Power Supply Requirements

[CharlesH]

I just built a computer. The components of the computer are as follows:

1. Asus H87M-Plus/CSM
2. Intel Core I5-4430 CPU
3. Kingston Tech Hyper 8 GB 1600 MHZ DDR3 Dual Channel Kit-240 Pin SDRAM (KHX1600C9D3K2/8GX)
4. Lite-On LightScribe 24X SATA DVD+/-RW Dual Layer Drive
5. Legacy Kingston VGA monitor
6. PS2 keyboard
7. USB mouse
8. Two USB external hard drives

Currently, I have a 485 watt power supply unit in the system. Because I may want to add a high level video card to the system, at some point, that may require a higher wattage power supply, I measured the wattage usage of my system using a Kill-A-Watt meter. According to what my meter tells me, the maximum wattage my system is using is less than 65 watts.

But when I use Website power supply unit calculators, I usually get wattage estimates between 300 and 450 watts for what's required for my system. That is, I'm told that I need a PSU rated between 300 and 450 watts.

Here is my question: Why is there such a discrepancy between what my system is apparently using and what the PSU calculators say I need?

 

[TechyInAZ]

You always want at least 50 to 100W minimum of extra power wattage on your PSU. So get a 500W to 600W psu.

 

[numanator]

You typically get a power supply based on the maximum possible power usage, this typically lets you allow for the power supply to age (power output typically drops as a power supply gets older). Quality of components and brand make a huge difference.

The measured usage you got was probably not at maximum CPU usage. The i5 4xxx series can use a max of around 100W and other minor components would be another 30-50 tops. With your current system a decent 200w power supply would be enough.

If you plan to get a graphics card in the build you will need more power. The highest power usage (single card) graphics card uses about 350w max by itself, add to that the max CPU usage and you get a 500w needed. But, you want to run your power supplies at 80% load or less for improved efficiency an a longer life for the PSU so for the highest power usage card in your system, a 650w power supply would be recommended from a notable brand model (good ones: any XFX, any Seasonic, Corsair AX/HX/TX series, Antec HCG series, for 750w+ EVGA G2 Series)

If you know what GPU you are considering I can figure out how large of a power supply you would actually need, most work with a 550w, a few work on around 450w.

Edit: To actually answer your question The reason that the power supply calculators are typically displaying more than you acutally need is because many people purchase bad power supplies which often use shady marketing techniques like not outputting the stated power, outputting the power on a different rail (CPU + GPU draw power from a 12v rail and that should be a majority of the power supply's watts), displaying a maximum powr output instead of a minimum. Because of this the power supply calculators and gpu manufacturers tend to recommend higher wattage than what the parts actually draw to try to mitigate complaints to themselves.