Offline UPSs have high efficiency, small dimensions and are denoted by the abbreviation VFD (output voltage and frequency depend on their input values). In normal operating conditions, the source is filtered voltage from the mains and the rectifier charges the battery. In the event of a power outage or a drop in voltage below a specified limit (usually at 230 V +10% / -15%), the source becomes the accumulator, which powers the appliance via the inverter. The switching time of the switch is usually around 4 to 8 milliseconds. After the power is restored, the battery will start charging. The offline system is used for appliances that don’t mind voltage interruptions in units of milliseconds, such as backup lighting or personal computers.