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If you get a PDF and a EPUB of the same document, and read it on the same device with the same version of Android, which would use up less battery power?

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Theoretically ePub should have the advantage over PDF as it is a smaller file that is basially HTML, CSS and image files with a few XML files for defining the books metadata, chapter order and table of contents navigation zipped into 'ePub' format. It is in many ways like a website. The PDF files will often be larger as they have embedded fonts, image optimisation and their own encryption to deal with as overheads.

However, while ePub will reflow text easily it is not very good for more technical things like code and mathematical equations etc, so really what format to have the book in will depend on your need rather than an impact on the battery.

Also the more optimised the eBook reader the less overhead on your battery. The screen is the biggest drain on nearly all mobile devices and will consume far more battery than any eBook reader.

Conversely, an ePub with a LOT of markup and very, very large unoptimised image files could be a bigger overhead to render than a slimmed down, lower image quality PDF.

As per @Chahk 's comments below, the actual saving for a black background is quite low for an AMOLED screen where black is unpowered, and non existant on LCD based technologies. This XDA user did an experiment which should be taken as anecdotal evidence and his finding was basically after 1 hour of screen on time he had saved around 6% battery life.

How to Geek article on black wallpaper and battery which states:

On most computer displays, such as the computer screen you’re probably reading this article on, using a black background won’t save you any battery life. No matter what color a pixel is – whether it’s dark black or blinding white – there’s a backlight at the back of your screen and it’s constantly outputting light. The black pixels block more of the backlight, but it’s still behind the black pixel, using power.

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    I second tis very good explanation (+1). Having the choice between ePub and PDF (my favorite reader supports both), I decide based on the book's content. If it's just a good story (novel etc.), or is mainly text with no special formattings, I always pick the ePub. But if special formattings are important, or a lot of graphics are used (mainly with technical references, as RossC pointed out), PDF is the better choice: Even if an ePub theoretically might save some juice, it wouldn't if I need to read the same parts multiple times, as formatting issues (text re-flow) made understanding difficult. – Izzy Jan 31 '14 at 12:32
  • What reader do you use? – warsong Jan 31 '14 at 13:19
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    Honestly, the difference in processing power between various formats is minuscule in comparison with the power drain from having the screen on. The reading app itself will probably not even register on Settings -> Battery graph. – Chahk Jan 31 '14 at 15:49
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    @Chahk completely right, that's why I had to point out the screen brightness and having a white background will drain battery far faster than a PDF reader or an ePub reader The screen will burn the battery faster than nearly anything on a phone, the only exception is that unoptimised mess in Google Play Services with the location etc, that they are trying to fix. – RossC Jan 31 '14 at 15:52
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    @Chahk thanks, I've put the screen part in bold, and elaborated on the Black Screen, which I was incorrect on. Many thanks again, that's great! I think the answer is better now. – RossC Jan 31 '14 at 16:01
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EPUB uses less battery because pdf files heavy compared epubs. It also depends on what app you are using, CPU speed and RAM of device. Reading EPUB file with black background n white text saves a lot of battery.

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    Black backgrounds only have (marginal) impact on battery life on AMOLED screens, where truly black pixels are unpowered. LCD based screens don't have this feature, and so they don't benefit at all from dark backgrounds. – Chahk Jan 31 '14 at 15:47
  • There is a difference on AMOLED, but it's not as much as one (myself included) would have thought. You raise a good point. This thread: forum.xda-developers.com/showthread.php?t=660853 is one person's experiment and he found that after 1 hour of screen time he had a saving of 6% which frankly isn't worth having a dark background. Very interesting. Cheers for the heads up @Chahk – RossC Jan 31 '14 at 15:55

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