The Network Off Battery Saver Android app turns off wifi and mobile data when the screen is off. They turn back on when the screen is on, and also whenever the phone is plugged in.
This can save large amounts of battery while affecting phone function very little.
Installable Android app (.apk) included, as well as a version runnable under Tasker (.prj.xml). On your mobile device you can download the project directly into Tasker using this TaskerNet link.
Since version 2.26: Bug fix for Periodic Wake-Up; Bug fix for Temporary Network Off; refining permissions needed; fixed formatting of Help text; numerous other minor bugs & refinements.
Wiki with description, installation help, and app general help: https://github.com/bhugh/Network-Off-Battery-Saver/wiki
Root required, though there is an experimental method to run without root outlined in the wiki.