A five-second power flicker can cost you an unsaved document, a corrupted file, or a dropped client call. An uninterruptible power supply (UPS) is a battery between the wall and your gear that rides through short outages and gives you time to save and shut down properly. This guide explains the types, how to size one, and the details that decide whether it actually protects your computer.
Quick answer
- Desktop PC: a line-interactive UPS with pure sine wave output, around 1000–1500VA.
- Laptop user: your laptop already has a battery — put the UPS on the modem, router and any NAS instead.
- Home lab, NAS or servers: line-interactive with USB monitoring so devices shut down automatically.
- Everyone: replace the UPS battery every three to five years.
The three types
| Type | How it works | Best for |
|---|---|---|
| Standby (offline) | Passes wall power through; switches to battery when power fails | Cheapest; basic protection for simple electronics |
| Line-interactive | Adds automatic voltage regulation (AVR) to correct sags and surges without using the battery | The right choice for home offices and PCs |
| Online (double conversion) | Always runs the load from the inverter; zero transfer time | Servers and sensitive equipment; expensive and noisier |
Pure sine wave matters more than you’d think
Cheaper units output a “simulated” or stepped waveform on battery. Many modern computer power supplies use active power factor correction (active PFC) and can react badly to that waveform — clicking, shutting down or refusing to switch over, which defeats the purpose. If you’re protecting a desktop PC or a gaming machine, buy a pure sine wave model. Check your power supply’s documentation if you’re unsure.
How to size it
- Add up the watts your gear actually draws — a smart plug with energy monitoring makes this easy (see smart plugs). A typical home office PC plus monitor might draw 150–300W.
- Compare with the UPS’s watt rating, not just its VA number. The two differ, and the watt figure is the binding limit.
- Leave headroom: aim to use no more than about 60–70% of the rating, which also lengthens runtime.
- Decide your runtime goal. Most home UPS units give a few minutes at full load — enough to save and shut down, not to keep working through a long outage.
Want to keep working? Size for the small stuff: a laptop, modem, router and an LED lamp can run far longer than a desktop.
What to plug in — and what not to
| Battery-backed outlets | Surge-only outlets | Never |
|---|---|---|
| Computer, monitor, NAS, modem, router, VoIP phone | Printer, speakers, desk lamp, chargers | Laser printers, space heaters, anything with a heating element or a motor |
A laser printer’s fuser draws a large surge that can overload or damage a UPS. Keep it on a surge outlet or a separate strip.
Features worth having
- USB data port and software (for example APC PowerChute or CyberPower PowerPanel) so the computer shuts down cleanly when the battery runs low. Synology, QNAP and TrueNAS systems can read a UPS over USB too — useful if you own a NAS.
- An LCD display showing load, runtime and battery health.
- Replaceable battery — check that the replacement pack is available and reasonably priced.
- Enough outlets, properly spaced for chunky power bricks.
- AVR for areas with brownouts and voltage swings.
Maintenance
- Batteries age whether you use them or not — plan on replacement every 3–5 years.
- Run a self-test occasionally and watch the reported runtime drop over time.
- Keep it ventilated and off carpet.
- Recycle old lead-acid batteries at a proper drop-off point.
A UPS protects against power loss, not data loss. Pair it with a real backup routine — see our external SSD guide and the 3-2-1 rule in NAS for home offices.