FRANKFURT (Reuters) - Private sector investments in Germany's electricity networks must be incentivised by better returns, the chief executive of utility E.ON, Europe's leading operator of energy networks, said on Wednesday.

"Grid operators now need a long-term, but quickly effective, framework with provisions for adequate interest on spending on existing and new networks," CEO Leonhard Birnbaum said in an online call after presenting financial results for January through June.

"Otherwise, the energy transition with millions of decentralised units and protagonists will fail," he added, referring to small wind and power generation plants.

(Reporting by Vera Eckert, editing by Miranda Murray)