Installed capacity fell roughly 26%, UNEF's Chief Executive Jose Donoso told an event in Madrid.
"The fall in residential self-consumption has hit the bottom in the first quarter, but it is now spreading to the industrial sector," Donoso said.
New installed capacity in households fell by 15% from January to March this year, while among industrial customers it fell by 30%, UNEF said.
Lower energy prices and the fading impact of subsidies has been cooling the rooftop solar fever in Spain since last year, with fewer Spanish households setting up solar energy installations.
Donoso urged the government to implement tax deductions, scupper the value-added tax for residential consumption and provide fiscal relief for companies.
More than one-fifth of Spanish houseowners have solar panels installed or are in the process of installing them, UNEF said.
