Online vs Line-Interactive UPS: Which Do You Need?
- Line-interactive: handles everyday sags and surges efficiently and costs less. Best for network closets, workstations and small servers.
- Online (double-conversion): rebuilds the power from scratch, so connected equipment never sees a transfer. The standard for servers, medical and industrial loads.
Choosing between an online (double-conversion) and a line-interactive UPS comes down to how clean your power needs to be, how sensitive your equipment is, and your budget.
How a line-interactive UPS works
A line-interactive UPS passes utility power through to your equipment while an automatic voltage regulator (AVR) boosts low voltage or trims high voltage. When power fails or drifts too far, the UPS switches to its battery and inverter, usually in a few milliseconds.
How an online (double-conversion) UPS works
An online UPS converts incoming AC power to DC, then back to clean AC, all the time. The battery is always connected to the DC bus, so when utility power fails there's zero transfer time. Your equipment is isolated from sags, spikes, frequency drift and generator noise.
Best for
Line-interactive
- Network closets and workstations
- Small servers, POS systems and edge equipment
- Areas with reasonably stable power
Examples: APC Smart-UPS (SMT/SMX), Eaton 5P/5PX, Vertiv Liebert PSI5.
Online (double-conversion)
- Server rooms and data centers
- Medical and lab equipment, industrial controls
- Sites on generators or with poor utility power
Examples: APC Smart-UPS SRT, Symmetra LX and PX, Eaton 9PX/9SX, Vertiv Liebert GXT5.
Side-by-side comparison
| Feature | Line-interactive | Online (double-conversion) |
|---|---|---|
| Transfer time to battery | ~2–10 ms | 0 ms |
| Voltage regulation | AVR boost/trim | Full regeneration |
| Frequency / generator noise protection | Limited | Full |
| Efficiency | High (~95–98%) | Lower (~90–95%; higher in eco mode) |
| Heat output and fan noise | Lower | Higher |
| Up-front cost | Lower | Higher |
| Typical sizes | 500 VA – 3 kVA | 1 kVA – 500 kVA+ |
Heat and placement
Online UPS systems run their inverter constantly, so they give off more heat.
See our UPS battery replacement service for more.
Which should you choose?
Choose line-interactive if:
- Your load is under about 3 kVA (networking, desktops, small servers)
- Your utility power is mostly stable
- Efficiency, quiet operation and cost matter most
Choose online double-conversion if:
- You're protecting servers, storage or anything that can't tolerate even a brief transfer
- You run on a generator or have frequent brownouts
- You need 5 kVA or more, or redundancy (N+1) and hot-swap modules
- The equipment is medical, lab or industrial
Not sure about capacity? Start with our UPS sizing guide.
Save with new or refurbished
GreenlightUPS carries both topologies from APC, Eaton, Vertiv and more, in New or Refurbished condition. You can save up to 70–80% off MSRP. Every unit includes a 1-year GreenlightUPS warranty, and in-stock orders ship within 2–3 business days. Comparing conditions? Read about our refurbishment process and warranty.
GreenlightUPS was founded in 1981. We're headquartered at 800 NE 42nd Street, Deerfield Beach, FL 33064.