Dutch energy firms produced less energy from sustainable sources last year, and their use of coal was up 5% on 2013, the Volkskrant reports on Tuesday.

Falling coal prices were behind the shift towards coal, which means 29% of Dutch energy is now derived from the fossil fuel, the survey showed. Coal produces twice as much carbon dioxide as gas.

As a result many power firms have gone down in the annual green energy rankings published by green groups and the consumers’ association Consumentenbond.

The biggest Dutch energy firms Nuon (owned by Vattenfall) and Essent (RWE) are among the least sustainable producers and are at the bottom of the rankings. Eneco, which was a good performer, saw its score go down from 6.8 to 6.2, the Volkskrant says.

Small firms, which focus 100% on sustainable energy, top the list but their combined market share was just 3.3% last year, the Volkskrant says. Three firms, all of which are wind power only – DE Unie, Pure Energie and Qurrent Nederland – scored 10.

Most of the 37 companies in the ranking do not produce energy themselves but act as energy traders, buying in power in the Netherlands and abroad. Their marks are based on their performance in terms of investment in sustainable energy production and delivery, the ranking’s compilers said.

Seven companies, including the biggest players in the Dutch market – Essent, Nuon, GDF Suez and EON – refused to cooperate with the survey. However, the researchers said there is enough public information about their performance available to give an accurate picture.