Tesla Powerwall in the United States and Portugal - a Comparative Analysis on the use of Storage with Small Scale Photovoltaic Systems

Recently Tesla announced their Powerwall solution to store energy in the residential sector, focusing on low cost storage, the use of photovoltaic systems and reduction of the electricity bills. The work presented in this paper analysis the use of this storage solution by comparing different scenarios in which Powerwall can be used. These scenarios compare different states of the United States of America and Portugal, considering the electricity prices and billing system in each of the regions. The potential savings provided by using the Powerwall result mainly from different hourly tariffs and in California it can be of 0.21€/kWh in the summer and 0.12€/kWh in the winter, in Hawaii it can be of 0.26€/kWh plus approximately 800€/year on fixed bill savings, in Alabama it can be of 0.10€/kWh and finally in Portugal it can be up to 0.19€/kWh, since all the cites or states practice different electricity prices as well as grid injection schemes. The scenarios considered for this work involve a 100% self-consumption regime in which grid injection is not considered even though it is allowed in some cities or states. To evaluate the economic impact of Power wall, different economic methods are used, such as the Profitability Index, the Internal Return Rate, and the Discounted Payback Period of the Investment. Other base economical parameters such as interest rate, maintenance and operations rate, inverter replacement rate, Powerwall Battery replacement, VAT tax rate, electricity evolution rate, electricity price, prices for a PV system connected to a Powerwall system, and PV degradation rate are also considered. The settings used for the evaluation are those of a residential location using a 2kW PV system connected to a 7kWh Powerwall Battery and they are considered for three scenarios as follows: Scenario 1 considers an on-grid PV system connected to a Powerwall battery both in the USA and in Portugal. In this scenario, Alabama state is used since only the selfconsumption scheme is practiced and there is no net-metering nor feed-in tariff schemes. The city of Aljustrel in Portugal is used in this scenario since a 100% self-consumption regime can be practiced there that is comparable with Alabama and this city has the highest solar radiation in the country. Scenario 2 contemplates an off-grid PV system connected to a Powerwall battery both in USA and in Portugal. In this scenario the state of Hawaii, which practices the highest electricity tariffs, is compared with the city of Aljustrel. Lastly, scenario 3 considers charging the Powerwall battery directly from the grid (without resorting to a PV system) in the super off-peak hours and using the Powerwall battery to feed the household grid in the on-peak hours both in USA and in Portugal. The state of California was chosen for USA since it practices the highest tri-hourly tariffs while Aljustrel city in Portugal practices the highest bi-hourly tariffs. The overall results show that Portugal is not attractive in any scenario while in several scenarios for the United States, considering the different states, the investment can be doubled or tripled, include with the use of the Powerwall without a PV system.