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
Both of my USBc power banks have passthrough charging and would work for this. That’s the feature you need to look for.
However, keeping a battery bank charged to 100% isn’t good for longevity, make sure you get a good name brand one so it doesn’t catch fire.