Over the past two decades, electricity markets in Europe and many other countries have been substantially re-regulated – from vertically integrated, usually publicly owned monopolies to more market-like structures with a separation of generation, supply and grid operation, and the privatization of utilities. Under this principle, suppliers have the choice to offer a diversified portfolio of electricity products (differentiated by price or origin; e.g. from renewable energy carriers), while consumers – in theory at least – have the freedom to choose their supplier and product on the market. Breaking up the hierarchically integrated structure of past electricity systems, however, also leads to higher fragmentation of the sector and an increased number of actors involved (independent generators, new sellers and re-sellers, and so on).