How much does a flash disk increase battery life?

How much does a flash disk increase battery life?

Summary: Notebooks need longer battery lifeI used a flash-disk based notebook for much of the '90s and loved its 10-hour battery life. You just have so much more freedom when you don't need to worry about keeping a battery charged.


Notebooks need longer battery life I used a flash-disk based notebook for much of the '90s and loved its 10-hour battery life. You just have so much more freedom when you don't need to worry about keeping a battery charged.

I've been very interested in how flash drives could extend notebook battery life. Using a Kill-a-watt power meter I ran some experiments with an Intel Core Duo notebook. Power use is a little more complex than I'd thought. Here's what I found.

The config I used a MacBook with the 2 GHz Core Duo, 2 GB RAM, Intel 950 integrated graphics, 13 inch screen, Bluetooth, Wi-FI, 8x DVD and a 160 GB Western Digital Scorpio 2.5", 5400 RPM disk drive.

One of the nicest things about the MacBook is that disk drive removal is easy (see the 1 minute video here). I ran the tests with the internal Scorpio drive removed and ran the MacBook off an externally powered FireWire drive.

Instrumentation In addition to the Kill-a-watt power meter, which sits between the wall power and the MacBook's power adapter, I also used the most excellent open source MenuMeters utility. MM shows CPU, memory, disk and network usage, typically sampled over a user-selectable 1-2 second period.

The tests I booted the system while video taping power usage with both the internal and external disks. I also ran a disk defragmentation tool, the estimable iDefrag to create heavy disk I/O.

Since I had the system up, I also checked some of the other subsystems. Display power consumption was measured with the display off and at minimum and maximum brightness. Optical drive consumption was checked by watching a DVD and ripping a CD. Idle power consumption with the screen off and CPU at 3-4% was also checked. I even removed a 1 GB SO-DIMM.

The results The surprise is that you can't just turn features on and off to judge power use. For example, performing I/O is a CPU involving business whether to disk or over Wi-Fi. In my defrag workload the CPU utilization ran from 80-95% on both cores, for an 8 watt hit - almost 3x of a busy disk.

What this means is that the busier you keep your system, the shorter your battery life. Therefore the less benefit you get from a flash drive's lower power consumption.

Here's a chart of the results. These measurements were taken with the battery and disk removed, except for the disk value.


Playing a DVD doesn't use much CPU. The wattage is mostly the drive itself.

The Storage Bits take The maximum power difference between a flash drive and a 2.5" disk is 3 watts. If you average about 3 hours battery life, a flash drive would save at most 9 watt hours (wh). That's 29 minutes with a 55 wh battery. Less than 20 if it isn't seeking constantly - and less than 10 minutes if the drive spends half its time in standby mode.

The biggest power sink in my notebook is the "everything else" that stays on when nothing is happening - 13 watts. Next is the CPU when it is busy. Then the display if you keep it above minimum brightness, the DVD/CD player and finally, just above Wi-Fi, a busy disk.

Flash drives have a real advantage in shock resistance over disks. But the performance is about the same as a disk, the power savings minimal and the cost disadvantage huge. They make the most sense for premium ultra-light notebooks with low power CPUs and small screens as well as hand-held devices.

Comments welcome, of course.

Topics: Apple, Hardware, Laptops, Mobility, Processors

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  • Playing DVD and CPU usage

    "Playing a DVD doesn?t use much CPU. The wattage is mostly the drive itself"

    Could you please clarify if playing a DVD doesn't use much CPU, or whether it doesn't use much power? I would like to know... because I'm not surprised that playing a DVD doesn't use much CPU, but I would expect to put the load on your video card, specially if you play MPG4 videos. So this would mean low CPU usage, but still some power consumption.
    • Playing videos never used much power

      We've been playing full screen, full resolution videos since the early '90s, when CPUs were doing mere hundreds of MHz. The only thing that changed - and the only thing that eats up CPU cycles - is the heavy-duty DRM that companies are shoving down our throats.

      This idea that video is somehow a heavy-duty operation is totally bogus for non-DRM video. We did it 10+ years ago, and now they want us to believe that after 10 years of technological improvement it's still going to be an intensive operation? Baloney. Don't tell me that after 10 years of advancements video is still a problem. It's not. It's that stupid DRM decryption stuff that is causing our video to use a lot of CPU.
      • not my experience

        It has been my experience, and still is in my pentium 4 CPU at work, that if you don't have a decent video card, playing video (specially MPEG4 but also MPEG2) eats your CPU cycles. My system can't even play MPEG4 without skipping some frames.

        When the first computer DVD systems came out, like the Creative DVD 3X kit, it came with a hardware accelerator called DXR2 I think. This made playing DVDs on your computer possible, because your video card probably didn't have DVD support and otherwise playing DVD was very CPU intensive.

        So although I agree with DRM being an expensive HD-DVD tax, my experience has been somehow different from yours and therefore my original question.
  • Conclusion?

    Am I correct that it might only gain you 30 minutes? No wonder no one is moving to them.

    What about speed tests?
  • Reply to 1st 2 comments

    IIRC, the power usage included a 16% CPU load. If I had a way to smoothly ramp up
    CPU load I could figure out how much load puts the Core Duo into max power
    mode. I couldn't figure out a way to turn on each function, so the 6 watts for
    playing a DVD is the whole thing - optical drive, CPU, graphics, fan - whatever.
    Hadn't thought about the graphics mode - thanks for the reminder.<br>
    Flash drive performance - despite the statistics the vendors offer - isn't very good.
    See <a href="" >Drives get a fight from
    disks</a> for some particulars.<br>
    R Harris
    • CPU rampup

      There are a raft of numerical benchmarks you could run. Some of them allow for running multiple simultaneous instances. A lot of them don't do much I/O. Be careful though, if you get too many memory hungry benchmarks running, then you'll start to swap to disk.

      Your tests make it obvious that the endurence gain for the swap is minimal.

      Another comparision that would be useful: Follow a scenario the way a person would actually use a laptop that requires a full day. E.g. A businessman on an oversees flight modifying his presentation; students taking notes all day on campus.

      Under those sorts of use patterns (when a person is spending 80% of his time looking at a screen, and not doing much else) what is the normal power consumption of the laptop.

      Suppose that it is 15 watts. Then changing your HD for a flash disk only gives you 20%.

      However consider this: If the MAIN ram is non-volotile then the entire computer can shut off and wake up in split seconds. If we come up with a screen that only uses power to change it's pixels; if the computer's clock frequency can be slowed down to match the workload, then maybe we can get a computer that runs a month on a charge.

      (FWIW: My Nikon D70 does two week long canoe trips without ever being shut off on a single battery. My palm pilog is instant on and goes for about 2 months on a single charge.)
  • What Notebook ???

    "I used a flash-disk based notebook for much of the ???90s and loved its 10-hour battery life."
  • Really, we also need a low pwer screen and cpu. How about some OLPC tech

    for a black and white mode screen that uses ambient light when you want low power, but can switch to color when needed. Also, all of the tricks of switching off the CPU and waking it in less than 100 ms when needed. But, of course everything sized for adults (14 inch screen and normal keyboard.