If I work more than 20 hours a week with work limit of 20 hours, what will happen?

Hi
I am a PhD student with Tier4 visa. Currently, I am working at university about 10 hours a week. If I want to work outside the university for 20 hours a week. Is there any system here that can count my hours? Would be any problems for me from home office?
My work limit is 20 hours a week.