Introduction

If you’re choosing between online, line‑interactive, and offline UPS, the right answer depends on how sensitive your load is to power disturbances and how much downtime you can tolerate. This guide gives you a plain‑English explanation of each topology, how they behave during sags, surges, and outages, and how to make an informed, defensible purchase.

The short answer

Online (double‑conversion, VFI) continuously powers the load through an inverter, so transfer time is effectively zero and power quality is the most stable. Line‑interactive (VI) corrects moderate voltage swings with an AVR and switches to battery during deeper events; there is a brief transfer. Offline/standby (VFD) waits until the mains fails and then switches to battery; protection is basic.

What each topology actually does

Online (VFI). AC is rectified to DC and then inverted back to AC 24/7. Input anomalies—sags, swells, harmonics—are largely isolated from the output. Ideal for servers, storage, medical, testing, and any process where even momentary drops are risky.

Line‑interactive (VI). The load normally runs from the mains via an automatic voltage regulator (AVR) that boosts or bucks the voltage. During bigger sags or outages, it transfers to battery and an inverter picks up the load. Good for PCs, POS, network gear, CCTV, and non‑critical IT.

Offline/standby (VFD). The load is directly on mains until a failure, then a relay/inverter combo supplies battery power. Best for very basic home or single‑device use where brief interruptions are acceptable.

Transfer time and why it matters

Online UPS keeps the inverter in the power path, so transfer is practically zero. Line‑interactive transfers within a brief window when crossing to battery. Offline is typically longer and more noticeable. Sensitive equipment with active PFC PSUs, databases, or medical devices is at higher risk from even small gaps or repeated brownouts. If your business cost of an interruption is real—even a few seconds of lost transactions—online UPS earns its keep.

Power quality and protection level

Online gives the most consistent voltage and frequency and also reduces noise and harmonics at the output. Line‑interactive moderates common sags and swells through AVR but passes more of the input waveform characteristics. Offline passes almost everything while mains is present and steps in only during an outage.

Sizing quickly and correctly

Express your load in watts, then pick a UPS with clear kW capacity (not just kVA). Modern online UPS often state both. Maintain a headroom of 20–30% for startup surges, growth, and battery aging. For runtime, multiply battery voltage by Ah to estimate energy and use a runtime curve from the manufacturer; be conservative because real‑world loads rarely match lab curves.

Cost, efficiency, and TCO

Line‑interactive is cheaper and typically more efficient at light loads. Online costs more upfront and draws slightly more power, but protects against a wider set of power problems and reduces hidden downtime costs. For offices and labs where any outage forces reboots or data checks, online’s TCO often wins across three years.

Compliance: India and international

If your UPS is ≤10 kVA single‑phase and sold in India, check BIS IS 16242 compliance. Also look for IEC 62040 topology labels—VFI (online), VI (line‑interactive), and VFD (offline)—used worldwide. This shared vocabulary helps compare apples to apples.

When to choose what

Use online for servers, storage, lab instruments, medical gear, calibration benches, and any 24×7 system. Use line‑interactive for PCs, POS, CCTV, network switches/routers, printers, and general office loads. Use offline only for the simplest home devices where brief interruptions are acceptable.

FAQs

Is “eco‑mode” on online UPS safe to use? It bypasses double‑conversion to gain efficiency, re‑introducing a transfer during events. Use only when the load can tolerate brief transfers.

Do I need a stabilizer with a line‑interactive UPS? Usually no; AVR already boosts/bucks. In extreme fluctuation areas, a separate stabilizer may still be helpful upstream.

How do I read kVA vs kW on spec sheets? kVA × power factor ≈ kW. Prefer UPS that publish continuous kW; that’s the real usable capacity.

How much runtime do I need? Size for safe shutdown plus buffer for brownouts. Many offices target 10–15 minutes; labs/servers often want 20–30 minutes or more.

Will a UPS fix wiring/earthing problems? No. UPS is not a substitute for proper earthing and protection.