A UPS is sold in VA (volt-amperes) or kVA, but your equipment is rated in watts. Buying on the VA number alone can leave you with a UPS that is too small or far too large. Here is how the two relate and how to size one.
Key takeaways
- Watts are the real power; VA is the apparent power. The UPS has both limits, and you must stay within both.
- Watts equal VA multiplied by the power factor.
- Add up the watts of all loads, include a margin, then check the VA and the surge.
- Plan the backup time separately; see the next guide.
1. VA and watts
Alternating current has a voltage and a current that may not rise and fall together, and the result is a power factor (PF) between 0 and 1. VA is the voltage times the current, and watts are VA times PF. A UPS rated 1,000 VA with a power factor of 0.6 gives 600 W, while a modern online UPS rated 1,000 VA at a PF of 0.9 gives 900 W. Always read the watt rating on the datasheet.
2. Add up your loads
List each device and its power in watts from its label or manual. Include computers, monitors, routers, switches, printers and so on. Equipment often lists the maximum, but the real use is lower. If a device lists only amps and volts, multiply them to get VA.
3. Be careful with surges and motors
Laser printers, motors, compressors and pumps draw a large current when starting, and some UPS units trip because of it. It is usually best not to connect a laser printer to a UPS, or to use a larger UPS built for it. Check the surge or overload rating of the UPS.
4. Add a margin
Leave room for growth and ageing, usually 20 to 30 per cent above the total load. A UPS running at or near its limit has no room for a new device and runs hotter.
5. A worked example
A small office has loads totalling 2,400 W. Adding 25 per cent gives 3,000 W. If the UPS has a power factor of 0.9, the VA needed is 3,000 ÷ 0.9, about 3,333 VA, so a 4 kVA UPS is the next sensible size. A line-interactive 3,000 VA unit with a PF of 0.6 would give only 1,800 W, which is not enough, so always check the watt rating.
6. Check the supply
Choose single-phase or three-phase UPS to match the supply and the load; see single-phase vs three-phase, and check the type with online, line-interactive and offline UPS.
Next step
Send us your equipment list and the backup time you want through the quotation form, and we will size the UPS and battery.
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