You plug in your iPhone every night to give it a full charge. But all that battery charging costs you just 47 cents -- a year.

That's according to a study conducted by Opower, a software provider for utility companies. Opower's estimate might even be too high -- it assumes you charge your iPhone 6 from 0% to 100% once a day every day, but most people don't empty their batteries of juice every day.

Don't believe it's that cheap? The calculation is actually pretty simple.

Apple's (AAPL) new iPhone 6 charges from 0% to 100% in 1 hour and 48 minutes, which is about 10.5 watt-hours of energy. Multiply that by 365 days in a year, and you get 3.83 kilowatt-hours. Multiply that by the average U.S. residential electricity price (12.29 cents per kWh), and you get 47 cents per year.

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Did you buy an iPhone 6 Plus? You'd better factor in another nickel: It costs 52 cents to power up every year. The iPhone 5, meanwhile, costs just 41 cents for a year's worth of charging.

They cost so little to charge, because smartphones -- built to last a day(ish) on a tiny battery -- are incredibly low-power devices.

By comparison, a laptop uses 14-times more electricity per year, and a desktop computer uses 49 times more, according to the Fraunhofer USA Center for Sustainable Energy Systems. As smartphones continue to replace PCs, consumer energy usage -- and bills could sink.

Smartphones have also eaten in to video game sales. Playing games on a smartphone instead of a television can save you even more money: An Xbox One uses 61 time more power than an iPhone 6, according to the Natural Resources Defense Council, and a TV sucks up 72 times more electricity.