Cortana always listens in order to hear its name and be a smart digital assistant. This is Microsoft answer to Siri and Google Now that is making its way to Windows 10.
Unfortunately, this will affect your notebook battery life. We have spoken with a few industry sources and we can definitely confirm that Windows 10 with enabled Cortana will have an impact on the battery life. We are testing this as we speak to check how big the impact is.
We don’t know how significant the battery life decrease will be, but the good thing is that you will be able to switch Cortana off in case you don’t need it. We heard that many new Toshiba notebooks will come with a dedicated Cortana button, as this is the easiest way to save battery life. Cortana on Toshiba won't listen until you press the button.
It would be smart if Microsoft would come up with Cortana enable / disable keyboard shortcut. Win + Q will enable Cortana news while Win + S will bring you directly to the Cortana search engine.
Windows 10 seems to be a logical upgrade for anyone who has Windows 8.1 on their notebooks and misses the options from Windows 7, and some familiar UI elements. We use Windows 8.1 on some devices, while most of our computers still have Windows 7 and nothing more. Microsoft DirectX 12 will force us to Windows 10 but from what awe can tell from Preview release, the upgrade to Windows 10 from with 7 seems like quite seamless and logical step.
Just make sure to be aware that your notebook battery life might suffer because of Cortana. Have in mind that this "talk to your PC and expect a smart answer" option can be disabled.