By Jonny Gerow posted Aug 23rd 2012
First, let’s talk about what efficiency means and how it’s calculated. A computer power supply takes AC power from the wall and converts it into DC power. During this conversion, some power is lost and is exhausted as heat. If a power supply is more efficient, it requires less AC power to produce the same amount of DC power than a lesser efficient unit and less heat is produced.
The efficiency of the AX1200i PSU
Intel’s ATX specification only requires that a power supply is 60% efficient at 50% load. Most decent quality power supplies made in the last decade are around 70% efficient at 50% load.
In 2005, Tacoma, WA based Ecova Plug Load Solutions initiated the 80 PLUS® program. In 2007 Energy Star made 80 PLUS certification mandatory in order to bear the Energy Star logo.
The 80 PLUS program tests computer power supplies for efficiency at 20%, 50% and 100% loads. Initially, power supplies only needed to be 80% efficient to earn certification. Over the years as power supplies became more and more efficient, new standards such as Bronze, Silver, Gold, and Platinum were created.
Below are the different 80 PLUS levels and the efficiency requirements for each level:
|20% load||50% load||100% load|
|80 PLUS Bronze||82%||85%||82%|
|80 PLUS Silver||85%||88%||85%|
|80 PLUS Gold||87%||90%||87%|
|80 PLUS Platinum||90%||92%||89%|
In the following example, we’re going to look at a 500W power supply putting out 250W (50%) of its power:
|DC Output||AC Input||Efficiency||Watts Lost|
|PSU that meets minimum ATX efficiency requirements||250W||416W||60%||166W|
|Typical low cost PSU||250W||357W||70%||107W|
|80 PLUS PSU||250W||312.5W||80%||62.5W|
|80 PLUS Bronze PSU||250W||294W||85%||44W|
|80 PLUS Silver PSU||250W||284W||88%||34W|
|80 PLUS Gold PSU||250W||278W||90%||28W|
|80 PLUS Platinum||250W||272W||92%||22W|
As we can see, even at just 250W output, we’re saving 40.5W of AC power just going from a standard 80 PLUS power supply to an 80 PLUS Platinum unit! But how does that translate into dollars and cents?
To figure this out, we’re going to have to do some clever math. A power supply only puts out as much power as demanded of it. Different tasks require a different amount of power. If we’re merely surfing the Internet, even a PC with a high end graphics card installed may only consume 200W. If you have three-way SLI, you could be using almost 1200W of power when playing a current game with all of the eye candy turned on.
For this exercise, let’s use a fairly current machine with high end SLI (two high end graphics cards) and it’s powered by an 80 PLUS Platinum certified power supply like the Corsair AX1200i Digital power supply.
We’re going to assume that I shut my PC off for 8 hours a day. Although +5VSB is live, I don’t have anything plugged into the PC that requires power (no phones, etc. I used to have theNvidia 3D Vision glasses charging, but haven’t used those in a while). Even wake on USB (a feature that allows your keyboard or mouse to wake your PC and therefore requires power to the keyboard and mouse) is disabled. Let’s say I work from home, so I’m using the PC for usual work stuff (email, Internet, etc.) using an average of 200W for 8 hours of the day. During lunch, the PC goes into power saver mode. The hard drives spin down, the CPU and GPU cycles are minimized, but the CPU cooler’s pump, all the fans and everything plugged into the PC like the keyboard, mouse, etc. are all still getting power. Let’s say 50W. It does this too for the hour I spend outside grilling dinner. Once I start eating, the PC assumes I’m not coming back any time soon and goes to sleep. It stays there while I watchMasterChef on TV, run errands, do some laundry, feed the cats, get my daughter to bed… about 4 hours total until I’m to the point where I’m ready to go downstairs and start gaming. While gaming, power output is anywhere between 500W and 700W depending on whether I’m at a cut scene, building up an armory or fragging in every direction causing my FPS to drop into the double digits. Let’s average that out to 600W.
Now let’s put that into a table:
|Power Saver Mode||2 hours||50W|
|Sleep Mode||4 hours||5W|
|Normal usage||8 hours||200W|
This averages out to 182.5W for the 16 hours per day that the PC is on. Of course, the PC isn’t running like this for all seven days a week. I’m out of the house a lot on weekends. So let’s assume that on Saturday and Sunday the computer gets half as much as use as it does on the weekdays. That means we’re using an average of 182.5W for 96 hours a week. There are 52.1775 weeks in a year, so we can calculate that the PC is on about 5009.04 hours a year. Multiply that by 182.5W and we have 914149.8. If we divide this by 1000, we get the kWh (kilowatt-hour) consumption for the PC for the year: 914.15kWh.
The price of electricity varies by city and your utility company. In the U.S., if you live in Wyoming, you could pay as little as $0.0658 per kWh. If you live in Connecticut, you could be paying as much as $0.1633 per kWh. And until they can figure out how to harness the power of a volcano to produce electricity, a kWh in Hawaii can cost as much as $0.3154!
How much you pay per kWh is always on your electric bill. When looking at your electric bill to calculate your kWh charge, don’t just look at the “supply charge”. Factor in all of the additional charges, like transmission service charges, distribution charges and environmental cost recovery adjustments.
If we have a look at the U.S. Energy Information Administration's website, we find that the price of electricity has gone up EVERY YEAR since 2002! The average retail price for the United States is $0.0999 per kWh as of 2011. And that average includes industrial customers that get a much better rate. The average cost for a residential customer is $0.1180 per kWh. In Europe, the cost per kWh can be even more.
So how much am I saving by using an 80 PLUS Platinum power supply at home versus another power supply? Using the data we’ve accumulated and calculated, assuming the same PC getting the same usage, but with different power supplies with different efficiency levels, and a residential electricity price of $0.1180 per kWh, let’s do another table!
|Efficiency Level||kWh / year||Cost per year|
|PSU that meets minimum ATX efficiency requirements||1398.11kWh||$164.98|
|Typical low cost PSU||1199.82kWh||$141.58|
|80 PLUS PSU||1050.26kWh||$123.93|
|80 PLUS Bronze PSU||988.09kWh||$116.59|
|80 PLUS Silver PSU||954.48kWh||$112.63|
|80 PLUS Gold PSU||934.31kWh||$110.25|
|80 PLUS Platinum||914.15kWh||$107.87|
As we can see, this "average user" saved $16.06 a year just by building a computer with an 80 PLUS Platinum power supply versus a regular 80 PLUS power supply! Never mind how much he saved versus using a power supply that’s not 80 PLUS certified at all! And that’s just based on my example. If you have a third graphics card, you’re going to use even more power (at least an additional 100 to 150W or more during gaming). If you don’t shut your PC off at night like I do, you’re going to use more power. Even if your computer is in sleep mode while you’re sleeping, that’s potentially another 40W a day!
If you factor in the industrial grade components used within Corsair's TX, HX and AX power supplies, and that all of Corsair's power supplies are rated for continuous output at temperatures greater than room temperature, you can easily use the same power supply for at least 5 to 7 years! The accumulated savings in using an efficient Corsair power supply over a cheaper, inefficient power supply unit easily provides a return of investment... as well as being better for the environment!