The Cost of Running Your PC
by Christoph Katzer on November 14, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Actual System Power Costs
On the previous page, we estimated the efficiency for each system as being 82%. Obviously, that's not a true way of calculating power requirements or efficiency. Now we're going to shift to the real world and see what the three sample systems end up costing on an hourly basis.
Before we get to the tables, it's important to remember that just because a power supply advertises 90% efficiency doesn't mean you'll always reach that level. You can look at any of our power supply reviews -- or just read Debunking Power Supply Myths -- to understand this better. The short summary is that all power supplies have an efficiency curve, which depends on the load you place on the power supply.
At lower loads and maximum load, efficiency is lower than if you run at a medium load (relative to the PSU's rated output). If you're just surfing the Internet, writing a document, or viewing pictures your system will largely sit idle. Playing a game, doing 3D rendering, encoding a video, or other complex calculations will place a higher load on your PSU. The following tables use actual efficiency with a real power supply to calculate power costs.
System 1
Our entry-level system, System 1, will utilize the Thermaltake TR2 QFan 300W power supply we recommended in our last article. System 1 consumes 90W to 140W of power, depending on load -- those are best-case/worst-case figures. We haven't posted our review of the QFan yet, but it achieves 82% efficiency at 90W load and 84% efficiency at 140W load. The hourly power costs are:
| System 1 Power Costs with Real Efficiency | |||||
| Load | Efficiency | Outlet Power | Cost/hr NC | Cost/hr CA | Cost/hr GER |
| 90W | 82% | 110 | $0.008 | $0.014 | €0.024 ($0.031) |
| 140W | 84% | 167 | $0.013 | $0.021 | €0.037 ($0.048) |
System 2
System 2, our midrange system, will use the OCZ ModXStream Pro. This system requires between 160W and 350W of power. The OCZ power supply runs at 84% efficiency for 160W and 85% efficiency for 350W. That gives the following power costs:
| System 2 Power Costs with Real Efficiency | |||||
| Load | Efficiency | Outlet Power | Cost/hr NC | Cost/hr CA | Cost/hr GER |
| 160W | 84% | 190 | $0.014 | $0.024 | €0.042 ($0.054) |
| 350W | 85% | 412 | $0.031 | $0.053 | €0.091 ($0.118) |
System 3
Lastly, our high-end system is running two graphics cards for maximum performance. This time we selected the OCZ EliteXStream 800W PSU. Note that even this beefy system still only requires 550W at maximum load, whereas it idles at 310W. In this case, efficiency is 84% idle and 83% at full load.
| System 3 Power Costs with Real Efficiency | |||||
| Load | Efficiency | Outlet Power | Cost/hr NC | Cost/hr CA | Cost/hr GER |
| 310W | 84% | 369 | $0.028 | $0.047 | €0.081 ($0.106) |
| 550W | 83% | 663 | $0.050 | $0.085 | €0.146 ($0.190) |
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bob4432 - Tuesday, November 18, 2008 - link
exactly, and if you are not afraid of flashing your video bios, then you can really tweak the power setups for the lower end power "2d" mode to much less than what ati put in as a default. i think ati rushed the 4850 and sounds like 4870 bios when they first came out (extreme heat, lack of real power savings, etc) but hopefully now all that is fixed.fwiw - i do consider the amount of $$$ in cooling when getting a real measure on how much it cost to run the pc. i am in phx, az and ac is a must, so in my computer room/office it usually gets about 5-8F warmer than the rest of the condo and therefore that difference needs to be taken into account.
BitBodger - Friday, November 14, 2008 - link
One thing not considered here is the effect of the heat from the computer. That computer sitting beside your desk is also an electric heater constantly warming your home. Live in a cold climate and this is not entirely a bad thing since it takes some of the load from your main heating system. But if you live in a hot climate and depend on air conditioning don't forget that the heater never stops meaning that your AC works that much harder and consumes that much more energy getting rid of the extra heat. And given the inefficiency of AC technology, it costs more watts in the cooling process than are emitted by the heater.Hammarby - Friday, November 14, 2008 - link
In all these calculations shouldn't you also factor in how much extra it will cost to cool your house when you have a 100-500 watt space heater running for 8 hours/day??Christoph Katzer - Friday, November 14, 2008 - link
That's why the article calls "The Cost of Running Your PC"........Lifted - Saturday, November 15, 2008 - link
If "running your PC" increases the demand on your home cooling, then that is a cost directly resulting from running your PC. It is out of the scope of the article since there are too many variables to consider than just the cost of electricity.JarredWalton - Saturday, November 15, 2008 - link
But if it's cold and running your PC reduces the amount of time you run your heater, then running your PC would cost less in the winter. Besides which, plenty of people don't have AC, so even in the summer there's no added expense. Thus, we chose to limit the discussion specifically to how much your PC costs to run, and how PSU efficiency can play a role in those costs.7Enigma - Monday, November 17, 2008 - link
While electricity is 100% efficient (or near enough that we don't need to quibble), it is also true that very few people use it to heat their homes due to $/BTU. Until there comes a time when it is as cheap or cheaper to heat your home electrically I don't think your comment holds true.I also agree with the original poster that while not exactly in the scope of the article it is closely related and should have been mentioned. The common position of why spend more money on a more efficient PSU or any other component when the electricity costs show a ROI much greater than the usable life of that product is not accurate when cooling costs are not taken into account. While heating electrically is near 100% efficient, cooling is definitely not.
nilepez - Wednesday, November 19, 2008 - link
Lots of houses are heated with electricity. I've never lived in an Apartment with anything but electric Heat.Nevertheless, the heat from a PSU is negligible in the winter (even in the south) and an efficient PSU, by definition, produces less heat, which is especially true when the PC is idle.
ZoZo - Friday, November 14, 2008 - link
Or you can factor in how much less it costs to heat your home when you have a 150W space heater running for 8h/day.Staples - Friday, November 14, 2008 - link
I have not even read the article yet but I am surprised that was not even mentioned. That often adds 50% more to the cost if you live in a hot climate. Same thing with light bulbs. There is a double savings with CF bulbs.