I really like the trend of increased notebook battery life that has been going on for the past few years.

My MacBook Air can seriously sustain up to 12 hours of battery in a single charge. However, whenever I fire up Xcode, iOS Simulator, and other CPU-heavy apps, the battery usually lasts closer to 5 or 6 hours. It can even sometimes be less than that if I’m truly pushing it (e.g. playing games).

One of the reasons why the battery life can vary by up to half the advertised battery life is because the Haswell chip that ships with most MacBook Airs are the ULT (Ultra Low TDP) variant, which can consume up to 15 watts of power. The more powerful variant in the MacBook Pros can consume up to 28 watts. This means that the batteries in the notebooks will not last as advertised when we push the limits. When Apple advertised “12 hour battery life”, they don’t mean that it will last 12 hours when you use the CPU at 100%, but most people who just browse the web and check email would not even use 5% of the CPU consistently.

The fanless new Macbook that Apple just announced last week are using the new Intel Broadwell Core M CPU, which consumes only less than 5 watts of power. Here’s Ryan Smith, writing for AnandTech explaining Broadwell Core M:

When it comes to building and configuring Core M, Intel likes to refer to their efforts as the “Fanless Challenge,” reflecting the fact that their biggest goal with Core M is to comfortably get the processor in to 10” tablets under 10mm in thickness that are passively cooled. A big part of getting into such a device is meeting the heat dissipation limitations of the form factor – a 10” tablet under 10mm would require a sub-5W SoC – which is where a lot of Intel’s engineering efforts have gone. Broadwell’s performance optimizations, the GPU optimizations, GPU duty cycling, and the 14nm process all contribute to getting a Core CPU’s power consumption down to that level.

This means that even when pushing the CPU to its limit, it will not use more than 5 watts by design.

This means that there is no way you can drain the battery in the new MacBook that fast, no matter how hard you push the CPU. The MacBook Airs and MacBook Pros may only be getting 25% to 50% of the advertised battery life when you push it, but the new MacBook will have much closer to the advertised battery life.

When the new MacBook ships, we’ll see how correct or wrong I am about this theory. But if I’m right, then I’d very happy with a stable 9 hour battery life. The bad news is, I think Intel are still quite far away from making the Pro chips fanless.

Update: A few people told me that Intel Core M can consume more than 5W when doing Turbo Boost. While this is true, I presume that Intel Core M it will still yield more performance per watt compared to Haswell.