An uninterruptible power supply (UPS) keeps your computer, network gear, or audio equipment running briefly during a power outage and protects it from some electrical problems. UPS units differ in design, output waveform, and capacity. This guide explains what matters when choosing one.
Three common UPS types
- Standby (offline): passes wall power through normally and switches to battery when power fails. Affordable, with a short switchover time.
- Line-interactive: adds voltage regulation, correcting small sags and surges without using the battery. A popular choice for home offices.
- Online (double conversion): constantly converts power through the battery system, so output is always clean and there is no switchover. More expensive, louder, and less efficient, but suited to sensitive equipment.
Pure sine wave vs simulated sine wave
When running on battery, a UPS creates AC power:
- Pure sine wave output closely matches normal utility power.
- Simulated (stepped) sine wave output is a rougher approximation.
Many modern computer power supplies use active power factor correction (active PFC). Some of these may not work well with simulated sine wave output and may shut down or make noise during battery operation. A pure sine wave UPS avoids this issue and is a safer choice for desktops, workstations, and audio gear.
Sizing: VA and watts
UPS capacity is given in volt-amperes (VA) and watts (W). Use the watt rating for sizing.
- Add up the power draw of the devices you will connect. Use the power supply's typical load, not its maximum rating, if you can measure it.
- Choose a UPS with a watt rating comfortably above that total, leaving headroom.
- Check the runtime chart at your load.
A watt meter can show how much your setup actually uses.
Runtime
Most home UPS units are designed to give you a few minutes to save work and shut down safely, not to run equipment for hours. If you need longer runtime, look for models with extended battery packs or consider other backup solutions.
Useful features
- Automatic shutdown software via USB, to close your computer safely.
- Surge-only outlets for devices that do not need battery backup, like printers.
- LCD display showing load and battery status.
- Replaceable batteries that you can swap yourself.
- Network management for servers or remote setups.
Battery care
UPS batteries wear out over time, often within a few years depending on temperature and use.
- Keep the UPS in a cool, ventilated place.
- Run the self-test periodically.
- Replace batteries when the UPS warns you.
- Recycle old batteries properly.
Placement and safety
- Do not plug a UPS into a power strip or surge protector.
- Avoid overloading outlets.
- Keep the unit off carpet and away from heat sources.
- Laser printers and space heaters should not be connected to battery outlets.
Quick checklist
- Choose line-interactive or online based on your needs.
- Prefer pure sine wave output for modern PCs and audio gear.
- Size by watts with headroom.
- Check runtime at your load.
- Look for shutdown software and replaceable batteries.
- Plan for battery replacement.
FAQ
Do I need an online UPS? Most home offices are well served by a line-interactive pure sine wave UPS. Online units suit very sensitive or critical equipment.
Can a UPS protect against lightning? It helps with surges, but no device fully protects against a direct strike. Unplug equipment during severe storms if possible.
How long do UPS batteries last? Often a few years. Heat shortens battery life.
