Designed for Rpi2 & 3, Best UPS I have found is definitively UPS PIco HV3.0B HAT Stack with Battery 450 available under below link.
It accepts most technologies of batteries and capacities, but with supplied version is good enough to manage supply of RPI3 and HDD running domoticz at least for 1h and if not sufficient will proceed with clean shutdown and automatic restart on main supply is back.
Nice industrial design, including user configurable Leds and button, relay and temperature monitoring with FAN control.
I bought one 2 month ago, and I'm very happy with it.
I have not any incentive nor interest in this product, just to highlight was quickly delivered to me, technical manual is well documented ( not chinese like) and firmware is professionally maintained with a dedicated forum also available.