Hi and welcome to Babypips. I am by no means an expert, but I have lived in the Philippines where brownouts were common in summer (decades ago) but more of a challenge was Syria, where outages could be up to 6 hours per day (20 years ago). Now you are lucky if you get 4 hours ON per day in the city, and an hour a day in the sticks.
Before you believe my answer at face value please always check with an Italian supplier or engineer to validate my calculations and statements.
First advice. Buy a UPS made in Italy, and preferably from a local dealer who can offer break/fix support. Alternatively a global brand like APC. Locals know best. The line voltage to domestic supplies in Italy is 220-230V EU standard and outlets should be two pin round + external earth - a Schuko socket. If the property is old, that may not be the case. Use a Schulo adapter in that case. Depending on where you are living you may also wish to buy a surge protected UPS or an extension lead with in-built surge protection if lightning is a common occurrence or building ground rod is not good.
Second, find out how much power your UPS consumes. You could do this yourself by plugging an inline watt meter and measuring the power consumption over a few days.
Third, figure out the frequency of power outages (or power dips that cause your electronic equipment to reset) and how long the outages last. A good retailer of electronic goods should be able to tell you.
Fourth, work out how many hours you really need to stay online before losing connection. You may also try to find out how long the local telephone exchange (or street side box) has power autonomy designed for. This used to be 12 to 24 hours for a local exchange but has fast reduced in recent times. It may only be 2 to 4 hours anyway which means even if your router is protected, the local exchange or streetside cabinet may not be.
Fifth, figure out what duration of autonomy you want to design for, and remember in the worst case you can always either trust your stop loss or close all trades out.
Your broadband router may typically consume 70W. If you want the router to be powered for four hours, that is 70W x 4 hours = 280WattHours or 0.28kWh. UPS units are rated in kVA (kilovoltamperes) not kW. A safe conversion factor for kW to kVA is to divide the kWh consumed by 0.7 (or multiply the kW load by 1/0.7 = 1.4. So if you are consuming 240Wh in 4 hours that is 240 x 1.4 kVA = 336kVA. A 500kVA UPS should be more than adequate to power your router for 4 hours autonomy. Your laptop batteries should last from 4 to 8 hours depending on model. A desktop machine would need powering from the UPS, so best to use a laptop.
Finally you can subscribe to always on services that are accessed via a browser. Check out the cost of such services, it may be cheaper than gearing up to cope with power cuts for long periods of autonomy.