What size UPS do I need?
List what is going on the UPS, set the power factor and the headroom, and this gives you the standard rating to specify — along with the working, so you can check it against your consultant’s figure.
Or start from a common item
Our units are rated at 0.8 lag, which is the default here. Raise it only if you know your load runs at a higher power factor.
25% covers future load and inrush. Go higher if motors start direct-on-line.
Sizing questions
Anything not covered here, call us — most of it is faster to answer on the phone.
How do you calculate the UPS size needed?
Total the continuous wattage of every load, divide by the power factor to convert watts to VA, add headroom for future load and inrush, then round up to the next standard rating. A 6,000 W load at 0.9 power factor is 6.67 kVA; with 25% headroom that is 8.3 kVA, so you specify a 10 kVA unit.
What power factor should I use?
0.9 is a safe default for mixed IT and electronic loads. Modern power supplies with active PFC run close to 1.0. Motors, older transformers and fluorescent lighting sit lower, often 0.7 to 0.8. If the load is mostly motors, use the lower figure — assuming 0.9 will undersize the UPS.
How much headroom should I add?
20–25% is standard. It covers load growth and the transient current when equipment powers up. Add more if anything on the list is motor-driven and starts direct-on-line, because inrush can be six to eight times running current and the UPS has to survive it.
Does this calculator account for backup time?
No. Capacity and runtime are separate specifications — capacity is set by the UPS rating, runtime by the battery bank behind it. A 10 kVA unit can hold ten minutes or two hours depending on the bank you put behind it. Tell us the runtime you need and we size the bank against it.