The tax is just enough to make sure that homeowners cannot gather and store solar energy cheaper than state-sponsored providers.
Via Mish-modified Google Translate from Energias Renovables, please consider Photovoltaic Sector, Stunned
The Secretary of State for Energy, Alberto Nadal, signed a draft royal decree in which consumption taxes are levied on those who want to start solar power systems on their rooftops. The tax, labeled a "backup toll" is high enough to ensure that it will be cheaper to keep buying energy from current providers.
Spain Privatizes the Sun
Via Google translate from El Pais, please consider Spain Privatizes The Sun
If you get caught collecting photons of sunlight for your own use, you can be fined as much as 30 million euros.