Eaton 3S Gen2 UPS 3S550B review

The 3S550B is a 550 VA / 330 W UPS with six UK outlets. Eaton lists it as offline/standby. The unit is small at 325 × 140 × 86 mm and weighs 3.2 kg.

The 330 W rating comes from an output power factor of 0.6. That is enough for a small NAS, a router and a switch together. It isn't a fit for a multi-server rack or a power-hungry workstation. Check the combined draw of your kit against the 330 W figure before buying.

This is the weakest area on paper. Eaton lists the topology as offline/standby, and the manufacturer doesn't confirm the waveform for the Gen2 3S550B. The research can't say whether the output is a pure sine wave or something else.

That is an unknown you would be buying into. If a stated waveform matters to you, look for a unit where the manufacturer clearly states it. For a NAS, router and switch, the missing detail is less of a worry.

Input and output are both 230 V. The unit is rated for use up to 2000 m altitude, in humidity from 0 to 85% non-condensing, and its noise level is listed as under 25 dB at 1 m.

The USB port is HID-compliant, and Eaton supplies UPS Companion software with an automatic shutdown function. A USB cable comes in the box. That gives you the standard route for a connected NAS or server to shut down cleanly when the battery runs low.

Owners report that a Synology NAS recognised the unit for shutdown. One caveat: the exact model variant in that report is not certain, so treat it as encouraging rather than confirmed for this Gen2 model. The research doesn't mention QNAP, TrueNAS or NUT.

There is no Ethernet port or network card. If you want several machines to react to a power cut, you will need another way to share the UPS status around your network. The research doesn't cover how to do that with this model.

The battery is a single 12 V / 5 Ah sealed VRLA unit, and it is user replaceable. For a small UPS that will outlast its first battery, that is a practical point. You can swap the cell yourself rather than replace the whole unit.

Runtime is the gap. Eaton doesn't publish a half-load figure, and the runtime graph could not be read. So there is no basis here for quoting minutes. With a 5 Ah battery and a 330 W rating, this looks like a unit meant to give a NAS time to shut down, not to ride out long outages. That is a judgement from the battery size and the shutdown feature, not a measured result.

There are six UK outlets. Three have battery backup and surge protection, and three are surge-only. That layout works well for a NAS, router and switch on the battery side. Put less critical items, such as a monitor or a charger, on the surge-only sockets.

Owners praise easy installation and the small footprint. The package contains the USB cable, a quick-start guide and a safety leaflet, with no bundled accessories. The research doesn't give a warranty term, so check it before buying.

The 3S550B covers the basics for a compact setup: HID USB, a replaceable battery, UK sockets and a small body. Its limits are an offline/standby design, an unconfirmed waveform, no published runtime and no network connectivity. It fits a single NAS-centred setup best. If you need a confirmed waveform or network-level monitoring, look at other models.