Online vs Offline UPS

There are two different types of UPSes: online mode and offline mode.

Offline mode is the simplest and cheapest: it has a relay switch that supplies your appliances directly from Eskom utility power while it's available, then switches over to battery+inverter power when utility power fails.

The process of switching over takes a bit of time, only a fraction of a second, but in this time your stuff will have a brief power cut. Usually it's not a big deal but it could cause your computer to reset, your microwave timer to reset etc.

A fast changeover could also cause a surge: appliances with motors in them typically store a bit of power as a magnetic field, and release that power later. 

AC power is delivered in waves, where power cycles back and forth, but normally a cheap inverter and the utility won't be synchronised. If your washing machine is spinning, the current it generates once power is cut could be added on top of the power the inverter generates as it kicks in and cause a surge. Newer appliances are protected against this sort of thing though. I'd only worry if I had an ancient old fridge or washing machine.

A safer, more expensive option is an online UPS where the appliances are always run off the inverter, and the inverter takes its power from the utility when available, or from battery when its not. This means power is never interrupted, and everything stays nice and smooth.

Offline UPSes are cheaper because they only need a small charger circuit. They usually take quite a long time to recharge after a power failure.

Higher end inverter units will usually let you run in either mode. The tradeoff with running in online mode is that it uses more standby power because all the power you use needs to be converted from AC to DC back to AC again. My cheapo Axpert aka MPPSolar unit uses about 50W in online mode, which is about R40 a month's worth. Decent inverters might only draw 10 to 20W.

I tend to leave my inverter completely disconnected because I know when the load shedding is coming anyway. It's important to keep the batteries topped up regularly though.