Introduction

Line‑interactive UPS sits in the middle ground: smarter than standby, leaner than online. It’s popular because it fixes everyday voltage swings without the price tag of double‑conversion. But it isn’t universal. This page explains how it works, its limits, and how to decide confidently.

What “line‑interactive (VI)” means

The UPS lets the mains feed the load most of the time. An automatic voltage regulator trims low or high voltage by switching taps or using buck/boost circuitry. When the input goes outside correction limits or fails outright, the UPS transfers to battery and the inverter takes over.

AVR benefits and boundaries

AVR smooths frequent brownouts and mild over‑voltage without burning battery life. That preserves runtime for true outages. However, since mains remains the primary source while present, line noise and harmonics are not cleaned the way online UPS can. During a cutover to battery, there is a short transfer that sensitive, high‑performance power supplies may notice.

Sine wave vs simulated sine

Some line‑interactive models output a simulated sine wave on battery. Active‑PFC computer power supplies can behave noisily—or even misbehave—on simulated waveforms. Prefer models that specify a pure sine output on battery if you’re protecting PCs with active PFC, NAS devices, or audio gear.

When line‑interactive is a good choice

General office PCs, routers, switches, small PoE loads, POS terminals, attendance systems, and CCTV DVR/NVRs usually run happily on line‑interactive UPS, especially in areas with regular but moderate voltage swings.

When you should step up to online

If your equipment is intolerant to brief supply gaps, you maintain databases or transaction systems, you run lab/medical instruments, or your site has deep sags, frequent brownouts, or unstable frequency, choose online UPS instead.

Runtime and sizing

Convert your load to watts and leave 20–30% headroom. Don’t chase unrealistic runtime from small units; aim for enough time to ride through common brownouts plus a graceful shutdown. As batteries age or ambient temperatures rise, available runtime drops, so sanity‑check with a conservative margin.

Practical selection checklist

Prioritize a pure sine output on battery, clearly published watt capacity, replaceable batteries, audible/visual alarms, USB/SNMP for shutdown, and Indian socket compatibility. Ensure good after‑sales support and easy access to spares.

FAQs

Does a line‑interactive UPS need a stabilizer? AVR usually replaces a stabilizer for IT loads. Avoid stacking devices unless your site has extreme fluctuations.

How long is the transfer to battery? It’s brief. Exact figures vary by model. If the application cannot tolerate any interruption, you need online UPS.

Will it protect against spikes and surges? It includes basic surge suppression and AVR. For sensitive or mission‑critical systems, online UPS offers stronger isolation.

Can I extend runtime by adding bigger batteries? Only if the model is designed for external battery packs and charging capacity. Oversized batteries without proper charging shorten life and can be unsafe.