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In many parts of the world (where power cuts are relatively frequent), we use "Uninterrupted Power Supply" (or UPS), which is an inverter connected to one fraction of the household circuit (loads like lights, computer etc..) and a battery.
I wonder what stops this current from going to the rest of the household circuit (heavy loads like boiler and refridgerator) or even the wider grid (ie power supply)?
A domestic inverter is intended to power a portion of the loads (mainly lighting), when the utility supply is down.
While other loads would be permanently connected to the utility supply, a contactor would connect specific loads to it, when it's up, but transfer them to the inverter, when it is down.
Thus the inverter, which comes on when the utility supply is down, can only cater to specific loads, while remaining isolated from the others and the grid.
Figure 1. Source: etchnog.com.
A UPS converts mains AC to DC to charge the battery. The battery, when required powers the inverter and feeds the load.
I wonder what stops this current from going to the rest of the household circuit ...
The rectifier diodes are electrical one-way valves. They block current from flowing in the reverse direction. The LED -> Diode -> non-return valve of my article What is an LED may be of interest.
I wonder what stops this current from going to the rest of the household circuit
This is assured by using a specific wiring spur from your UPS AC output to the circuit in which your critical AC appliances (those that are important to keep powered) are connected.
A connection to the other household circuits and the grid is prevented by a switch. It is likely an electromechanical "changeover" or "double-throw" relay. The method and its operating circuitry very likely is required to conform to standards set by the agency having jurisdiction over connections to "the grid."
