A standard UPS should work fine. You can choose the power (obviously the higher power ones are also more expensive) based on how long you think the power outage may be. Each solenoid valve, when running, draws about 6 Watt, and the OSPi+RPi at maximum draws about 5 to 10 Watt.
One for the router, NAS and modem (and eventually for the OSPi) and another for the tv and retro consoles (don’t want to lose my progress in games that don’t auto save). Works great. There’s enough juice in there to run a raspi for a really long time.
I damaged the power supply IC on OSPi by miswiring so removed it. I inject 12V into the unused ethernet cable wires at my router from my UPS and drop it to 5V at the OSPi and put it into the standard Pi power connector. Power the 24VAC input of the OSPi in the normal way for valve outputs. During a power outage the Pi is powered but I can’t run the zones. I use a pump for water supply so I couldn’t water anyway. All the injectors and 12 to 5 V converter are standard stuff from Amazon.