The most accurate and reliable way of measuring the power consumption of any device is by using an external measurement device connected to the battery line of the device under test.
Our solution is to add a known low ohmic shunt resistor R to the battery line and measure both the battery voltage and the resistor voltage drop. The measurement is performed with a data acquisition card capable of collecting 50 kSamples per second with high accuracy. A PC software converts the voltage data into current and power figures in real-time.
An easy alternative solution with mobile phones would be to run a power consumption meter app in the measured device, but there are several issues with that method.
All the device settings are left in the default state during measurements, unless mentioned otherwise. Display brightness has been adjusted with luxmeter to match 200Nits (typically ~50% brightness).