Online UPS vs Line-Interactive UPS: kVA to kW Conversion and Battery Runtime
Online vs line-interactive UPS by IEC 62040-3 class (VFI, VI, VFD), kVA to kW conversion, load headroom and battery runtime estimation from Ah rating.
The difference between an online UPS and a line-interactive UPS is whether the load runs from the mains or from a continuously operating inverter. IEC 62040-3 defines three classes: `VFI` (online double conversion), `VI` (line-interactive) and `VFD` (offline). A correct choice weighs the class, the `kVA`/`kW` rating and the battery runtime together.
> Related Category: Browse models by rating and topology in UPS Uninterruptible Power Supplies, Online UPS and Line-Interactive UPS.
1. IEC 62040-3 classes: VFI, VI and VFD
| Class | Common name | Load supply in normal mode | Transfer time to battery | Typical use |
|---|---|---|---|---|
| VFI (voltage and frequency independent) | Online, double conversion | Rectifier + inverter, always | 0 ms | Servers, medical devices, PLC and process panels |
| VI (voltage independent) | Line-interactive | Mains, corrected by an AVR stage | Typically 2–6 ms | PCs, point-of-sale, network gear, video recorders |
| VFD (voltage and frequency dependent) | Offline, standby | Mains directly | Typically 5–10 ms | Single PC, modem |
An online UPS always feeds the load from its inverter, so sags, frequency drift and harmonics on the mains never reach the load. A line-interactive unit switches to battery only when the mains leaves its window; most switch-mode power supplies ride through that gap, but contactor coils and sensitive instruments may drop out. Where a generator is part of the system, the online topology is preferred because generator frequency wanders.
2. kVA to kW conversion and UPS sizing
The `kVA` figure is apparent power; `kW` is the active power the load can really draw:
kW = kVA × output power factor (cos φ)
| Nameplate | Output power factor | Usable active power |
|---|---|---|
| 1 kVA line-interactive | 0.6 | 600 W |
| 1 kVA online | 0.9 | 900 W |
| 3 kVA online | 0.9–1.0 | 2,700–3,000 W |
| 10 kVA online | 0.9–1.0 | 9–10 kW |
Sizing steps:
- Add the nameplate watts of every connected device (multiply `VA` values by their power factor).
- Check the total against both the `kW` and the `kVA` rating of the UPS.
- Keep 20–30% headroom; a UPS running at 70–80% load avoids overload alarms and gains runtime.
- Keep laser printers, compressors and motors off the UPS, or size separately for their inrush current.
Example: 4 PCs (4 × 250 W) plus a switch and modem (100 W) = 1,100 W. Target `1,100 / 0.8 ≈ 1,375 W`; a 2 kVA unit with 0.9 power factor (1,800 W) fits. For three-phase input ratings, see Single-Phase vs Three-Phase Power and AVR kVA Sizing.
3. Battery runtime (Ah) estimate
t (hours) ≈ (number of batteries × battery voltage × Ah × η × k) / load (W)
- `η`: inverter efficiency (typically 0.85–0.95)
- `k`: usable share of capacity at high discharge rates. Lead-acid (VRLA) Ah ratings refer to a 20-hour discharge; over 10–30 minutes only about half is available (`k ≈ 0.5–0.6`).
Example: a 1 kVA online UPS with three 12 V 9 Ah batteries (36 V) and a 600 W load:
`36 × 9 = 324 Wh` → `324 × 0.9 × 0.5 ≈ 146 Wh` → `146 / 600 ≈ 0.24 h ≈ 15 minutes`.
At 300 W the same UPS runs more than twice as long because `k` rises at lower current. For firm figures use the battery maker's constant-power discharge table. For hours of autonomy choose long-runtime models with external battery cabinets and matching UPS accessories, and confirm the charger can refill the larger bank in a reasonable time.
4. Installation notes
- If an RCD is fitted on the UPS output, neutral continuity and the UPS earthing arrangement must follow the manufacturer diagram.
- VRLA service life is typically 3–5 years at 20–25 °C; every 10 °C rise roughly halves it.
- The bypass line of an online UPS transfers the load to mains on fault or overload; fuse the bypass input for the UPS rating.
- Surge protection does not replace a UPS; see Surge Protector Socket vs SPD and UPS.
Frequently asked questions
Should I choose an online or a line-interactive UPS?
Choose online (VFI) with zero transfer time for servers, medical devices, automation panels and sites with generators. A line-interactive (VI) unit with AVR is usually enough for home and office PCs, point-of-sale and network gear.
How many watts is a 1 kVA UPS?
It depends on the output power factor: 600 W at 0.6 and 900 W at 0.9. Read both the kVA and the W figure on the nameplate.
How long does a 12 V 7 Ah battery run a UPS?
In a single-battery 600 VA unit expect about 10 minutes at 200 W: 12 × 7 = 84 Wh, of which roughly 38 Wh is usable after efficiency and fast-discharge losses. Runtime grows more than proportionally as load falls.
Can a laser printer or air conditioner run on a UPS?
It is not recommended. Fuser heaters and compressors draw several times their rated power at start, pushing the UPS into overload or bypass.
When should UPS batteries be replaced?
When runtime falls to half of its original value or the unit reports a battery test warning. VRLA batteries typically serve 3–5 years at room temperature.