Hydropower is the most used renewable energy technology with over 4000 TWh electricity generated worldwide in 2017, corresponding to almost 16% of the total electricity generation. In the Nordic countries, hydropower provides an even larger share of the electricity generation with about 50% of the total electricity generation coming from hydro. In other words, hydropower plays a significant role in power systems worldwide in general and in the Nordic power system in particular. Typically the hydropower included in larger power system models are simplified to reduce computation time. These simplifications can be denoted as a hydropower Equivalent which aims to mimic the behaviour of a more detailed description of the hydropower system.

Here some of the most common power system models of the Nordic system are summarized including a shorter description specifically describing the modelling of hydropower. The models included are Apollo developed by Sweco, Balmorel which is an open-source alternative, EMPS created by Sintef and BID3 developed by Pöyry.

All four models utilizes so called hydropower Equivalents with one or two stations per geographical area. In BID and EMPS the inflow is divided into regulated and unregulated inflow and only include one hydropower station and associated reservoir per area. Apollo and Balmorel on the other hand include two hydropower stations per area, one regulated, with an associated reservoir, and one unregulated.