In the way to reduce the power usage of my iPhone 5 (iOS 8), I'm simulating the power saving mode of Samsung Galaxy Note 4 by turning on the "Greyscale mode"
but does it really reduce the power usage and why ?
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There are two approaches that could lead to less power consumption:
Whilst the first is definitely not applicable as the iPhone features an LCD screen, the latter is something I once inspected. I made an iPhone draw a couple of dozen lines per second and then print how many it could render in 30 seconds.
Having the accessibility feature enabled I hoped for a higher score in my little "benchmark". However, it resulted in no improvement at all. Actually the iPhone was slower having grayscale activated.
It might be misleading but a lower benchmark, here, indicates more CPU cycles, hence more power consumption.
So unless the iPhone actually caps CPU power on grayscale mode (which I doubt) your iPhone consumes even more power in grayscale more. This is probably due to the fact that it first renders the view in color, then caches it and puts the gray-filter on it.
Side fact: on iOS9 you will have a "low power mode" available for your iPhone. I already use iOS9 on my main device on a daily basis and the power saving functionality is awesome! My iPhone survived a 6-day music festival whilst taking pictures, checking mail in the mornings and sometimes calling people. So don't worry about it too much, in a few weeks your iPhone will last surprisingly long.
No, I'm afraid it does not reduce the power consumption of your device.
LCD screens (which the iPhones use) use - almost - the same amount of power no matter what colour is being displayed, and the majority of this power is used by the backlight, so if you lower the brightness of the display you can actually save power.
As far as I know, the Galaxy Note uses an AMOLED screen, which actually uses less power when displaying black pixels, so that's why it is a useful feature on that device.