Ørsted and RWE, the world's two largest offshore wind players, both suffered from weaker winds in the first half, highlighting that profits in the booming industry remain tied to weather conditions.
Denmark's Ørsted said it "experienced significantly lower wind speeds than normal, especially across our offshore portfolio" in the first half, adding it would likely hit the lower end of its guided core profit range in 2021.
The company said quarterly wind speeds amounted to an average of 7.8 meters per second (m/s) across its offshore portfolio, which was lower than the 8.4 m/s seen in the second quarter last year and the normal wind speeds of 8.6 m/s it had expected.
Germany's RWE also cited much lower wind volumes in Northern and Central Europe compared with the very high level last year, reporting a 22% decline in core profits at its offshore unit to 459 million euros ($539 million) in the first half.
"As our offshore wind farms are located in the coastal waters of the United Kingdom, Germany, and Sweden, all RWE sites were affected," Germany's largest power producer said.
The results show that despite strong fundamentals, renewables continue to be an intermittent technology where swings in the level of winds and sunshine have a direct impact on earnings.
Shares in Ørsted traded 1.75% lower at 0815 GMT, while RWE slipped 0.3%.
The whims of the wind are not the only challenge faced by firms in the industry, after some of the world's largest turbine makers, including Denmark's Vestas and Spanish-listed Siemens Gamesa, pointed to high raw materials prices.
Denmark's Vestas, the world's largest wind turbines maker, this week cut its 2021 outlook, citing constraints in the freight market related to the impact of COVID-19 which have driven global supply chains towards breaking point.
($1 = 0.8522 euros)
(Reporting by Stine Jacobsen and Christoph Steitz, editing by Keith Weir)