Everyone is saying that Wi-Fi is more power-efficient than 3G on smartphones.
But, I don't think so, because two radios are turned on at the same time, the 3G radio is still active when enabling Wi-Fi
There is some reliable proof to support this?
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Yes, WiFi is significantly more efficient, as the protocol is completely different. Once association with access point is established, WiFi only uses energy for actual transmission of data. On the other hand 3G remains in high-power state even after transmission is done.
source: "Energy Consumption in Mobile Phones: A Measurement Study and Implications for Network Applications" Niranjan Balasubramanian, Aruna Balasubramanian, Arun Venkataramani, Department of Computer Science, University of Massachusetts Amherst
It's also worth mentioning, that WiFi has legal limit of power output of 200mW (0.2W), while 3G devices legal power limit is 2W.