  The increasing transmission capacity needs in a future energy system raise
the question how associated costs should be allocated to the users of a
strengthened power grid. In contrast to straightforward oversimplified methods,
a flow tracing based approach provides a fair and consistent nodal usage and
thus cost assignment of transmission investments. This technique follows the
power flow through the network and assigns the link capacity usage to the
respective sources or sinks using a diffusion-like process, thus taking into
account the underlying network structure and injection pattern. As a showcase,
we apply power flow tracing to a simplified model of the European electricity
grid with a high share of renewable wind and solar power generation, based on
long-term weather and load data with an hourly temporal resolution.
