Voyager 1 and Voyager 2 both have enough power to keep transmitting its findings to scientists here on Earth until the 2020s.

FORBES: Voyager 1 Flies Through 'Magnetic Highway' On Its Way Out Of The Solar System

Each stage of converting and transmitting power results in a loss of efficiency, but with technological improvements these losses are being reduced.

ECONOMIST: Solar power from space

Apple also provides a setting in the Airport Extreme software that let us diminish the base station's transmitting power should we want to restrict the range of transmission to, say, a conference room for security reasons.

FORBES: Magazine Article

That way grid operators could, say, automatically lower the cost of transmitting power from a region with excess to capacity to one with a shortage, instead of letting the problem fester until the peaker plants turn on and phone calls and faxes fly.

FORBES: Juicing the System

"If you switch on a cellphone on board, and there's no network, it will gradually increase the power it's transmitting because it's trying to get hooked on a network which doesn't exist, " he said.

CNN: In-flight phone calls to become standard, say experts

Recent work has also demonstrated that at suitably low temperatures they can act as superconductors transmitting an electric current indefinitely, without power loss.

ECONOMIST: The Nobel prizes: Informed choices | The