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Smart phones are very energy efficient because they are designed to run on a battery for a long period of time. For a smartphone battery with 1650mAh and a voltage of 3.7V (typical cell phone battery voltage 3-5V) the Wattage for an 8 hour talk time on 3G as is 0.76 W.
Assuming the smartphone is charged once a day from 0% to 100% a Galaxy SIII: consumed 12.3 watt-hours to charge, taking 2 hours and 26 minutes, the maximum wattage was 6.6 watts, with an average of approximately 5.0 W.
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