Raspberry Pi UPS

The Raspberry Pi has very moderate power requirements, depending on the devices connected. The standard micro-USB power supply supplies 2.5A; the standard USB-C power supply (for the 4B) supplies 3.0A. These power supplies are sized to provide power to the USB ports as well. On a headless Pi (without screen, keyboard, or mouse), considerably less power is consumed.

This means that a Pi can be powered from a USB power bank. Using a 1A port might be pushing it, but using a 2A port should work just fine. Check the Pi for under voltage to be sure, see Hardware Monitoring.

If you use a power bank that supports pass-through charging (charges the connected device while being charged itself), it can serve as a UPS (uninterruptible power supply) for your Pi. See Raspberry Pi UPS for more info. Make sure the power bank continues to supply power ("charge the connected device") when connecting or disconnecting the input.

I've been using a 16000mAh TeckNet PowerZen that I got from Amazon in 2017.

It powers my production Pi 3B+ for over 18 hours, with RaspBee Zigbee radio add-on board, and P1 serial USB cable. The Pi runs deCONZ in graphical mode (accessed over VNC), two Homebridge instances, and the OpenTherm Monitor.