The 1% Rule for Mobile App Power Consumption

Gabor Szanto

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Xcode 7.0 (with iOS 9) comes with a new tool: Energy Impact measurement. We launched the SuperpoweredPerformance project so developers can use the Superpowered Audio SDK to check how Superpowered’s energy impact compares to Core Audio.

The first image (below) shows that there is no visible difference in energy impact for normal audio playback between Superpowered and Core Audio:

But when time stretching/pitch shifting is enabled, Superpowered’s audio quality is notably better with lower CPU usage and noticeably better energy impact:

Don't be fooled by graphs demonstrating "low" energy impact in every case. In this case, we are playing just one (1) audio stream here.

Faster processing mean less power consumption

Faster processing means the CPU completes the same task in a shorter amount of time, allowing for lowered CPU clock frequency. Lower CPU clock frequency affects power consumption, thus battery life.

Power consumption is proportional to the CPU clock frequency (f) and the square of the voltage (V):

(power consumption) = (CPU clock frequency) * (Voltage)²

Voltage is proportional to the CPU clock frequency. Higher CPU clock frequency requires higher voltage, lower CPU clock frequency requires lower voltage.

Conclusion: Superpowered faster processing algorithms improve mobile device battery life, as they require lower CPU clock frequencies, meaning lower voltage => lower power consumption.

Side effect: as mobile devices make use of passive cooling; lower voltage and lower power consumption means less heat as well.

In other words, faster processing means improved battery life AND a cooler mobile device.

The questions remain: How much of an impact? If an algorithm decreases CPU load by 1%, how is battery life affected? Fortunately, Apple has already done the research for us.

In the “Writing Energy Efficient Code” session of WWDC 2014 (Part 1), Apple notes:

1% CPU - 1.1x power draw
10% CPU - 2x power draw
100% CPU - 10x power draw

Translating that provides this rule of thumb: 1% increase in CPU load means 10% higher power draw.

Summary: Superpowering apps with Superpowered audio significantly improves battery life, which is paramount in user satisfaction factor on mobile devices.