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Accuracy of PSU calculator?

Last response: in Systems
October 15, 2009 8:18:34 AM

I tried this PSU calculator:
and it seems like it is giving me a low estimate for the power supply I would need to run the following system:

Core i7 920 stock
"high end desktop"
2 sticks ddr3
one sata hard drive
one radeon hd 5870
dvd burner
1 PCI card (I did this to represent a sound card)
2 regular performance 250mm fans
1 regular performance 120 mm fan
capacitor aging of 20%
90% system load

It said I needed a 395 watt (continuous I am assuming) to run it. I have an Antec TruePower Trio 550 watt (continuous) right now and the following are my specs:

Intel Core 2 Duo E6600 2.4 overclocked to 3.15GHz, 1.35 vcore (just guessed, don't remember)
8800GT 512MB
3 sticks ddr2
one sata drive
one dvd burner
asus p5b deluxe wifi-ap (don't know if that qualifies for "high end desktop but I will use that anyway)
one sound card
3 120mm fans (LED)
2 80mm fans (LED)
floppy drive
35% capacitor aging (3 years old)

The one for my current system says 383. Why then do I have a 550 watt? Is that accurate?

Also, I know the PSU rails have a lot to do with requirements as well. Let's say I have one 12V rail. In what situation would only one be bad and why? I know very basic stuff like if you have parallel circuits the voltage is the same on all of the wires, does this have to do with the amperage available per rail?

Thanks. I was building a mock i7 system and it got me thinking.

a b B Homebuilt system
October 15, 2009 3:40:30 PM

Those two systems sound as though they would be quite close in power draw. I don't think I should comment on the accuracy of that particular calculator as I am unfamiliar with it.
But those specs do not sound unreasonable.
You didn't say if this was fully loaded, but it does not sound like idle specs.

Considering an 80% efficiency (which is high for a power supply), I'd say a 550W is the minimum PSU I would recommend for that set up. If it were me, I would probably choose a 650 watt for a little extra headroom.

Some folks swear by one powerful 12 volt rail, and I don't disagree with that. Preferentailly, I suppose I would get two rails. My understanding is the multiple rails is somewhat of a marketing gimmick, and what's truly important is to have enough amps on one rail to handle your video card(s).

I believe I used an Antec calculator for my system. For comparison:
AMD X4 940 @ 3.8Ghz
Biostar 790GX mobo
4 gig 1066 OCZ reaper
Coolit Domino ALC
2 sata HD's
2 dvd drives
lighting X4
Antec 900 with 5 fans
Antec 650 Blue PSU

I think fully loaded I was looking at about 485Watts.
a c 90 B Homebuilt system
October 15, 2009 4:08:48 PM

buzznut said:
Considering an 80% efficiency (which is high for a power supply), I'd say a 550W is the minimum PSU I would recommend for that set up. If it were me, I would probably choose a 650 watt for a little extra headroom.
If you have a 550W PSU that rating is 550W DC power output for the system.
Where the 80% efficiency comes into play is that it will take an input of 660W AC power from the wall socket to create the full 550W worth of DC output.

sgt matt;
The PSU calculator is pretty accurate. Everyone is surprised that the powerful systems don't suck huge amounts of power considering the 1000W and 1200W PSUs out on the market now.
You can usually find a few review sites to back up the PSU calculator.
i7-920 @ 3.33GHz / Intel DX58SO (Intel X58) / Intel X25-M SSD (80GB) / Patriot Viper DDR3-1333 3 x 2GB

The power draw measured at the wall socket of 401watts means the system is using 320W @ 80% efficiency, 328W @ 82% efficiency or 340W @ 85% efficiency depending on the PSU in use and its efficiency rating in that power band.

I am less concerned about how many rails the PSUs have as about how much +12V amperage they provide.

November 13, 2009 1:33:51 AM

It works fine don't worry too much.

Just input your cpu, memory,

and video card. Calc showing 252w (12v)

for your setup. PSUs are rated in total watts

which mean nothing as most everything runs

on 12v. Match this number to the 12v watt number

(V * A not W given) on the power supply. The

is on the PSU label. PSUs are usually rated

@ 30oC so subtract 10% for 40oC operation.

280w (12v) @ 30oC = 252w (12v) @ 40oC

Look at this PSU 12V * 30A = 360w

"Rated up to 400W of continuous power output at 40ºC"