ComputerStore.joburg journal
Size a UPS for a desktop by its load and the runtime you need
A small uninterruptible power supply can keep a desktop running through a short outage and give you time to save work and shut down properly. Buying one is confusing because boxes quote VA while computers quote watts, and runtime claims are rarely what you get at your own load. A few minutes with a calculator gets you to a sensible size.
By ComputerStore.joburg editorial · · 2 min read
Start with what you want to protect
List every device that must stay on during a cut: the tower, the monitor you need to save your work, perhaps a router so a video call can end politely. Leave out printers and speakers that do not need backup. Note each device's power use in watts from its label or specification sheet.
Convert watts to VA and add headroom
A UPS should have a VA rating higher than the total watt load it protects. VA is usually higher than watts for computers, and the ratio between them, the power factor, is typically between 0.6 and 0.9. Dividing your watts by a cautious figure such as 0.8 gives a safer estimate: a 200 W load is roughly 250 VA. Then leave about 20 to 30 percent of extra capacity on top of that.
If your estimate lands near the top of a model’s rating, step up a size. A unit working close to its limit has little room for a new monitor or a later upgrade, and its battery time will be shorter.
- Total watts of everything that must stay on.
- Divide by a cautious power factor, such as 0.8, to estimate VA.
- Add about 20 to 30 percent for headroom.
- Choose a UPS rated comfortably above the result.
Be realistic about runtime
Runtime depends heavily on load: the more you plug in, the shorter it gets. Home and small office units typically give somewhere between about five and fifteen minutes, which is enough for an orderly shutdown, not for working through a long outage.
Manufacturers publish runtime charts. Read the chart at your load, not the headline figure.
Pick the right type
Home and small office units are usually standby, also called offline, models, which switch to battery within a fraction of a second when power drops. Line-interactive models add voltage regulation that handles minor sags and spikes without using the battery. Online units are a third type, usually chosen for more demanding setups.
Whatever you choose, ask the seller for the exact model's specification sheet, warranty terms and battery replacement options before you buy. Availability and price vary; this site does not confirm either.