The Planetary Reserve was an auxiliary power grid located on the planet Earth. Starfleet had access to this grid, and would, during a crisis, coordinate with other entities on the planet in ensuring that emergency and medical facilities were receiving power. If a city lost main power, they could draw power from the grid's reserve banks. The people of Earth could not survive for long on this gird.
In 2286, the Planetary Reserve was taxed to its maximum during the Whale Probe crisis. In that crisis, the whole planet was experiencing an increase in cloud cover which brought about severe weather and many cities losing main power. (Star Trek IV: The Voyage Home)