Add up what you actually want to keep running and this works out the UPS capacity you need — load, headroom, and the VA rating that follows from them. It is the same arithmetic we use when someone sends us an equipment list.
What are you backing up?
Your result
How the calculation works
Three steps, and you can check every one of them:
1. Add up the real watts. Use what your equipment actually draws, not the rating printed on its power supply — a 650 W PSU rarely pulls anything close to 650 W.
2. Add headroom. 30% covers start-up spikes and gives the battery an easier life.
3. Divide by the power factor. Back-UPS families are rated at roughly 0.6, so 660 W of load needs about 1100VA of UPS. Online Smart-UPS units are nearer 0.9.
One rule that catches people out: never put a laser printer on the battery outlets. Its warm-up surge alone can overload the unit.
Other tools
Frequently asked questions
What size UPS do I need?
Add up the real wattage of everything you want to back up, add about 30% headroom, then divide by the power factor of the UPS type — roughly 0.6 for an APC Back-UPS and 0.9 for an online Smart-UPS. A typical desktop and monitor comes to around 130 W, which lands on a 600VA Back-UPS. Use the calculator on this page and it does all three steps for you.
Should I size for peak or average load?
Always for peak. A gaming PC idles at a fraction of what it draws when a game loads, and it is during that spike that an undersized UPS drops the load. Size for the highest figure your equipment reaches, then add headroom on top.
Can I plug a laser printer into a UPS?
Not into the battery-backed outlets. A laser printer draws a very large surge when its fuser warms up, often more than the UPS can supply, which will trip the unit or shorten the battery life. Use the surge-only outlets or a wall socket.
Does a bigger UPS give longer backup?
Up to a point, and less than people expect. A larger unit has a larger battery, but backup time is driven mainly by how much load you put on it. Halving your load does far more for runtime than moving up one capacity tier.