Introduction

Have you ever thought about how much it cost to run your PC -- the one you're using to read this article? What does it cost to play games, surf the Internet, or download files? It all costs money -- money that you, your parents, or whoever is in charge of the monthly electricity will have to pay. Those of you in charge of paying this bill will surely be interested in keeping costs down, which is why you might want to pay a little more attention to what sort of hardware you are using in your computer.

Many users -- especially computer enthusiasts -- put together a new PC that can easily handle any task, without much thought for power efficiency. If you intend to use the computer primarily for gaming, buying a high-end processor and graphics card makes sense. Likewise, if you intend to do complex three animations or movie encoding, you'll probably want to have as much processor power as possible. If all you're going to do is watch movies, run Microsoft Office, and surf the Internet, you're not going to put a big load on any of the components. In that case, your PC will typically be idle and waiting for user input, while any high-power components will still go merrily along sucking down extra power.

We recently looked at the topic of power consumption for each component in the PC. Of course the numbers were merely a rough estimate for our specific setup, programs, and tasks, so that article could serve as a baseline for the amount of power your system might require. We also discussed how power requirements affect the type of power supply that you will want to purchase. In this article, we want to focus more specifically on the costs of running a computer (not counting anything like broken components and upgrades). We look at electricity prices in the US and Europe to calculate how much various types of PCs actually cost to run. Perhaps you're one of those people with multiple systems -- one for gaming, one for office work, maybe one or two for the kids, and perhaps a few extras running distributed computing tasks 24/7. We will look at several different workloads to see how much various types of systems actually end up costing on a hourly, daily, and yearly basis.

KWh prices in the U.S and EU

When we started researching prices of electricity (measured in kilowatts hours/kWh) for the different countries, we were surprised by the huge differences in price. In the US prices range from $0.05 to $0.21, according to the Energy Information Administration -- the average price is $0.089 per kWh. European prices are different for each country, so we will just take Germany as an example. Prices there are high relative to the US but about average for Europe. In 2008, Germany has an average of 17 to 22 Cents (€) -- about $0.22 to $0.29 USD! That's anywhere from 1.5 to 6 times as expensive in the old world depending on where you live; obviously, areas where costs are higher will probably be more interested in PC power consumption, but that is a separate issue from what we are looking at today.

Calculating Power Requirements and Costs

To find out now how much your PC actually costs to run, you will first need to know your power consumption. For this article, we will use three sample systems representing differing levels of hardware and performance. The specifications for the sample systems can be found in our previous article on power supply units. Power consumption is as follows:

Electricity providers report power use in kilowatts hours, since the power consumption of your entire house is going to be large compared to a single PC. Every light bulb, TV, microwave, refrigerator, vacuum cleaner, etc. requires power. Unless you are running a lot of computers, it may not even be necessary to think much about how much your computer uses without addressing those other areas first. Still, there's a large difference between an entry-level PC with EIST/Cool & Quiet sitting at the desktop and a high-end PC running the latest 3D game.

For our comparisons, we will look at two states in the US (North Carolina and California) and Germany will represent Europe. We used an exchange rate of $1.30 per Euro. Power use is calculated by the above chart, factoring in the efficiency of the power supply. For simplicity's sake, we will start by assuming 82% efficiency on all systems and loads. Divide the power consumption by the power supply efficiency and you end up with the actual power use in Watts. Converting Watts into kWh requires a bit more math: take the power draw in Watts and multiply that by the number of hours a device is running, and then divide that number by 1000. The results are as follows:

If you've ever wondered why Europe seems to be pushing for higher efficiency devices than the US, the above charts should provide an easy answer. Sure, very few systems actually consume 400W or more continually, but plenty of businesses run hundreds of 100W-200W PCs 24/7. Of course, other business expenses generally far outweigh power costs if you have that many PCs -- for example, the hundreds of employees sitting in front of those PCs likely cost 100 times as much per year, give or take. Still, the cost of leaving a high-end system running even eight hours a day at your house is not trivial, with idle power consumption costs ranging from around $100 to $300 per year. So let's delve a little deeper.

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 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 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.

Using a Higher Efficiency PSU to Reduce Costs

These days manufacturers are all promoting their high efficiency power supplies, and we have organizations and certifications like 80 Plus encouraging even small boosts in efficiency. Not surprisingly, plenty of users have been sucked in by the marketing and are now convinced that they need to purchase a new power supply in order to save money each year. Does it really make that much of a difference? The answer as usual depends on how you use your system. The previous page provided a baseline measurement, but now let's look at how much money you can save if you go out and purchase a new 80 Plus Bronze or Silver certified power supply as an upgrade to a slightly older ~80% efficiency PSU.

