Which UPS you need in VA and how much battery for X minutes.
With 300 W and 25% headroom you need 375 W; with the usual 0.6 power factor that is 625 VA, so a 650 VA UPS. For 10 minutes of runtime you need about 59 Wh of usable battery.
Add up the real draw of everything plugged into the UPS. Use a power factor of 0.6 if the maker does not list watts. Real runtime drops with load and battery age.
VA = W × (1 + headroom/100) / power factor → next common size; Wh = W × minutes / 60 / 0.85Add up the real draw of everything plugged into the UPS. Use a power factor of 0.6 if the maker does not list watts. Real runtime drops with load and battery age.
All amounts use the same currency of your choice. These are estimates, not measurements or offers.
Choose a UPS that keeps your PC, NAS or router running through a power cut and gives you time to shut down calmly. Measure or estimate the real draw of everything you will plug in: the monitor and router count too. UPS units are sold in VA; the watts they handle depend on their power factor, usually between 0.6 and 0.9.
Check that the UPS meets both figures on its spec sheet: VA and watts. For runtime, look at the maker runtime chart at your load, because lead-acid batteries deliver less the faster they discharge. For a NAS, pick a model with a USB port so the NAS shuts down before the battery runs out; for PC power supplies with active PFC, a pure sine wave UPS is the safer choice.
VA is apparent power and W is real power. A 650 VA UPS with a 0.6 power factor delivers at most 390 W. Always check the watts on the spec sheet.
Lead-acid batteries usually last 3 to 5 years. If runtime drops a lot, replace it: most UPS units let you swap the battery.
It is not recommended: when the fuser heats up it draws very high peaks that can overload the UPS.