I use this
Power Supply Unit with Battery Backup • 12VDC 3A UPS • with Battery Compartment for 7Ah Battery (Battery NOT Included) Overview Features Input voltage can vary from 90VAC to 265VAC without affecting the output voltage. (output voltage remains constant) Input and output fused Short circuit...
www.communica.co.za
With a normal 7ah battery. For a bit more you can get a 9ah battery to last slightly longer. Auto switches over between power and battery with no downtime so everything stays on ALWAYS.
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www.communica.co.za
You can buy different brands should not matter. Very good solution for extended outages. Speaking of outages Centurion is experiencing an outage since about 4am already due to substation fault so I'm on backup since then and battery Voltage is still 12.2V
I added a cheap DC to DC buck to keep and eye on the voltage just to make sure battery health so you can play around with the setup. The PSU I linked does not have any alarms and just has a light green = main power red = battery. It has extra connectors if you want to connect a buzzer which is nice.