Originally posted by: JackSpadesSI
Does anyone know of a calculator program - or even a rule of thumb - that can estimate, based on which parts you've compiled, how much power you'll need (and then you could add a safety buffer for peace of mind)?
Find the approximate power needed by your CPU and video card. For CPUs, the TDP will work acceptably and is generally easy enough to find. For video cards, you will generally need to find a review that does some power testing (such as most Anandtech video card reviews) and make some inferences. Or guess.
Add those numbers together. Add 50 to cover miscellaneous hardware. Divide the result by 10. Buy a power supply with a 12 V rail rated for
at least that many amps. This is an adeqaute (and relatively easy to compute) rule of thumb for most purposes, although it is not at all precise (and on top of that, it deliberately tries to over-estimate!).
Example:
TDP of Pentium E2160: 65 W
Est. power draw of GeForce 8600 GT: 43 W (
link)
Miscellaneous: let's say 50 W
Sum: about 160 W
Sum/10 = 16
Suggested power supply rating would be 16 A on the 12 V rail or better.
Originally posted by: JackSpadesSI
Nice link! Could I use that total power number for a different brand of PSU (say, Corsair)?
To the extent that you can use it for anything, yes. Power supply estimators that provide only a total wattage rating recommendation (such as Outervision) are limited in their utility simply because they have to make some significant assumptions. Two different power supplies with a total rating of 500 W do not necessarily have the same rating for the 12 V rail, which is where power for the CPU and video card (the most power-hungry components) comes from.
For example, if it told me I need 630W would a 650W do it or is that too close for comfort?
You can always go
higher; the real concern is if you want to go lower. Anyway there is a great deal of slack in that type of recommendation, so don't take it as gospel.
Given two absolutely identical systems (except for their PSUs) running at absolutely identical loads on every component, would the system with a 750W PSU draw more power from the wall than the system with the 650W PSU (assuming that the total system load was within the capabilities of both PSUs)?
It depends. The system with a 750 W supply will not draw an extra 100 W, if that is what you are asking.
The efficiency of a power supply follows a curve, often expressed as a percentage that varies based on how much load is put on the supply (as a percentage of the supply's rated output). The peak generally occurs when the supply is loaded at about 50% of its rated output.
Example. Additionally, some designs are simply more efficient, at any load, than others.
The bottom line is that you can't predict the outcome of that kind of comparison unless you have two specific models of power supply in mind.
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