## 4 Discussion  
The adopted methodology enables the examination of the impact of bilateral contracts of the wholesale markets. The Public Power Corporation (PPC) represents about 90% during off-peak hours (1-8), which determines the minimum load that will be covered by bilateral contracts. This leads at about three up to nine PPC thermal units to operate for meeting bilateral contracts, in case of low load/10% and high load/25% respectively. Besides this deviation in the committed units, the results both in the prices and energy mix are not strongly influenced. This is attributed to the fact that the new market algorithm on the voluntary energy exchange [15] enables bidding for thermal units below variable cost. The main reasoning behind that, besides the liberal assumption that every participant can adopt its strategy without constraints, is that thermal units in the European target model cannot remunerate shut-down costs through supplementary mechanisms that existed in the mandatory pool. This fact, in practice, enables PPC, as well as all thermal units, to dispatch their technical minimum with limited risk, as the remuneration of shutdown costs is difficult from the day-ahead, intra-day, and balancing markets. This is exceptionally difficult for lignite-fired units that have a high shut-down cost, as well as a long-time for the (de)synchronization of the units in case of start-up/shut-down respectively. Therefore, unless there is a specific regulatory decision on limitations in the bidding of thermal units, the dispatch of units is not strongly influenced by the introduction of bilateral contracts. In fact, the impact on the wholesale price is negligible, as the differences at the annual level are within 0.1 \(\upmu\)/MWh. Moreover, the non-linear evolution of SMP, as shown is Figure 2 (where the case of 15% has higher SMP compared to other cases) results from the conditions of specific dates and availability of units, which is negligible to enable the provision of a conclusion that this constitutes as an optimum level of bilateral contacts. In the case of the energy mix, the impact is again limited, as shown in Figure 3. However, the dispatch of units is different, namely different natural gas units are dispatched at different capacities, although the sum of natural gas units is similar. This is attributed to the fact, that as the level of bilateral contracts is increased, the dominant producer PPC increases its capability to dispatch more units. Besides the lignite units, that is of high priority, due to the increased shut-down cost, that could force PPC to target at their operation at their technical minimum, the increase of bilateral contracts' level, enables the operation of higher capacity from PPC natural gas units compared to IPPs' natural gas units. However, again, the changes are relatively small but evident. Finally, the paper reveals, in alliance with the results of a recent paper [15], the impact of different order types, namely hourly and block order types. Block order types, although introduced for the needs of power producers, can lead to a reduction of wholesale price. However, as revealed in a recent paper [15] they can also lead to the evolution of wholesale price with high fluctuation, because they decrease flexibility in meeting hourly demand. Figure 4: Wholesale price for different scenarios, dates and different level of bilateral contracts: (**a**) 0%, (**b**) 10%, (**c**) 15%, (**d**) 20%, and (**e**) 25%. To conclude, the implemented market strategy, as described above, allowing all thermal units to bid its technical minimum (not meeting bilateral contracts) below their Minimum Average Variable Cost (MAVC) with almost zero price, practically leads to similar power units mix. So, the level of the allowed share in bilateral contracts (10% up to 25%) for PPC, does not affect considerably the energy and units' mix, as well as the system marginal price. PPC would favor a higher share for bilateral contracts, but this is not a game changer in the wholesale price signal.