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My idea is to load an operating system to a tmpfs at boot time from an USB drive or a SD card. All write operations would then be carried out on the ram. (Assume that I have a lot of RAM).

I would like to estimate how much battery time I would get by doing that.

I read that my battery's statistics are: 5600 mAh 83 Whrs

From various websites, I gathered that an average hard drive consumes between 6 and 9 Watts every hour.

Based on those figures, how can I estimate how much battery time I would gain?

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closed as not constructive by Breakthrough, Tog, Karan, Nifle, soandos May 28 '13 at 20:13

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The hard drive isn't draining the battery when it's turned off, and it draws less when idling than when actively accessing data. So a lot depends on your workload. (RAM, OTOH, is drawing pretty much the same power per GB all the time.) –  Daniel R Hicks May 27 '13 at 18:37
    
Your battery isn't going to last longer because you use a usb device ( which requires power ) or reading your sd card ( which requires power ). –  Ramhound May 27 '13 at 18:56
    
Why did I get a down vote on that? people pointed out that it was hard to evaluate: does it make my question invalid? –  qdii May 28 '13 at 11:41
    
@Ramhound: my battery would live longer because it no longer has a hard drive draining power. The USB thing is just to explain how I would boot my computer in the absence of a hard drive, it's irrelevant otherwise. –  qdii May 28 '13 at 11:43
    
@qdii - Where have you read your battery would last longer because it likely won't at least not enoguh to make a difference. –  Ramhound May 28 '13 at 11:58

2 Answers 2

Would be hard as different computer specs (CPU, GPU mainly) will drain battery differently.

Your best bet to actually benchmark it is to have 1 dedicated machine, and then run it with configuration as below (assumption is that all other peripherals - dvd drive, wifi, lan, etc all turned on):

  • With normal spinning HDD
  • With an SSD HDD (as SSD may use battery differently)
  • No HDD - Running purely on DVD drive (using LiveCD)
  • No HDD - Running purely on USB drive (or other external HDD)
  • No HDD - Running purely on RAM Disk (boot using USB, and USB creates a RAM disk) - The only possible issue with this is that you may have to re-install the OS every single time since RAM is volatile and will be wiped everytime you powered down your laptop.

With just based on the numbers you got there, I really doubt there is a way to calculate, not to mention that you do not provide any estimate of how long is the 'current' battery time with the HDD, and other specs of your laptop that may affect the battery usage.

For CPU Power Usage (assuming intel) you can check on ark.intel.com For GPU power usage some manufacturer will have the information of how much power they need to run

Also usually the biggest power drain (to my experience) is CPU and GPU running more than HDD usage. If you have a program that need 100% CPU and/or GPU power (eg. Gaming, BOINC or bitcoin mining or running other research tools like matlab / mathematica / etc) then those will chew through your battery like crazy, which again as I said, is a bigger concern than simply read/write operation of the HDD. Unless you don't do any CPU/GPU intensive action, and do lots of continuous moving of big chunks of files.

Hope this helps.

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I guess you are right, that depends on the work load. If my computer uses 20Wh on heavy load and the disk is 6Wh, the battery life I gain is more than if it uses 40Wh. Yet I am sure there is a way to make that variable enter the equation. I am looking for back-of-the-envelope math here, not precise computations. –  qdii May 28 '13 at 12:14

Darius is right, there is no method to estimate, knowing capacity and drain figures won't help much. My wild guess you get 5-50%% increase, depending heavily on usage pattern.

The best thing to do is to run some tests by your own.

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