Google Inc. Wednesday unveiled a $4.4 trillion plan for reducing U.S. dependence on fossil fuels, as the Internet giant tries to emerge as a leading advocate for greater adoption of alternative energy.

The proposal is based on the U.S. halting the generation of electricity from coal and oil by 2030 and relying on power from wind, nuclear and geothermal sources instead. It also entails cutting oil use for cars by 40%.

While...