Tab Suspender

Tab Suspender for Safari reduces your memory and CPU usage by up to 60% and indirectly, saves your battery consumption by automatically suspending the tabs you aren't using.

The tool allows you to set how many minutes to wait before suspending inactive tabs. However, you can create a Whitelist with the the URLS or domains you do not want to suspend. In addition, the app can detect tabs that contain active forms which will not be suspended if you don't want it.