Optimum sizing of PV-attached electricity storage according to power market signals – A case study for Germany and Italy

• PV-attached battery unprofitable from German wholesale aggregator point of view.
• PV-attached battery profitability below utility costs of capital in Italy.
• Profitability can be reached if energy CAPEX reaches 100–150 EUR/kW h.
• Optimum sizing depends on usage.
• Maximum 5 h of storage and power rating exceeding 40% of PV power required.

This paper investigates the business case of power storage attached to PV generation from the perspective of an aggregator trading power on wholesale markets and possibly supplying household customers. The profitability and an optimum storage configuration are determined for two European regions: Baden-Wuerttemberg in Germany and Puglia in Italy based on wholesale price data and solar irradiation data for the years 2007–2011. During this period of time and under the assumptions made, adding storage to a portfolio of PV generators would not have constituted a business case for Baden-Wuerttemberg while profitability could have been reached for Puglia. However, the return of PV-attached storage that could have been achieved in Puglia during the years 2007–2011 is below levels typically required by companies operating on wholesale markets (deregulated power generators or traders) as those market participants’ capital costs are usually significantly higher than borrowing costs of the state. Storage proved to be financially more attractive in cases where severe grid constraints lead to significant levels of curtailment provided that the associated losses would not be financially compensated. This could pose a risk to any investor as grid upgrades would eventually erode the revenues. Restricting the storage to PV energy only (i.e. without the possibility to do arbitrage on markets) depresses the business case and is generally unprofitable, except but for situations of severe grid bottlenecks. The picture does not change significantly if a consumption portfolio is added. In order to reach profitability, the energy related CAPEX will have to fall to a range of 100–150 €/kW h. The optimal storage configuration depends on its usage and does not exceed 5 h of discharge at full power with the discharging power limited to 40% of the nominal PV capacity.