In short. An online UPS continuously converts input AC to DC and back to AC, so the load always runs from the inverter with no transfer time on mains failure. It isolates the load from the supply and suits critical equipment, at the cost of efficiency and heat.
Power path
Input → rectifier/charger → DC bus with battery → inverter → output. Static bypass is the alternative path. On mains failure the battery feeds the DC bus; the load sees no break.
Characteristics
| Feature | Effect |
|---|---|
| Zero transfer time | No break on mains failure |
| Output regulation | Voltage and frequency regulated |
| Efficiency | Lower than line-interactive; eco modes can raise it |
| Heat | Losses become room heat; input = output ÷ efficiency |
| Input harmonics | Rectifier type decides |
Input window
An online UPS tolerates input voltage variation within a window without using the battery. A narrow window on a weak supply drains the battery. A stabiliser upstream can help. See stabiliser vs UPS.
Generator compatibility
- Rectifier walk-in avoids a sudden load step on the generator.
- Input harmonics can distort generator voltage. See UPS and nonlinear loads.
- Input frequency window must cover generator drift.
Modes of operation
- Normal double conversion.
- Battery mode on mains failure.
- Bypass on overload or fault.
- Eco mode where used, with clear understanding of the transfer behaviour.
Illustrative example
A trading office uses an online UPS for servers. A mains drop to zero causes no interruption; the generator starts and takes over, the rectifier walks in, and the battery recharges.
Efficiency and heat in detail
Input power = Output power ÷ Efficiency
A 20 kW load on a UPS of 94 percent efficiency draws about 21.3 kW; the difference of about 1.3 kW is heat in the room. At 90 percent efficiency, losses rise to about 2.2 kW. Over a year of continuous running, the energy lost is considerable, which is why efficiency at your typical load, not only at full load, should be checked in the datasheet.
Eco mode
In eco mode, the load runs on bypass while the supply is good, and the inverter takes over when it is not. Efficiency improves, but there is a short transfer, and the load sees the quality of the bypass supply meanwhile. Consider this carefully for critical loads and test the transfer time.
Input power factor and harmonics
| Rectifier type | Input current distortion | Generator impact |
|---|---|---|
| Six-pulse | High, needs filter | More voltage distortion |
| Twelve-pulse | Lower | Better |
| Active front end (IGBT) | Low, near unity power factor | Best |
Battery integration
- Match DC bus voltage and string arrangement.
- Provide DC protection and isolation.
- Check charging current limits and temperature compensation.
See batteries.
Failure modes
| Failure | Result |
|---|---|
| Inverter fault | Load transfers to static bypass |
| Rectifier fault | Battery supports until depleted |
| Battery fault | No ride-through |
| Fan or cooling fault | Thermal shutdown |
Design the system so each failure leads to a known, acceptable state, and alarm it. Test those states.
The master engineering question
Is the load critical enough to justify double conversion losses, or would a simpler topology suffice?
What to send us
Send load list and room details. Send your application details.
Questions engineers ask
Is online UPS always best?
For critical loads, yes; for others, consider cost and heat.
What is eco mode?
The load runs on bypass when input is good; efficiency rises with some risk trade-off.
Why does it run hot?
Conversion losses.
Does it clean harmonics?
It isolates the load but its input may produce harmonics.
Values are illustrative. Use manufacturer data.