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Question: How much of a difference do "green" drives actually make in a system build? Do you save enough power for it to be worthwhile, or is it just a marketing gimmick?

When a drive is "green," the designation usually just means that it runs on the slower side—5400 rotations per minute, as opposed to the more ubiquitous 7200 RPM. But in some cases, this slowdown can translate to drives that are quieter, cooler, and less power-hungry. We're not talking the same power savings as, say, switching to fluorescent light-bulbs from incandescent ones. But there are a few watts to be saved here, which makes green drives a decent choice for a platform that will see a lot of use, but doesn't necessarily need to be high-performance. (If you're really looking for power savings above all else, though, the absolute best option is a solid-state drive.)

The three features that are touted the most often by manufacturers of green drives, as we said, are their relatively quiet and cool operation and their lower power consumption. These specs are measured in decibels, degrees Celsius, and watts, respectively, and can usually be found on fact sheets for various drive models on the manufacturer's website (here's a Western Digital sampling) or from third-party benchmarks, if you don't trust Big Data Storage.

Some green hard drive models perform decently well in these areas over their desktop-standard counterparts. For example, a 1TB Western Digital Caviar Green model that runs at 3.0 Gb/s with a 32MB cache consumes an average of 4.8 watts when reading or writing, 2.82 watts when idle, and 0.38 watts when in standby or sleep mode. A WD Caviar Black with the same specs consumes 8.4 watts when reading or writing, 7.8 when idle, and 1 in standby or sleep. You're not exactly slashing your electric bill with a green drive, but it's something. You also drop a couple of decibels in loudness going from Black to Green.

Of course, what facilitates the difference between these two drives are their speeds— the Caviar Black runs at 7200 RPM, and the Green uses a mysterious system that WD calls IntelliPower. Theoretically, this is supposed to mean that it runs between 5400 and 7200 RPM depending on the demands being made, but most third-party tests have found the Caviar Greens pretty much stick to 5400 RPM.

In other lines of hard drives, the savings are even more modest. An EcoGreen 5400 RPM Samsung hard drive, the HD103SI, has power consumption figures of 5.6/4.4/1 watts, while an equivalent 7200 RPM drive, the HD103SJ, runs at 7.2/6.3/1 watts. Seagate Barracuda launched a line of 5900 RPM "Green" drives that still get decent savings over another of the same brand—the green version uses 5.4 watts during read/write and 4.8 when idle to a similarly specced Barracuda XT's 9.23 during read/write and 6.39 when idle.

A drive's power consumption can be bumped by metrics other than speed, such as a larger number of platters, so it can be hard to level the playing field between different makes and models. But if absolute power consumption is what you care about, you'll just want to look for the drive that consumes the smallest number of watts.

In terms of cost, using a green hard drive compared to a normal one makes very little difference. Assuming your drive spends 4 hours reading and writing and 20 hours idle per day, switching from the WD Black to Green saves you only 45 kilowatt-hours per year. The national average cost of a kilowatt-hour is 11.93 cents, netting you a whopping $5.38 per year for your sacrifice of 1800 RPM. For comparison, changing one 60-watt lightbulb used 4 hours a day to a 7-watt fluorescent one saves you more, about $9.23 per year.

Your mileage will vary if you are using more hard drives or a RAID setup, live in an area where power is expensive, and so forth. But if you're looking to curb your power consumption in a home or small business setting, there are usually larger power savings to be found elsewhere.

In conclusion, if you're looking to put together a desktop system build that gets even an average amount of performance, you shouldn't be looking at green drives, given that "green" is a relative marketing term meaning "slow, quiet-ish, and cool compared to our other products." You'll want to swallow the extra couple of watts a 7200 RPM drive needs for the performance boost.

But, if you are building a system that you're not placing all your high-performance hopes on, like an HTPC or a NAS, a "green" drive isn't a bad option. Quietness and coolness are valued characteristics in HTPCs when you're looking to avoid drowning out the audio with drive spinning and fans or overheating the surrounding set-top boxes. Likewise, home theater and storage are relatively low-demand operations, so for most consumer purposes you won't need a high-performance drive. And it doesn't hurt that "green" drives are also cheaper.

If you're really committed to going green, the drives with the lowest power consumption out there are solid state. On average, solid state drives consume between a few dozen and couple hundred milliwatts, which is one or two orders of magnitude less power than the average hard drive. They have no moving parts, so they generate much less heat and noise. The trade-off is that they're a good deal more expensive and much smaller, but if maximum coolness, quietness, and power efficiency are your goal, a solid state drive is currently the best you can do.