Granola runs in the background but if you install the GUI, you'll be able to see some statistics such as how much energy, money and CO2 you'll be saving by running Granola on your computer as well as the overall savings by all Granola users.

The energy saving depends on your computer activity: if you mostly surf the Internet, the Granola wiki states that you will probably save around 30% energy. If you are playing games without a pretty new graphics card, the savings are less: around 10%.