Abstract The emergency power supply functionality of photovoltaic battery energy storage systems (PV BESS) is evaluated based on a case study, which comprises a single-family house in Germany with defined electricity load profile and installed PV BESS. Key factors, which influence the emergency power functionality, are: begin and duration of the blackout, electricity load and PV production profile during blackout and BESS state of charge at the beginning of the blackout. The backup functionality especially depends on the available electricity generation from the PV system and shows therefore a strong seasonal dependency. In case of a blackout, a PV BESS generally makes electricity available, which would not be available to a household without PV BESS. However, the complete coverage of longer blackout periods from PV BESS under the assumption of a normal load profile (100% autarkic system operation) is limited to only a few high PV production periods during the year. In this context, load reduction and load shifting by adapted user behavior during a blackout shows high potential to increase the backup supply functionality and the overall security of energy supply by extending the period during which the reduced household electricity load can be covered from the PV BESS.