Our sample power supplies on the previous page are all relatively high-end choices for the specific market. Many (most) systems don't have power supplies anywhere near that nice, relatively speaking. So what happens when we switch to an older ATX 1.3 PSU -- something that would have been more or less state-of-the-art three years ago? Will a newer power supply really help you save the planet? Will it at least reduce your power costs and save you money? Let's find out, this time looking at power costs over the course of a full year: 24 hours a day, seven days a week.

For reference, we looked at some PSU efficiency results stashed away in our files and estimated ATX1.3 PSU efficiency at 75% idle and 78% load. That represents a high-end ATX1.3 PSU, and in some cases the discussion is hypothetical as it wouldn't be possible to find an older PSU with the necessary output rating. (That applies specifically to the high-end system.)

Now we can see exactly how much money you might save during the course of a year by purchasing a new high efficiency power supply. Obviously, the more power your computer uses, the better your monetary savings. Looking at these tables, you might begin to think there's actually a point in upgrading power supplies -- and there is, provided you're running your computer a large portion of the time.

What happens if we change our usage model to something more realistic for most families? Instead of looking at 24/7 usage, let's change it to three hours of use per day on average, with two hours at idle and one hour at load.

The need to upgrade power supplies suddenly doesn't seem as dire once we switch to a more realistic usage model. Particularly on low-end systems that only use 100W of power give or take, even an extremely inefficient PSU probably doesn't matter too much if the system isn't on more than a few hours per day. Even with power costs that are up to three times higher in some parts of Europe compared to areas in the US, the savings don't make sense.

If you happen to be the type of user that leaves your system on all the time, certainly you can save a fair amount of money by purchasing a better power supply. An easier solution would simply be to turn off your computer when it's not in use, unless you have a really good reason to leave it running overnight. Similarly, if your current PSU happens to fail, it might be worthwhile to spend a little bit more money to get a higher efficiency, better quality power supply. If you figure on a moderate amount of use and a five-year lifespan, you might want to spend as much as $50-$100 extra. Otherwise, there's very little incentive to go out and spend $150 on a top quality power supply just so you can save $10-$15 per year (or less).

The Difference a Few Percent Makes

Hopefully we've made it clear that upgrading an existing power supply to a higher efficiency model purely for the power savings doesn't make sense. However, there are times when you need to buy a new power supply, so we will wrap things up with a closer examination of how efficiency impacts power costs. Should you really care about the difference between 85%, 87%, or 90% efficiency?

This time, we don't need to worry about specific systems, but instead we will focus on efficiency and monetary savings at various power loads. The following table is again a best-case scenario for saving money -- i.e. you are running the system 24/7. Efficiency 1 is the base value and we compare the savings you would gain by selecting a power supply that achieves Efficiency 2. Efficiency ratings at the various loads represent what you might realistically find in various high-end power supplies currently on the market -- so getting 90% efficiency with a load of only 50W isn't going to happen.

Obviously, the higher the load the better your savings, since a difference of 1W hardly matters. Your best course of action would be to select a power supply that offers the best efficiency at the load you will use most frequently. So for example, if you only play games on your computer and otherwise have it shut off, you might seriously consider a power supply with optimal efficiency at the 500W-600W range. On the other hand, if you typically just surf the Internet you'll probably be more interested in the efficiency at 100W-200W.

At the maximum load of 700W, and going with German power costs, the difference between an 85% and 89% efficiency power supply could be as much as €71. That's enough to get a significantly better power supply, but of course that sort of savings is unrealistic since it will be extremely difficult to achieve a 700W load all the time. The 400W load represents a more realistic maximum, as something like an overclocked quad-core system running Folding@Home could actually draw that much power around the clock. In that case, your savings could still be a pretty significant €30 per year, so over three years you could save almost €100. If you only run the system eight hours per day, however, the difference in cost drops off quickly.

Obviously, spending $20 more just to increase efficiency by 1% isn't necessary. You'll probably use a power supply for at least three years, so all other things being equal higher efficiency is good. That "all other things" is the problem, however, since rarely are the other areas the same. Pay attention to the other features like noise levels, voltage regulation, and the number and type of connector as well. Also keep in mind that we still have changing ATX standards, and sometimes new connectors, so spending a small fortune on a top quality PSU that might be outdated in a year or two might not be the best course of action either.

The bottom line ends up being a simple case of common sense: don't buy more power supply than you actually need, and don't spend a lot of money for a small increase in efficiency. Figure out how much power your system will normally use, and then choose a power supply appropriate for that sort of workload. If you routinely stress your system (i.e. workstation loads or intense gaming), an extra $100 for a high-end power supply might be a good idea. For most users, however, moderation will be the better course of action.

Finally, we spent quite a bit of time putting together the spreadsheet that we used to generate the tables in this article. We selected a few different markets for our power costs, and then we selected several different systems. Obviously, we couldn't cover everything, but for those who are interested in running their own calculations we thought you might appreciate our spreadsheet. Feel free to insert your own KWh costs, efficiency, and system power requirements to see how things change. (The highlighted fields should be the only areas you need to modify.)