Line-Interactive vs Online UPS: Which Backup Is Right?

Both designs keep equipment running through an outage. The difference is what happens the rest of the time: a line-interactive UPS passes utility power to the load and corrects its voltage, while a double-conversion UPS rebuilds every cycle of the output from its own inverter. This guide explains that difference with the manufacturer's definitions, then compares two units sold here, the APC Smart-UPS SMT1500C and the Smart-UPS On-Line SRT1500XLA, on transfer time, power and runtime.

Written by the UPSPLUSBATTERY technical team. Sources checked 22 September 2026.

Scope and terms

This article covers single-phase UPS units from about 500 VA to 3 kVA at 120 V, the range where the two topologies overlap and buyers actually have to choose. Above 10 kVA the choice is largely made for you: Schneider Electric notes that double conversion is the most popular type in that range. Standby (offline) units and three-phase systems are out of scope.

• Line-interactive: the load runs on utility power, conditioned by an automatic voltage regulator (AVR). On a power failure a transfer switch opens and the always-connected inverter carries the load from the battery.
• Online double conversion: the load always runs from the inverter, fed by a rectifier. On a power failure the rectifier stops and the battery feeds the same inverter, so nothing switches.
• Transfer time: the gap between the utility failing and the battery-fed inverter carrying the load.

How each topology moves power

Figure 1 shows the two power paths in normal operation. The line-interactive path is short and efficient but relies on a switch at the moment of failure. The double-conversion path passes every watt through two conversion stages, which is why it isolates the load from the utility and why it costs more to build and to run.

What the IEC 62040-3 classes tell you

The international standard IEC 62040-3 classifies a UPS by how its output depends on its input. Schneider Electric's White Paper 1 applies it to both designs: a line-interactive UPS is VI (voltage independent) because its AVR regulates voltage but cannot regulate frequency, and a double-conversion UPS in on-line mode is VFI (voltage and frequency independent) because the inverter sets both. That single letter explains most of the practical differences in Table 1.

Transfer time: when 2 to 10 milliseconds matters

For most IT equipment a transfer of a few milliseconds passes unnoticed; APC notes that a zero-transfer requirement is very uncommon with IT equipment. That is why line-interactive is the default for closets and small server rooms. It becomes a problem in three situations:

• Equipment that drops or reboots on transfer. APC suggests trying high sensitivity, which gives the 2 to 4 ms transfer, to see whether the load then withstands it; where zero transfer time is required, it points to a double-conversion online model.
• Frequent transfers. White Paper 1, explaining why line-interactive units carry an AVR, notes that more frequent battery use can cause premature battery failure. Sags deeper than the AVR can correct send a line-interactive unit to battery each time; a double-conversion unit's wider input window rides out most of them without touching the battery.
• Generator power. Frequency is the weak point of a VI design, which passes input frequency through. For its Back-UPS range, APC states that frequency deviations beyond about ±5% put the unit on battery and that a generator should be three to five times the total load to hold frequency steady; the documents cited here give no equivalent figure for the SMT, so check its input frequency tolerance before relying on it with a generator. A VFI unit regulates output frequency itself, although the generator still has to be sized for the load.

Whatever the topology, APC recommends never loading a Smart-UPS above 80% of its rating, which leaves margin for inrush and growth.

Two real 1500 VA units compared

The APC Smart-UPS SMT1500C and Smart-UPS On-Line SRT1500XLA both carry the Smart-UPS name and sit in the same size class, one of each topology, so they isolate the cost of double conversion. Table 2 uses the specifications on each product page in our store and APC's published transfer times; the current price is on each product page.

How the numbers work. Double conversion costs more per watt of capacity; compare the current prices on the two product pages below. The planned load is 80% of the watt rating, following APC's loading advice. The runtime figures are each unit's own half-load and full-load values, so they are not measured at the same wattage: the SRT's half load is 675 W against the SMT's 500 W. Where runtime is the constraint rather than power quality, the SRT's external battery packs are the lever the SMT does not have. For your own load, the UPS runtime calculator gives an estimate at your real draw.

Both units are sold here: the APC Smart-UPS SMT1500C and the APC Smart-UPS On-Line SRT1500XLA. For a rack, the line-interactive equivalent is the SMT1500RM2UC 2U rack model. The full ranges are in our APC line-interactive UPS and APC double conversion online UPS collections, with every SRT model grouped under APC Smart-UPS SRT.

Canadian conditions that tip the choice

In Canada, utilities maintain supply voltage at the service entrance within the guidelines of CSA CAN3-C235-83. For a 120/240 V service its Table 3 sets a normal range of 110 to 125 V and extreme limits of 106 to 127 V. Drift inside that band normally passes straight through a line-interactive UPS; its AVR corrects sags and swells beyond it, and deeper ones push it onto battery.

In our experience, three Canadian situations favour double conversion: sites that run on a generator during outages or in remote locations, because of the frequency point above; industrial sites with motors, welders or variable-speed drives on the same feeder, where distorted power is common and White Paper 1 notes that within the VI category only on-line UPS address waveform distortion; and critical loads in clinics, labs and small data rooms where the equipment maker specifies zero transfer. Offices and retail on urban feeders, with modern IT loads, are line-interactive territory.

Decision checklist

1. Does anything on the load drop or reboot during a transfer, or is it specified for zero transfer? If yes, double conversion, with eco or bypass mode left off. For lab or clinical equipment, follow the equipment maker's UPS requirement; a UPS does not change a device's medical rating.
2. Will the UPS run from a generator, or does the site see frequent sags and transfers? If yes, double conversion.
3. Is runtime beyond the internal battery needed? If yes, choose a model that accepts external packs, such as the SRT.
4. Otherwise, is it IT equipment on a stable urban supply? Line-interactive, with high sensitivity set if transfers are ever noticed.
5. Size either unit so the planned load stays at or below 80% of the watt rating.

Batteries and upkeep

Both topologies use sealed lead-acid batteries that wear out and need replacing. We sell a compatible replacement battery pack for the SMT1500C and a compatible replacement battery pack for the SRT1500XLA; neither is an APC-made part. For fleets or critical sites, our service partner GDF Technologies runs UPS preventive maintenance programs across Canada and explains the trade-offs of online double-conversion UPS for larger installations.

Sources

1. Schneider Electric, White Paper 1, The Different Types of UPS Systems, version 10, 2024 (power paths, IEC 62040-3 classes, typical power ranges, frequent battery use).
2. Schneider Electric, FAQ000268416, What is the transfer time of APC Smart-UPS and Symmetra units?
3. Schneider Electric, FA156513, Why is my equipment dropped or rebooted while connected to my Smart-UPS? (80% loading, sensitivity, zero transfer very uncommon with IT equipment).
4. Schneider Electric, FA158827, Why might my APC Back-UPS be beeping? (generator frequency note).
5. Schneider Electric, SMT1500C product page.
6. Schneider Electric, SRT1500XLA product page (zero transfer time, up to 10 SRT48BP packs).
7. CSA Group, CAN3-C235-83, Preferred voltage levels for AC systems, 0 to 50 000 V, Table 3, as reproduced in Newmarket-Tay Power Distribution, Conditions of Service COS-230-05.
8. UPSPLUSBATTERY product specifications for the SMT1500C, SRT1500XLA and their battery packs, read 22 September 2026.

APC, Smart-UPS, Back-UPS and SRT are trademarks of Schneider Electric. UPSPLUSBATTERY is a reseller and is not affiliated with, sponsored by or endorsed by Schneider Electric. Compatible batteries are not made or endorsed by Schneider Electric.