Next time you want to go buy a super smart phone, ask one question, what is the "Thermal Envelope" for the phone that you are buying. It will be expressed in terms of watts. The higher the number the better it is.
Taking a typical value of 3W (fairly aggresive for a lot of the devices), lets try to allocate this to the various components on the device. The LCD screen takes an order of 200-400mW of power. The modem can take about 600-700mW. That leaves you anywhere between 2.2W -1.9W for the rest of the system. If you allocate about 200 to 300mW for other peripherals like the eMMC, SD Card etc then you are left with a 2W to 1.6W for the complete SoC which includes the CPU, Memory, the Graphics engine, the audio engine and video encode/decode.
Applying the 80/20 rule for power taken by Graphics & CPU vs rest of the SoC, you will have about 1.6W to 1.3W of power that can be allocated to the CPU and graphics combined. Since the graphics nowdays take about the same amount of power as CPU iteself (Snapdragon Krati takes about 0.8W of power while the graphics can take upto 1.2W), you see that the CPU gets about 800mW to 600mW it can take most.
So even though you have a big honking CPU dual core/quad core etc, the performance is really limited by the thermal envelope of the phone you are buying. Also the phone performace depends heavily on all the components not being loaded at the same time. Which means you should know what your usage is going to be and how that will change the power equation. A quick benchmark for that will exercise most of the components together is if you can stream a HD video clip over the air for about 20min and see how hot the phone is getting, or try video conferencing.
Disclaimer: The numbers are typical and change from one phone to another. An OLED display will give you a little more headroom where there are more black pixels than white.
Taking a typical value of 3W (fairly aggresive for a lot of the devices), lets try to allocate this to the various components on the device. The LCD screen takes an order of 200-400mW of power. The modem can take about 600-700mW. That leaves you anywhere between 2.2W -1.9W for the rest of the system. If you allocate about 200 to 300mW for other peripherals like the eMMC, SD Card etc then you are left with a 2W to 1.6W for the complete SoC which includes the CPU, Memory, the Graphics engine, the audio engine and video encode/decode.
Applying the 80/20 rule for power taken by Graphics & CPU vs rest of the SoC, you will have about 1.6W to 1.3W of power that can be allocated to the CPU and graphics combined. Since the graphics nowdays take about the same amount of power as CPU iteself (Snapdragon Krati takes about 0.8W of power while the graphics can take upto 1.2W), you see that the CPU gets about 800mW to 600mW it can take most.
So even though you have a big honking CPU dual core/quad core etc, the performance is really limited by the thermal envelope of the phone you are buying. Also the phone performace depends heavily on all the components not being loaded at the same time. Which means you should know what your usage is going to be and how that will change the power equation. A quick benchmark for that will exercise most of the components together is if you can stream a HD video clip over the air for about 20min and see how hot the phone is getting, or try video conferencing.
Disclaimer: The numbers are typical and change from one phone to another. An OLED display will give you a little more headroom where there are more black pixels than white.
