What is the ERC (European Research Council)? Find more information here:

-ERC website
-Euresearch: the Swiss guide to European research & innovation

Q: The PI of a StG has to devote at least 50% of his/her working time to the ERC project. This means that if he/she works 50%, he/she has to devote at least 25% of a total working time to the project?

If the 50% part-time work is the PI's total working time, in order to fulfil the minimum requirement of the WP, the PI would have to devote at least 50% of his/her total working time on the project. This would corresond to 25% of a full-time work. In any case, the working time has to be properly documented. Furthermore, please note that it also has to be assessed on a case-by-case basis whether this amount of time is sufficient to carry out the project as proposed.