Today it means: A battery box, capable of providing power in amounts like a generator. It is not a generator, not generating any electricity from fuel, merely storing power previously charged in the unit. The power stored in the unit does not necessarily need to be provided by solar panels, mains power can also be used to charge the unit, or any other electricity source. Input power may also be streamed directly as output power. Full definition →
solar + generator. From being a device designed to be hooked into solar electric photovoltaic power panels. And, from being a device that would in turn provide electric power on demand any time of day.
Etymology from Wiktionary, CC BY-SA 4.0.