How to choose a UPS: VA and W, topologies, runtime and battery life

What a UPS is for

An uninterruptible power supply (UPS) keeps equipment running during a power cut and protects it from surges and mains fluctuations. It is not meant to replace the grid: its job is to give you minutes to save work and shut down cleanly, to let a network and a server ride out short cuts without a restart, and to prevent data damage on a NAS.

VA is not the same as W

A UPS shows two numbers: apparent power in VA and real power in W. Equipment loads the UPS with real power, so go by watts. The W/VA ratio (power factor) is around 0.6 on cheap models (a 1,000 VA UPS = 600 W), 0.9 and above on better ones. The procedure is simple:

1. Add up the real draw of the protected equipment (label or a power meter; a PC typically 100–300 W, a gaming PC with a strong card 500 W and more, a monitor 20–40 W, router and NAS 10–30 W each).
2. Add 20–30 % headroom – the battery ages and equipment accumulates.
3. Choose a UPS whose rating in W is above the result.

An overloaded UPS does not last a second in a power cut, which is the most common disappointment.

Three topologies

• Offline (standby) – the cheapest; switches to battery within a few milliseconds of a cut, the battery output is usually a stepped sine wave. For a router, modem and an office PC with an ordinary power supply.
• Line-interactive – has automatic voltage regulation (AVR), so it corrects under- and over-voltage without going to battery, which then lasts longer. The most common choice for PCs, NAS and small servers. Models with a pure sine wave output are required for power supplies with active PFC (most of today's PC power supplies) and for sensitive electronics.
• Online (double conversion) – the equipment runs permanently from the inverter, with no switch-over at all; always a pure sine wave. For servers, medical equipment and places with poor mains; more expensive and fan-cooled.

Runtime and how to extend it

Runtime depends on the load: the same UPS keeps a 100 W load running many times longer than 400 W. Makers state runtime at full and half load; for an ordinary desktop PC with a monitor expect 5–15 minutes from a UPS around 1,000 VA. If you want hours, you need a model with extended battery modules or a large lithium battery. The more practical route: connect only what must keep running (PC, NAS, router) through the UPS and leave the printer and chargers on an ordinary socket.

The battery: a consumable

The lead-acid battery in most UPS units lasts 3–5 years, a lithium (LiFePO4) one roughly twice as long. You notice the end when runtime shrinks and the UPS reports a replacement; most models allow hot-swapping the battery. Replacement batteries by UPS type are in the UPS batteries category. Place the UPS in a ventilated spot at room temperature – heat shortens battery life the most.

Communication and automatic shutdown

A UPS with a USB or network interface can tell the PC or NAS to shut down safely during a longer cut – without it, protection lasts only until the battery is empty. Makers' software and NAS systems support this as standard; for servers and multiple devices a UPS network card is useful.

What to protect and what not to

Yes: PC and monitor, NAS and disk arrays, router, modem and switch (without them there is no internet even with a working PC), camera systems, a point of sale. No: laser printers (they briefly draw over 1,000 W while heating and overload the UPS), heaters, coffee machines. A laptop needs no UPS – it has its own battery; a surge protector is enough.

Buying from us

In the UPS category you filter by rating in VA and W, topology, output waveform and number of outlets. UPS units in stock ship within 1–2 working days, delivery is free from CZK 6,000 and a 24-month warranty applies; if you are unsure what rating you need, send us a list of the equipment and we will advise.