This report provides an overview of plug-in battery energy storage systems (plug-in BESS) highlighting opportunities and challenges. Plug-in BESS, which are often connected with plug-in solar photovoltaic or ‘balcony solar’ systems, are the latest innovation in distributed energy resources technology. Plug-in BESS are smaller capacity batteries usually under 3kWh in storage capacity per unit. They are a do-it-yourself solution to home solar generation and storage that can be plugged into standard wall sockets and enabling consumers to start generating and storing renewable, affordable electricity. The ability for consumers to plug-in solar PV and BESS at home unlocks more equitable access to clean, affordable, and reliable energy. This is particularly important for energy consumers that may have previously been shut out of the market such as renters or apartment dwellers. Plug-in systems have the added benefit of reducing pressure on qualified electricians and solar PV installers, skilled occupations in high demand and in shortage globally. Due to the emerging nature of the plug-in BESS market, the technical capabilities, limitations, and performance characteristics are not well understood. The report indicates that there is a need for policy action to improve transparency and comparability of the performance of plug-in BESS units.