For 12volts most fuses are 7.5, 10, 20 and 30 amps. If you use something that needs a crap load of amps or wattage then 40 or 50 amp fuses are needed. For examples my carputer power supply uses a 10amp fuse and I also modded my gaming PC's power supply to use 10 amps for 12 volts and 7.5 amps for 5 volts for the DC outputs. My car amp uses a 40amp fuse for the main power but the amp has a 15 amp fuse. So if theres a power surge the 15 will blow first and if it effects my main power the 40amp will blow.

Your UPS is nothing more then a power inverter with a battery. It turns 12 volts DC into 120 volts AC. When it's plugged into the wall it's going into two way. Straight power to the device and also into a battery charger keeping the battery charged.

The UPS should have a voltage regulator so you shouldn't really even need a fuse but I would add one just for as a safety. Last thing you need is to test it or wait for a power outage and too many amps end up into the PC or even into the battery.
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