Investment flexibility is particularly valuable in deregulated electricity markets, where electricity market prices may have higher variability. In order to continuously re-establish the balance between load and generation, a certain amount of active power is kept in reserves: such reserves are usually referred as “spinning reserves”, but definitions vary among countries (Rebours and Kirshner, 2005, Kirby, 200, Bovo et al., In Chapter 2 those reserves were subdivided into primary, secondary and tertiary reserves depending upon the time window of availability.