PSU amps vs watts

martian_aphid · 6 May 2008 at 13:31

Thanks to the guidance of the users of this forum I learnt that the most important thing to look for in getting the right PSU is the amp rating on the +12volt rails.

My question, is there any difference in a two PSUs that suplpy the same amps on the +12v rails but are rated with different wattages

For examples, this Tagan 900W:
http://www.overclockers.co.uk/showproduct.php?prodid=CA-017-TG&groupid=701&catid=123&subcat=1100

Its spec. confirms 64A on the +12v rails, now they also do a 800W version of that PSU which has exactly the same modular connectivity and the same 64A on the +12v rails but delivers a different wattage. Are these basically the same PSU but by getting the 800W you can supply the ~??A you require but at reduced cost.

Thanks

Danny

errata · 6 May 2008 at 13:43

if the basis of quoting is the same then the diffence is on the non 12v outputs, if you dont need the power on those other voltages (for drives say) then get the low rated supply.

make sure the number of rails etc is the same, as the continous rating of rails may be a different reason for the wattage being lower on the cheaper supply (more rails is not neccesarily better)

amkan13 · 6 May 2008 at 13:50

power (watts) = current (amps) * voltage (volts)

the wattage is different but has the same amps on +12v rail. that just means the other rails 5v, 3.3v, etc have more amps on them with the 900w PSU.

the 800W will have less amps on these other rails. these other rails are usually for hdds, dvd drives etc

Super_Sonic · 6 May 2008 at 18:59

P = I x V x P.F

P.F = kW/kVA aka True Power/Apparent Power

/pedant

p4radox · 6 May 2008 at 19:04

Super_Sonic said:
P = I x V x P.F

P.F = kW/kVA aka True Power/Apparent Power

/pedant

Ahh, but all modern PSUs have active PFC, so in theory at least the reactive power is zero.

Super_Sonic · 6 May 2008 at 20:24

p4radox said:
Ahh, but all modern PSUs have active PFC, so in theory at least the reactive power is zero.

Indeed but that's in theory, as we all know a reactive power of zero would mean a unity power factor which in reality is not possible even with active PFC.
The best active PFC can do is 0.99 and an 80PLUS PC Power supply must have a power factor of 0.9 or more to obtain a certificate.
Either way be it active or passive PFC it's a great thing as a computer PSU without any power factor correction has an awful power factor of around 0.55-0.65 :eek:

Think of all the wasted power if every computer PSU out their had no PFC