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According to measurements from Lawrence Berkeley National Laboratory, the average cell phone draws 3.68 watts of power from the outlet while it's charging and 2.24 watts when charged.
If you do that 365 times a year, that's 3.8 kilowatt-hours in total. At an average US retail price of 12.29 cents per kilowatt-hour, it will cost iPhone 6 users just $0.47 per year to charge their device. The iPhone 6 Plus has a larger battery and thus draws more electricity per fill-up (16.8 watt-hours).