Spotlight search in OS X uses background processes to index the content of your Mac. Indexing is the key to achieve the quick search capability when you hit Command-Space to search with Spotlight. To save CPU cycle and thus battery usage, you should prevent Spotlight from indexing those folders whose content you do not intend to be searched.
Go to System Preferences > Spotlight and click on the “Privacy” tab to reveal the list of folders Spotlight will not index for search. Click the “+” button or drag and drop to add folders to the list.
Typically you will add folders whose content you do not care to appear in Spotlight search results. It is also useful to add in folders with frequently-changing content as Spotlight will need to reindex whenever content is changed. Examples would be the “Desktop”, “Downloads” or folders used by your backup software.