In other terms, electricity systems are locked-in with their existing generation mix and cannot quickly adapt it to changing market conditions. In discussions about the costs of back-up capacity this is also referred to the ex post case. In the long term instead electricity companies can adapt their generation mix to new market conditions, resulting from changes in demand, fuel prices as well as the introduction of new capacity into the electricity mix. This was referred to as the ex ante case, as the analyst places him/herself at the very beginning of the investment cycle which can now be optimised.