Ok, i understand that. for example if i watch a 2 hours movie at 720p.
then same movie at 1440p they will consume same amount of battery.
Yes and No (in your case, only no).
Here's the thing:
-The screen has enough pixels for a 1440p resolution to display 1:1
-All these pixels will be used, regardless of the resolution
-This means that 1080p will RENDER things at 1080p, and then UPSCALE it to 1440p
-Upscaling required power is neglegible
So this works by making your phone render pixels at 1080p, which is 25% less than 1440p. This means a 25% computational power that can lead to higher FPS. After rendering, it upscales to 1440p.
Yes, it will consume SLIGHTLY less battery in two cases:
-in 1440p you're using 100% of the hardware and on 1080p you use less than 100%. Our displays can't go over 60fps (vsync is on), so if a game runs at 40 fps in 1440p and 50 fps on 1080p, it will be using the same thing, but will be rendering faster
-In low consumption scenarios (like UI draws) where the UI will have to render 25% less pixels, which means 25% less power used in the long run.
BUT, what I just said isn't the big power drainer. That alone would probably account for a 1-2% increase at most. What really drains the battery is the screen. 1080p upscaled to 1440p is still the same amount of pixels powered up as 1440p. This means that the battery will be drained at the same speed.
So, yes, using 1080p instead of 1440p will improve your battery, but absolutely not enough for you to notice a difference.
In your case: if you play a 720p movie on whatever screen resolution, it will consume less battery than the same movie on 1440p. This is because you have to decode the movie and render it to the screen. 1440p movies require double the computational power to decode and display. I'm not sure how much the difference is, but it's there.