Leif does not store any personal information about you. Leif does not track your activities nor make use of cookies.
Leif stores the wattage data and a cache of API responses. This can be accessed at ~/Library/Containers/com.fetimo.leif. You are free to remove all data in here at any time.
Leif uses HTTPS to securely get data about the current carbon mix in the UK from a third-party service called carbonintensity.org.uk run by National Grid.
Leif is open-source so it is relatively easy to see if the code deviates from this privacy policy. If you believe that a clause has been violated or is out-of-date please raise an issue.