Power Supply Selection: Watts or VA?
 

My current power supply is rated at 250 Watts, but seems to be near its end-of-life. A power-measuring meter says that the computer is drawing 130 Watts and has a Power Factor of 0.60. I understand that that means that in order for the computer to get 130 Watts of power, it has to draw 130 / 0.6 = 217 Volt-Amps, and that 87 Watts of power ( = 217 - 130 ) gets returned to the power station.

1. When chosing a replacement power supply, which number should I exceed, 130 or 217?

2. What safety factor is recommended for a computer that is used primarily as a desktop machine? Specifically, if the answer to #1 (above) is 130, and a safety factor of 50% is recommended, then that would call for a power supply rated for at least
130 * ( 1 + 0.50 ) = 260 Watts.

3. If the 217 Volt-amps number does not play a role in the selection of a power supply, why would anyone buy a more expensive high-efficiency power supply, many of which have power factors of nearly 0.85?

TIA...

I do not know enough to answer your questions, but I do have a problem with your math, 1.5*130=195, not 260.

Don't go for a powersupply that is just what you need, get a big "buffer", if you know you need 130W, you should get a 300+ W PSU, then you have power for more powerful hardware, should you ever upgrade your computer.

Search the web for "power supply calculator" plug in your components in one of the numerous ones out there and it will tell you the power supply size you need. I would suggest getting one by Antec, OCZ, Enermax .... something from a good quality manufacturer not a cheap generic brand they are nothing but problems in the long run. Depending on what you have for a machine (you should tells us BTW) get at least 24a on the +12v rail the 3.3v and 5v rails usually have more than enough amps than they will ever need anyways.

I think there is a difference between the power factor and efficiency.

To measure the efficiency you need to compare the input and the output(s) of the power supply.

If your power meter does not do that, I think that the power factor is something completely different. My knowledge is a bit rusty, but this article explains it.

Concluding: your understanding to draw 130 / 0.6 = 217 is not correct.

A3:
The higher the efficiency, the more power comes out for the same input power (or less power is converted to heat), so
- the less you pay to the elctricity company
- the less you waste energy and polute the environment

This is a very good PSU calculator you can use. It gets updated a lot.
http://www.extreme.outervision.com/psucalculator.jsp

I'm no electrical engineer, but if you currently are replacing your PSU, buy a minimum 350W or 400W model from a quality, name-brand manufacturer. That should be sufficient for the vast majority of typical desktops; if you have a particularly large number of peripherals that draw power you might want to take that up to 450W, but if you're running 250W now a 350W replacement should be fine. Some of the latest high-end graphics cards require a certain minimum power rating, so keep that in mind if you will likewise be upgrading the video card

I visit a gaming forum a lot and the best I recommend is a 500-550W PSU for high-demanding systems that have the latest GPU's. So for a general desktop PC which is future proof and can stand an upgrade in the future, 480W should be more then enough.

1) It depends on PF. Read http://www.lmphotonics.com/pwrfact.htm. This is for motors but it goes true for switch-mode power supplies (computer power supplies) too. I suggest buying the next model up. The 87 watts gets released as heat instead being return to power companies.
2) Do not make this confusing then is already is. Like I said in 1, buy the next model up.
3) When power supplies age, their ability to provide the advertise wattage gets worst. Switch-mode power supplies will only consume the power that the load needs plus the power that the power supply needs to work.

I recommend buying power supplies that have active power factor correction and universal voltage. Active power factor correction increases efficiency and decreases reactance load. Universal voltage makes the power supply a little immune to brown outs and surges because they have a wide voltage range usually between 100 volts to 240 volts. I have a Seasonic S12-430 (430 watts) that has such features. It is also more efficient than other power supplies. My Seasonic power supply regulates voltage very, very well at about 1%.

power supply
 

Like the previous poster I would also recommend the
Seasonic 430. Its very quiet and very efficient. Not
cheap but many more expensive power supplies are not
worth the money.

Well, I must admit that since reading a bit more on this, the wattage doesn't make much of a difference. You should be more focused on power efficiency, volts and the amp load each can take/deliver then focusing on watts.
Some sites do compile lists with the best PSU's, and those all seem to be Zippy, Seasonic, Enermax, OCZ, PCP&P, Antec or Fortron (FSP)