In an example embodiment, a battery unit comprises a battery unit housing; and a battery unit circuit. In this example embodiment, the battery unit housing contains at least a portion of the battery unit circuit, and the battery unit circuit further comprises: a battery cell, an inverter to control the charging and discharging of the battery cell, a processor to provide control signals to the inverter for controlling the charging and discharging of the battery cell, and one of: a power plug for coupling to and uncoupling from a power outlet assembly, and a luminaire base for coupling to and uncoupling from a luminaire socket in a light fixture. In this example embodiment, the battery unit is rated at less than or equal to 2400 Volt-Amperes. The battery unit may further comprise a transceiver.