I tried to find some examples of the amount of energy consumed in popular apps.

This paper detailed not only energy consumption in joules of a number of apps on a single phone, but also the energy consumption and duration of common system calls: https://www.researchgate.net/publication/234057253_Comparing_Mobile_Applications'_Energy_Consumption/link/0fcfd50eacc9602f24000000/download

In line with Apple Developer's documentation which encourages programmers to minimize certain functions, networking system calls were the most costly and slowest. One shocking finding was that The New York Times app has a higher medium power rate than YouTube.

@lexicologist Thanks for sharing that paper! I thought it was very interesting. The most amazing thing for me was that advertisements have an extremely harmful impact on energy consumption. I was wondering where it detailed the system call energy usage? I was unable to find any information on that but am interested in knowing more!

Also, I found two beautiful slide decks from talks by Apple engineers about writing energy efficient code:

1. https://devstreaming-cdn.apple.com/videos/wwdc/2014/710xxwwk9jiqtu2/710/710_writing_energy_efficient_code_part_1.pdf
2. https://devstreaming-cdn.apple.com/videos/wwdc/2014/712xx1pl2u942g2/712/712_writing_energy_efficient_code_part_2.pdf

Hope somebody finds it useful!