How much wattage do you actually need?

[BroFister420]

When people look for power supplies, they usually have some arbitrary wattage number in mind. When they don't, they'll ask around and many people will recommend a similar number. Many will even recommend higher range for that rare case where you decide to "upgrade." But they never have a calculated number when determining their build for optimal efficiency.

 

2 parts: efficiency and $ cost $

 

First off, we know a few things:

-Running below 20% capacity is the least efficient.

-Running around 50-60% capacity is the most efficient.

 

I'll start with one of my rigs. After seeing the parts, guess how much wattage is used before looking at the answer.

This is on a Seasonic 550w 80+Gold

 

AMD 3670k OC 3ghz

Gigabyte Radeon 7870 OC 1.1ghz

5 * HDDs

1 * SSD

3 * 120mm fans

1 * PCI sound card

1 * PCIe USB3.0 card

1 * USB3 card reader

 

 

 

 

 

 

 

idle / load

76w / 328w

 

Sound right? Or surprised? Let's recap the TDP on the parts from wikipedia charts:

AMD 3670k = 100W

Radeon 7870 1ghz edition = 175w

5 HDDs = ~40w (8w each, I always keep my HDDs spinning)

With this, we already have 315w. That leaves about ~13w to cover the rest of the minor stuff, which is about right (SSD, fans, etc.).

 

Now that load # will rarely ever be hit in actual usage; I was maxing out prime95 and Furmark at once.

In fact, I'd say about a good 80% or more of my personal usage will be at idle. Pretty much, whenever I'm not gaming or doing something that's clearly going to require some hefty power. Could be listening to music, browsing, doing homework, etc.; still considered idle.

 

So, 76w. That's about 13-14% of my PSUs capacity. That's a really inefficient area! Considering that usage is what my PC is mostly at, I'm getting very bad efficiency. When you hit really low numbers, the efficiency scale can go way up in %. But at the same time, the amount becomes closer and closer to being negligible. (Need 1w, but pulls 2w)

 

However, on the load side, I'm at a nice 59%, which is in the most efficient bracket. This is the perfect sweet spot, as the most power you can pull will also be the most efficient.

 

-----------------------

 

The above covers the efficiency part. So let's look at the cost side of things. There are 2 targets to aim for:

Sufficient wattage

or

Optimal efficiency

 

For reference, I'm going to refer to these 2 PSUs. 1 is 80+gold 550w for $100. The other is just 80+ 350w $40.

http://www.ncix.com/products/?sku=76942&vpn=SSR-550RM&manufacture=Seasonic%20Electronics

 

http://www.ncix.com/products/?sku=30144&vpn=SS-350ET%20APFC&manufacture=Seasonic%20Electronics

 

Optimal efficiency would be hitting the 50-60% sweet spot to get the most of out your PSU. Pretty straightfoward.

 

Sufficient wattage is basically what you need, and only that. For my system, I'm pulling 328w on 100% load. And while they don't make that exact wattage in PSUs, a 350w psu is what I would use.

"OC CPU and a 7870 on a 350w PSU?! You're obviously crazy!"

Well, no. This may have been the case in the past, when quality OEMs were hard to come by. The rated wattage is there for a reason. Quality PSUs will go through vigorous testing to make sure they don't explode or catch fire. That's what you pay for.

 

So that's a $60 difference between extreme efficiency vs. necessity.

For this example, let's say I pay $0.15/kW and the system wattage required is 80w/330w (idle/load).

The computer is used 8 hours a day, every day for a year.

50/50 time spent in idle and load.

550w 80+Gold @ 87/90% efficiency.

350W 80+ @ 80%.

 

Here's the formula used the gold efficiency:

8 * 365 = 2920 / 2 = 1860

330 * 0.1 = 33w load waste

80 * 0.13 = 10.3w idle waste

(((33+10.3)*1860)/1000)*0.15 = $12.08 / year

 

8 * 365 = 2920 / 2 = 1860

330 * 0.2 = 66w load waste

80 * 0.2 = 16w idle waste

(((66+16)*1860)/1000)*0.15 = $22.88 / year

 

If I were to get the 550w instead of the 350w, I would save $10.80 / year on electricity with this system. I'd have to use this system for almost 6 years to get my cost back from getting the better and higher wattage power supply.

 

So what does this all mean? Depending on how long you keep your PC, you may have just saved $30/$40 in that scenario.

In reality, my electricity is cheaper that the example, so I'd have to keep the system for 10+ years to make up the cost of buying that higher wattage and efficiency PSU.

 

 

Yeah...

[Bluethulhu]

I would get the 550w so you don't have to buy a new PSU if you plan to upgrade. 350 is pushing it for a lot of systems you want to upgrade IMO.

[wng_kingsley7]

I like this thread........500w-600w sounds pretty esteem

[Lord Pantaloons]

This is nonsense, you need at least 1200w 80+ platinum for an i3 and a GT 640, having more watts make your computer faster cause it's getting more power.

[miyabwah]

This is nonsense, you need at least 1200w 80+ platinum for an i3 and a GT 640, having more watts make your computer faster cause it's getting more power.

I agree. I have a 250 watt power supply that makes my computer slower.

[Lord Pantaloons]

I agree. I have a 250 watt power supply that makes my computer slower.

If you had a Rosewill Hercules 1600w your system would perform better than an i5 w/GTX 680, you should look into that as your next upgrade.

[Dr.Gordon]

This is nonsense, you need at least 1200w 80+ platinum for an i3 and a GT 640, having more watts make your computer faster cause it's getting more power.

not sure if serious or trolling.. :')

[noobhunter]

If you like upgrade your component,I have think you must have 700w or more

[Eomera]

Why whould you need a more powerfull PSU for upgrades? Often new components use less power then their previous versions.

Like a Core2Quad Q8200 95w TDP and a I7-2700K 84w TDP, and that's a huge upgrade when it comes to pefrormance.

[WoodenMarker]

A decent 500w bronze would be more than enough for your system. Save your money and don't get a more expensive psu. The amount of time it would take for your system to run on full load for 24/7 to save the extra cost for the more expensive psu is a bit ridiculous. 

[franklyong]

Seems good. You may be able to push with a CX430 since people have done a 7970, and i7 on it with no OC

[Ribozyme]

Great, been preaching this for a while now, Glad people are getting aboard. Me personally have a 3770k and a 680 overclocked and run it on a 400watt be quiet straight power e9. Draws 280 watt max during crysis 3 from the wall, so actual power use by the components is 10% lower.

 

I recommend this PSU to anyone because its low load efficiencies are amazing and it is truy quiet. Like really inaudible.