The EU only has power because the member states have agreed it should. The EU has three sorts of competence: exclusive, shared and supporting. Basically, this is the difference between whether the EU is in charge of an area (once the member state governments and the MEPs have agreed), whether it is an area that national governments and the EU co-operate in or an area where the EU supports Member States. These are as follows:

One of the most frequent myths you will hear is that the EU dominates domestic law. It doesn't. This is often associated with the inaccurate statistic that 80% of UK law is made in the EU.  Originally, this myth sprang from a speech by former Commission President Jacques Delors who stated, in July 1988, that “within ten years 80% of economic legislation would be of EU origin”.[1]

He was wrong.

Currently around 15% of UK laws are agreed in the EU or have an EU influence and a similar figure applies to regulations.[2] This is the norm across the EU; Jacque Delors was wrong across Europe, both inside and outside the Eurozone:

• 10% of French laws were directly transposing EU directives, while 25% of them included elements of EU origin;[3]
• 9.6% of all primary and secondary laws in Denmark had an EU origin.[4]