## 6 Conclusions and Future Work  
This paper presents an economic assessment of solar-powered residential battery energy storage systems on Madeira Island, where only the self-consumption regime is currently allowed. This study's main objective was to understand to what extent energy storage devices can be profitable under the legislation and energy tariffs currently in place. The assessment was conducted against six local UPACs using year-long solar PV production and electricity demand measurements and two different battery control strategies. Several inverter/battery combinations were considered for each UPAC based on their consumption and production patterns. The obtained results are in line with many other research works that state that storage for self-consumption only will not be profitable until the price of energy storage devices drops considerably. In our particular case, the energy storage devices only start to be profitable at an average price of 256 \(\upmu\)kWh, including the inverter price. At this point, it is important to remark that the 10-year projection assumes little change in terms of prosumers behavior as well as of the regulation framework and international markets. For example, sudden increases of domestic demand due to a pandemic situation such as COVID-19, or in the cost of energy from non-renewable energy sources would lead to very interesting trade-offs such as the increase of direct self-consumption (in the case of a pandemic situation), and added value for arbitrage (in the latter situation), which would very likely change the profitability of energy storage devices. Our results also highlight a significant trade-off among SC, pre-charge and profitability. More precisely, using pre-charge increases the chances of reaching profitability more quickly than a greedy strategy. However, this comes at the cost of a drop in the SC rates and, consequently, increases in the curtailment of solar PV production. This suggests the need for new legislation that make self-consumption more profitable that the alternatives. Nevertheless, it should be mentioned that any changes in legislation and pricing mechanisms require a deep understanding of the main drivers and barriers to such changes, particularly to what concerns the future adoption by the consumer/prosumers base. For example, while the local DSO already provides semi-flexible energy tariffs (2-TOU and 3-TOU) to regulate the demand and encourage consumption outside the peak hours, the vast majority of the domestic consumers/prosumers still prefer the single-rate tariff which is a major barrier to achieve higher levels of penetration of RES [27]. In future work, we aim to explore how more advanced storage control strategies can improve storage profitability without compromising self-consumption levels. For example, by introducingday-ahead forecasts, it may be possible to set the pre-charge SOC dynamically, thus avoiding solar PV curtailment in the early morning. Furthermore, a forecast based approach would also enable the possibility of using storage to decrease the PPC, which is another way of lowering the energy bill. For example, changing from a 6.9 kVA PPC to a 5.9 kVA one would reduce the fixed component by around 17 Euros after one year. Furthermore, we also aim at exploring the value proposition of PV-BESS in LV commercial UPACs such as small restaurants and offices. Initial steps towards this direction are taken in [28], where it is shown that, compared to domestic PV-BESS installations, the payback time of a small restaurant (yearly consumption around 24 MWh) is at least four times faster. The main reason for this is the simple fact that, due to the high electricity demand throughout the day, it is always possible to pre-charge the battery in the off-peak period and fully discharge it during the restaurant operation hours. Conceptualization, L.P. and J.C.; methodology, L.P. and J.C.; software, J.C.; validation, L.P., J.C. and L.B. ; formal analysis, L.P., J.C. and L.B. ; investigation, L.P., J.C. and L.B.