Collect all fifteen energy cells to get the "Powerhouse" achievement. The following areas contain cells that must be collected before you progress far enough and lock yourself out from returning there.

Mount Horu: 1 cell. Note: You must have fourteen cells collected before entering.
Ginso Tree: 1 cell.
Misty Woods: 1 cell.

The remaining cells are in the open area and can be found at any time before entering Mount Horu. Some will not become available until you have certain skills. All will become available after you have the "Charge Jump" skill. There is an ability which shows cell locations on the map, but some are hidden in secret areas.