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From what I can recall, both 3G and Wifi should consume about the same about of battery when in operation. However, my field testing seems to indicate that 3G is more battery consuming than Wifi.

Is this result general? If yes, why this is so?

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up vote 8 down vote accepted

First of all, all wireless systems use the most power when transmitting, ie actively powering the antenna to upload data.

Now, just think of the ranges the different systems have to bridge. WiFi works well for like 30 meters, or 100 meters outdoors. Usually, the nearest cell tower is considerably farther away than those 100 meters, therefore, 3G needs more power than WiFi.

Add to this that 3G is capable of opening multiple connections at once, and the fact that 3G autmatically boosts power output on weak signals, 3G needs more power.

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A lot more to be honest :) –  Martín Marconcini Nov 22 '10 at 17:36
    
According to wikipedia, cell tower range is measured in tens-of-kilometers, so we're talking about more than 100x difference in range. –  Ken Nov 22 '10 at 23:48
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RotHorseKid is mostly right, however it's not quite as simple as distance — the operating frequency and bandwidth also affect the power usage.

The wikipedia page on dBm provides some approximate power levels for various transmitters, including:

  • Maximum output from a UMTS/3G mobile phone as anywhere from 125 mW to 2 W (for various power classes — I'm not sure where the iPhone 4 fits, nor any of the iPhones)
  • Typical laptop wifi transmission power of 32 mW (and I'm fairly sure the iPhones are below most laptops) with a maximum allowable power of 200 mW
  • Bluetooth output is 1.3 mW, 2.5 mW or 100 mW depending on range (1m, 10m 100m)

Now it's not quite that simple, since the transmitters won't be 100% efficient, and there's associated electronics required for the communications to work (on top of the EM transmissions). Such electronics tend to use more power when they run at higher frequencies, and higher potential bandwidth will require a higher frequency (i.e. the supporting electronics of wifi probably uses more power than the supporting electronics of 3G, which in turn probably uses more power than the supporting electronics of Bluetooth).

At a guess, I'd say the supporting electronics would use between 10-100 mW (perhaps someone knows the actual comms chips and can provide specs?), so depending on the power class of the iPhone, it'd be comparable, but I'd still expect 3G to use significantly more power than wifi (and, anecdotally, my iPhone gets hotter when using 3G compared to wifi, which backs this up too).

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