The company swung to a loss of 1.86 billion kroner (250 million euros, $278 million) in the second quarter compared to a profit of 93 million kroner in the same period a year ago.
A 12 percent drop in revenue after stripping out one-off items "was mainly due to lower gas and oil prices," said the government-controlled group, which still has conventional energy assets.
The decline was partially offset by a doubling of revenue from construction contracts for offshore wind farms and transmission assets, it added.
Sydbank analyst Morten Imsgard said the results were slightly below his expectations, mainly as a result of the weather not being windy enough.
"It's literally because it has been less windy. That's the way it is when you operate wind farms," he told news agency Ritzau.
Excluding one-off items, Dong Energy made a profit of 814 million kroner, compared to a 502 million kroner profit in the same quarter last year.
"Performance for the first six months of the year has been very satisfactory, both financially and strategically," chief executive Henrik Poulsen said in a statement.
Dong Energy listed 17.4 percent of its shares on the Copenhagen Stock Exchange on June 9, after selling coal-fired assets in recent years and transforming itself into a global renewable energy leader.
The 2014 sale of an 18 percent stake in the company to a consortium led by US investment bank Goldman Sachs was controversial and nearly toppled the former centre-left government.
Shares in Dong Energy were 2.45 percent lower at 275 kroner in late morning trading on the Copenhagen bourse, but significantly higher than the June listing price of 235 kroner.