Offline UPS - also referred to as VFD (Voltage and Frequency Dependent) or Standby UPS - offer the most basic level of power protection.
When mains supply is present, the UPS output is supplied via a built-in EMI/RFI filter which provides the load with protection from spikes and transients by clamping peak voltage to pre-defined levels.
When the mains supply fails or fluctuates outside of the UPS's operating window, a relay connects the load to the inverter output (resulting in a 4-8ms transfer time). In normal operation, with mains supply present, both output voltage and frequency will track the input voltage and frequency respectively.
As the inverter is switched off when the UPS is operating normally, the term 'Offline' is given to any UPS of this design. The inverter output on Offline UPS is typically a square-wave.
Offline UPS are the most basic models and designed for use in small, non-critical applications that require protection against momentary loss of power. They are used to protect workstations, terminals, or equipment below 1 kVA.
Typical internal battery autonomy with Offline UPS lasts for just a few minutes and there doesn't tend to be the option to add external battery packs for additional autonomy.
It is not advised to use a VFD UPS to protect critical loads or sensitive electrical equipment.
What Power Problems Do The Different UPS Topologies Protect Against?
The table below offers an easy comparison of the protection provided by Offline UPS, Line Interactive UPS, and double-conversion Online UPS:
| Power Problem | Online (VFI) | Line Interactive (VI) | Offline (VFD) |
| Sags/Brownouts | Yes | Yes | - |
| Surges | Yes | Yes | - |
| Spikes/Transients | Yes | Yes | Yes |
| Electrical Noise | Yes | Yes | Yes |
| Harmonics | Yes | - | - |
| Frequency Variation | Yes | - | - |
| Mains Failure | Yes | Yes | Yes |