generates a cleaner sinewave output than the APC brand units.
Faster battery recharge than the APC.
Adequate lightning/spike protection at 480 joules (that's a lot).
PC safe shutdown software can be downloaded from their site (free).
SmartPRO is just the upscale version (USB etc.), same basic spec's., better insurance.
Go for it.

The belkin is ok. Intended more for home use with the lower connected equipment warantee of $75k. The triplite spec. looks more like commercial quality. There is a Belkin with 900VA, same list price as the Tripplite and it has a broadband protection input.

Either would be adequate so go for the pretty one in this case.

Note these are both BBS inspite of being called UPS. A UPS has "0" transfer time.

These units have a nice boost/buck function for voltages between 90-135Vac. That will keep them from going onto battery when it's just a power surge vs a true drop out.
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Bow Down to the King
Political User
I ordered the Tripp Lite SmartPro LCD UPS 1000VA. Final cost including shipping $147.99.

Thanks for your help.


Bow Down to the King
Political User
I received my Tripp Lite SmartPro LCD UPS 1000VA on Friday and had a chance to install it yesterday. Simply put, it is a very nice unit and appears to work quite well. I haven't installed the software yet but I will update everyone once I do.

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