In the meantime, I refreshed my memory with this review and comparison of the German and Spanish feed-in laws. The feed-in tariffs were first introduced following the Electric Power Act 54/1997 (which liberalised the electricity generation market in line with the EU line) with Royal Decree 2818/1998. The rates weren't fixed like in the German Stromeinspeisungsgesetz (1991) and the succeeding Erneuerbare-Energien-Gesetz (2000), instead, producers got a premium above the hourly market price -- a regime that prevents producers from predicting their future income reliably. The reform of the Spanish law I remembered was established with Royal Decrees 1432/2002 and 436/2004, which introduced fixed rates as an alternative option producers are free to choose.

The linked article mentions several differences in detail, one I see as significant (again relevant to predicting future income) is degression of the rates: in Germany, a producer gets to sell at the same rate for 20 years, but the rate for new installations is decreased by a fixed percentage; in Spain however, the rate is set each year anew, and affects both new and old installations.

_{*Lunatic*, n.
One whose delusions are out of fashion.}