The size, weight and functionality of all our mobile devices is greatly impacted by the size of battery inside them. So what is it that uses all that battery?

The folks at Microsoft wrote a great blog post on the Engineering Windows 7 blog:

Looking at a modern laptop, here is where the power goes:


It is clear that screens soaks up most of your battery. Since screen brightness is such a huge source of battery drain, mobile devices have special hardware to control it:

  1. Laptops (eg Macbooks) have special buttons to control screen brightness.
  2. The iPhone has an ambient light sensor so it can lower your screen brightness as soon as possible.


The next culprit is RAM.

Wonder why the iPhone/iPad still have only 1GB of ram, even though they carry 64 bit processors?

The problem lies with the fact that, unlike a LCD screen or a CPU, you cannot power off RAM. Doing so will cause it to lose its contents. There is a reason its called ‘volatile’ memory. RAM needs to be constantly refreshed every few milliseconds.

Even when your devices are sleeping, the RAM is still powered on. This is the reason why your Macbook, which has 8GB of RAM loses battery much faster in sleep than your iPad, with just 1GB.


But what about the CPU? Turns out it is the most complicated piece of this puzzle.

As stated in this Ars Technica article:

The trouble isn’t the minimum power required to keep things running, it’s the difference between the minimum and the potential maximum power consumed by the most demanding components of the system.

So while at idle stage, the CPU can use almost negligible power, when boosted up, the power consumption can go to crazy levels. How crazy you ask?

Again Ars Technica does a good job of explaining:

The chart below, used by Apple at WWDC to explain its energy-saving strategy in Mavericks, shows some power usage numbers for a typical modern laptop CPU with an integrated GPU.

That’s a 60x range between idle and turbo!

Turns out the CPU can be a worse culprit than any other part of your device…

No other component of the system has such a huge range between idle and maximum power combined with such a high maximum power level. A dimmed screen does not consume 60 times less power than a bright one; laptop RAM (soldered to the logic board in Apple’s modern MacBooks, Air and Pro) and SSDs will never use 25 watts even when running full-tilt.

It is for these reasons why:

  1. OSs like OS X Mavericks have so much focus on making the most efficient use of CPU.
  2. Intel’s Haswell chip is all about efficient power usage.
  3. Your laptop gets crazy hot when you do CPU intensive tasks.
  4. High end servers require special power outlets and data centers have energy as their biggest cost.


Hopefully this article has shed some light on why your mobile devices are limited the way they are and why they behave the way they do.