Short answer: Add up the watts of everything you want to run, add about 25% headroom, and divide by the power factor (often 0.6 to 0.8) to get the VA rating. Backup time depends on the battery, not the UPS.
Buying a UPS or home inverter is easier once you separate two questions: how much it can power at once, and how long it can keep going.
VA and watts
UPS and inverter boxes are rated in VA (volt-amperes), but your devices use watts. The two are linked by the power factor, usually printed in the specifications:
Watts = VA × power factor
Many small UPS units have a power factor around 0.6, so a 600 VA unit delivers only about 360 W. Always compare the watt figure, not just the VA.
Working out what you need
Say you want to run a Wi-Fi router (10 W), two LED bulbs (9 W each), a ceiling fan (60 W) and a laptop (65 W). That is 153 W.
- Add about 25% headroom: about 191 W.
- At a power factor of 0.6, that needs roughly 320 VA. A 500 or 600 VA unit gives comfortable room.
Warning: Motors and heaters draw far more than their label when they start. Fridges, pumps, irons, heaters and kettles can overload a small UPS. Check the inverter's rating for these before connecting them.
How long will it last?
Backup time depends on the battery's stored energy:
Battery energy (Wh) = battery voltage × amp-hours
A common 12 V, 150 Ah inverter battery stores about 1,800 Wh. With about 80% inverter efficiency, the 153 W load above would run for roughly 9 hours in theory. In practice expect less, perhaps 6 hours: lead-acid batteries should not be fully drained, and capacity falls as they age.
A small computer UPS with a tiny internal battery is different. It is designed to give you 5 to 15 minutes to save your work and shut down, not to run through a long power cut.
Questions people ask
Q: UPS or inverter: which do I need?
A: A UPS switches over almost instantly and protects computers from short cuts, but usually has a small battery. A home inverter with a large battery runs lights and fans for hours, and many now switch fast enough for computers too.
Q: How long do inverter batteries last?
A: Lead-acid batteries typically last a few years and need their water level topped up if they are the flooded type. Lithium batteries cost more but last longer and need no maintenance.