>Energy compagny buy the excedent of private owned solar panels at very
low price, and sell it back at the regular price, taking the same
margins as on other sources and having no problem to fonction.

This increases the cost of the last mile infrastructure, as it turns out a lot of older infrastructure doesn't have the capacity to handle whole neighborhoods putting electricity back onto the grid.

It also forces utilities to use more expensive peaker plants to generate electricity at the times when solar isn't providing as much.

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> taking the same margins as on other sources

If they take the same margins as from other sources, then there are no additional funds available to make up for the shortfall. They need to have a much higher margins (compared to other sources) to do that.