How to calculate power requirements

I still don't understand how to calculate power supply requirements. I know there are some, one or two that I know of, power supply calculators out there, but isn't there some way of looking up the watts yourself? Why does it seem so hard to find out how much power components need. I'm not interested in ballpark guessing anymore. :mad

 

Not sure why you would need exact power reguirements. You would need to know exactly the req'd wattage under max load for every single hard drive, CPU, Fan, MOBO...etc on the market. Reqd wattage would increase with age due to heat and dirt. The online caculators are great tools, its generally better to have the extra wattage if needed. You wouldn't want a 250W PSU if that is what your set up calls for.

 

Only way you could get exact current figures is to put a mulitimeter in line. Don't forget as the above poster stated, power supplies degrade %10 to %20 per year. I just use the calculator for a ballpark figure and double it when buying a PSU, an under powered PSU can destroy components.

 

Doesn't the manufacturer or designer test these things for how much power they need generally? If they have, haven't they posted the requirements somewhere?

The reason I'd like to know is because the more precisely I know how much wattage I need the less money I have to spend in "guesswork overhead wattage" when I'm purchasing a psu. For instance, if I'm looking at two psu's, one is 600 watts and the other 700 watts and there is a thirty dollar price difference, how am I supposed to know how badly I "need" the 700 watt? With thirty dollars on the line I'd like something more precise than just doubling, rounding, and estimating.

 

http://www.extreme.outervision.com/psucalculatorlite.jsp

 

This one is intresting... http://web.aanet.com.au/SnooP/psucalc.php

I checked my current system on three different sites and got 277, 291, and 416 Watts. I think some factor in what you should have vice what the componets draw.

I've had two sonys with 250W and an Emachine with a 300W and they all handled the addition of second HDD, CD-W or DVDs and a better video card plus doubling the ram during their lifetime. I built my latest PC and probably over did it with a 650W (wish I went smaller just for a quiter machine).

 

PSU calculators are all fine & dandy, but when you buy a PSU that the Manufacture Overrates/Fluffs it's specs, your Screwed :flip

 

@ Goran.P and Jerkyking: Great links! I'm in kind of the same spot as I'm looking into upgrading my PSU if needed. Both those PSU calcs pretty much had the same results when I ran though 'em with my numbers, so they're probably pretty accurate. And ASUS has a really good point; if some shady company puts bogus or 'best case scenario' specs on their sticker, you may end up with a peice of junk with great numbers on it. So it's probably best to buy from a reputable name brand that's been around for awhile. Many people have a problem with spending alot of money on a new PSU (and I'm one of 'em but I'm just cheap ) but if you think about it, the PSU is probably the MOST important part in the computer: it supplies all of the power to every component. A cheap power supply can spell disaster and instability and provide for a very frustrating experience. Many folks have PSU issues and think it's a motherboard or software or even a virus problem, when it's actually just an $18.00 PSU screaming "no more! help!" under the load....

While we're on the subject of calculating power numbers, why is it that I see some power supplies with 18a per 12v rail, with 4 rails total, but the combined amperage is listed at 52a which comes out to 13a per rail? 13x4=52... Why isn't the combined output 72a? 18x4=72...

Thanks again for the links guys!