How the calculator works
The calculator follows the same process we describe in how to size a UPS, in four steps:
- Total the real load in watts. Device figures come from our device power draw database, which lists typical and peak draw rather than nameplate maximums.
- Add headroom. The minimum UPS watt rating is your load divided by (1 minus headroom). At 25% headroom, a 600 W load needs an 800 W rating.
- Find the smallest common class that fits, using generic capacity classes from 600 VA / 360 W up to 3000 VA / 2700 W.
- Check runtime. For each class we model a typical internal battery with the same Peukert-style model as the runtime calculator. If the smallest fitting class falls short of your target, the calculator steps up.
Apparent power (VA) is estimated with a power factor of 0.95, which suits modern active-PFC electronics. If your load is mostly older equipment or motors, read VA vs watts and check the VA rating as well.
Worked example: home office with fiber internet
A home office with an office desktop (70 W), two 27-inch monitors (30 W each), a fiber ONT (8 W) and a Wi-Fi router (14 W) totals 152 W. With 25% headroom the minimum rating is 203 W, so even a 600 VA / 360 W unit fits. At this load the calculator estimates around 20 minutes from a small unit, enough to finish a call and shut down.
If the goal is to keep internet running through a two-hour outage, the right design changes: put the ONT and router on their own small UPS or a DC mini UPS. At 22 W, even a modest battery lasts hours, while the desktop gets a clean shutdown on the main unit. Splitting loads by runtime need is often cheaper than one large UPS.
Size by the job, not just the load
Ask what the UPS must achieve. Ride-through of short blips needs only seconds. A clean shutdown needs 5 to 10 minutes. Staying online through typical outages needs an hour or more, which is usually a job for a lithium UPS, extended battery packs or a power station rather than a bigger lead-acid box. See how much runtime do you need.
Limits of any calculator
- Real UPS models differ in battery size and inverter efficiency. Always compare with the maker's published runtime chart for the exact model.
- Batteries lose capacity with age and heat. A three-year-old battery in a warm cabinet may deliver half the new runtime; see battery lifespan.
- Motors, compressors and heaters are flagged because a UPS is rarely the right tool for them. See the use-case guides.
Frequently asked questions
What size UPS do I need for a desktop computer and monitor?
An office PC and one monitor usually draw 60 to 150 W, so a 600 VA / 360 W unit is enough for a clean shutdown and a 850 to 1000 VA unit gives 15 to 25 minutes. A gaming PC that peaks at 400 to 600 W needs about 1500 VA / 900 to 1000 W, preferably pure sine.
How much headroom should I leave on a UPS?
Plan to load the UPS to about 70 to 80% of its watt rating. That margin covers measurement error, transient spikes from graphics cards, battery aging and future additions. Running close to 100% also shortens runtime to just a few minutes.
Should I size a UPS by the power supply wattage?
No. A power supply's label is its maximum output, and most computers draw a fraction of it. Sizing by labels usually doubles or triples the UPS you buy. Measure with a plug-in wattmeter or use typical figures from our device database.
Is a bigger UPS always better?
A larger UPS gives more runtime and headroom but costs more, weighs more and has more battery to replace. Its self-consumption is also higher. Buy for your load plus headroom and your runtime goal, not for the largest number you can afford.