The annual cost to charge an iPad is just US$1.36, according to the Electric Power Research Institute, a non-profit research and development group funded by American electric utilities.
By comparison, a 60-watt compact fluorescent bulb costs $1.61, a desktop PC adds up to $28.21 and a refrigerator runs you $65.72.
The group, known as EPRI, studied the power consumption of Apple Inc.'s iPad to determine the effect that the newly-popular devices might have on the nation's electricity use.
The answer: not much.
If the number of iPads triples from the current 67 million, they would need the electricity from one small power plant operating at full strength.
But if people are using iPads instead of televisions to play video games, or ditching their desktop computers for iPads, the shift to tablets could mean lower overall power consumption. A desktop computer uses 20 times more power than an iPad.
Baskar Vairmohan, the EPRI researcher who conducted the iPad test, said the group is now studying usage to understand whether the explosion of tablets is adding to power consumption, or reducing it.
Residential power demand is on track to fall for the third straight year, according to the government. A weak economy is keeping people in smaller houses and shacked up with others. At the same time, efficiency programs are pushing more efficient light bulbs, air conditioners and other devices into homes. Refrigerators use a quarter of the power they used a generation ago, according to EPRI.
For the iPad test, Vairmohan measured the amount of power used to charge up an iPad with a drained battery. He assumed that users would charge up every other day. Over a year, the latest version of the iPad consumed 11.86 kilowatt-hours of electricity. (Older versions consume somewhat less power.)
The juice would cost $1.36 at the U.S. average residential price of 11.49 cents per kilowatt-hour.
But there's an even cheaper way to go than the iPad. EPRI calculated the cost of power needed to fuel an iPhone 4 for year: just 38 cents.
Follow Jonathan Fahey at http://twitter.com/JonathanFahey .