Basically as the title says. We have semi frequent power outages where I live. The noise machine in my daughter’s room goes out and wakes her up. If I were to buy a USB powered one, plug it into a power bank like one of those 10000ma ones you get for charging cell phones, would it have continuous power. Basically like a cheap UPS
I got my son a noise maker that has a built in battery for this same reason. It doesn’t seamlessly switch to battery I’m sure there is one that does.
We use the Hatch Rest. Same idea, uses battery if unplugged, no disruption in sound. Only lasts ~8 hours on battery alone tho.
This one does: https://www.techradar.com/reviews/lectrofan-micro2