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The Tesla Powerwall battery solution provides the user with the option to control how and when the energy is used by storing energy from the photovoltaic (PV) system and/or from the grid to then be used when the electricity is more expensive. This option helps save money on the electricity bills and cut down on the PV system payback times. Since the Powerwall battery is perceived as being a cheap energy storage solution and energy usage can be controlled, three scenarios were considered to determine which is the most attractive to invest in. One is an on-grid PV system and Powerwall solution without grid injection incentives, then an off-grid PV system and Powerwall solution, and lastly a Powerwall solution without a PV system. The United States of America and Portugal were considered in this work in which the States of California, Hawaii, Alabama and the city of Aljustrel were chosen for comparison because of their different billing and grid injection schemes. The results show that over a 25-year period an off-grid solution in Hawaii can triple the investment and a Powerwall without a PV system solution in California can double the investment while all other scenarios are not viable investments.

Abstract-Following recent trends in the developed countries, Portugal recently cut down the Feed-in tariffs for photovoltaic energy. They are now based on the Iberian market average (around 0.04€/kWh) while the final consumer electricity price is 0.16€/kWh. In this view, the PV system should be dimensioned for self-consumption since injecting into the grid produces less revenue. The motivation for the current work was to analyse the changes due to this new regulation, through the analysis of the economic feasibility of different size PV systems (1kW, 3kW, and 5kW), considering the selfconsumption regime with a battery bank as an option. We also compare the most profitable cities in Portugal with and without the use of a battery bank. The results show that the PV systems with battery banks are not as profitable as standalone PV systems in the Portuguese self-consumption regime. This is mainly due to the high price of the battery bank. In general, the mainland results are better than the islands for the 1kW and 3kW PV systems. In spite of the higher initial investment cost, the islands present better results for the 5kW PV system. In general, it can be stated that the investment in PV systems is highly conditioned by external economical factors that continuously change and the current work analyses the current situation in Portugal for different installed powers with or without battery banks.

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