Originally used for a nightlight on a solar powered electric gate setup, where you want to make sure the nightlight doesn't drain the battery so far that the gate motor doesn't operate.
Many electric gates run the electronics and motors off a 12v battery, so it will work even in a power outage, and only use the 120v from the building to trickle charge the battery.
It's been that way since before solar, so one of the first uses of solar was to add a small (they used to be "real expensive*") solar panel to an electric gate.
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