Ebitda in the renewables division totalled €819.6 million for the first half of 2016, down 2% year on year.
Revenues in the division grew 1.9% for the period, to €1.3 billion.
Iberdrola said the wind load factor in Spain fell 3.4 percentage points to 24.5% in H1, forcing a 12.8% fall in output, compounded by falling prices.
Iberdrola blamed lower wind generation in Spain for the fall, despite increased earnings from its US and UK businesses growing 31.7% and 2.8% respectively.
There was even a boost from its Latin American renewables unit, as higher prices and a revaluation of the Brazilian Real helped earnings increase by 13%, the company said.
Iberdrola currently has more than 6.3GW of renewables and contracted generation under construction, of which 1.35GW will be in operation during the second half of the year, mainly of renewable generation.
The company is currently installing the 350MW Wikinger offshore wind project in Germany, due to be online by the end of the year.
For the Iberdrola group overall, Ebitda fell 3.6% year on year, while net profit grew 4.2% in H1, to €1.52 billion.