Regarding legality: According to Commission Regulation EU 2016/631 systems under 800W don't qualify as any of the regulated types of power generating facilities

Regarding legality: According to Commission Regulation EU 2016/631 systems under 800W don't qualify as any of the regulated types of power generating facilities, so any law that would limit their installation must be local. In practice you need to have a two-way meter installed by the power company and you're good to go.