Enter your load as kW, HP, or Amp, pick single or three phase, and get the recommended UPS kVA rating plus input current — rounded to the nearest standard UPS size.
Fill in your connected load, phase type, and power factor below.
Standard size, rounded up for headroom
Enter your load and hit calculate to see the recommended UPS rating and how much headroom it gives you.
Choosing the right UPS rating is a balancing act that trips up a lot of first-time buyers, and even a few experienced panel builders when the load list changes late in a project. A UPS that is undersized will trip on inrush current the moment a compressor or a large monitor bank switches on, and a UPS that is grossly oversized wastes capital, takes up more floor space, and often runs less efficiently at light load than a correctly matched unit would. This calculator follows the same sizing logic used by electrical contractors and data-centre facilities teams: convert whatever load figure you have on hand into real power, translate that into the apparent power the UPS itself is rated in, and then round up to a standard, commercially available size.
The starting point is always the connected load in kilowatts, because kW is the unit almost every piece of equipment is ultimately rated in, even when the nameplate shows something else. If your load is already known in kW, the calculator uses that figure directly. If instead you have a motor or pump rated in horsepower, the tool first converts shaft power to electrical kW using the standard factor 1 HP = 0.746 kW. And if all you have is a current reading from a clamp meter or an existing panel ammeter, the calculator works backward from the supply voltage, the phase configuration, and the load's power factor. For a single-phase circuit, real power is kW = (V × I × PF) ÷ 1000; for a three-phase circuit, the same relationship picks up a √3 term to account for the phase geometry, giving kW = (√3 × V × I × PF) ÷ 1000. Whichever path you start from, the load ends up expressed in the same common unit, which is what makes UPS sizing consistent regardless of how the equipment happens to be labelled.
Once the load is known in kW, sizing the UPS itself comes down to a single, deliberately simple relationship: kVA = kW ÷ Power Factor. A UPS is rated in apparent power (kVA), not real power (kW), because its internal components — the inverter stage, the battery charger, the wiring — all have to be sized to handle the full current the load draws, regardless of how much of that current is doing useful work versus how much is reactive. A load with a poor power factor therefore demands a proportionally larger UPS even though the real power it consumes hasn't changed; this is exactly why data-centre and server-room loads, which often run at PF 0.9 or lower, need noticeably bigger UPS units than their kW rating alone would suggest.
The calculated kVA figure is rarely a number you can buy off the shelf, so the final step rounds it up to the nearest standard UPS rating — units are manufactured in fixed steps such as 1, 2, 3, 5, 7.5, 10, 15, 20, 30, 50, and on up into the hundreds of kVA, rather than to arbitrary custom values. Rounding up rather than down is intentional: it builds in headroom for inrush current when equipment first switches on, for any future load growth on the same circuit, and for the gradual capacity loss that battery-based UPS systems experience as they age. As a rule of thumb, aim for the standard size that gives you somewhere between 10% and 25% of spare capacity over your calculated demand — enough margin to be safe, without paying for capacity you'll never realistically use.
Alongside the kVA rating, the calculator also reports the input current the UPS will draw from the upstream supply, using the same three-phase or single-phase relationship in reverse. This figure matters for anyone sizing the incoming breaker, the supply cable, or an isolator switch feeding the UPS — it tells you what the panel upstream actually needs to be rated for, separate from what the UPS itself is rated to deliver to the protected load downstream.
As with any sizing exercise, treat the output here as an solid engineering starting point rather than a final purchase decision. Real installations should also account for future expansion plans, the specific inrush characteristics of the connected equipment, ambient temperature at the installation site, and any redundancy requirements (N+1 or 2N configurations) mandated by the facility's uptime requirements. Always have the final UPS selection reviewed by a qualified electrical engineer before procurement, particularly for critical-load installations such as server rooms, hospitals, or industrial control systems.
A UPS's internal components — the inverter, charger, and wiring — must handle the full current the load draws, not just the portion doing useful work. kVA (apparent power) captures that total current demand, while kW (real power) only reflects the working portion, so UPS units are always rated in kVA.
A margin of roughly 10–25% is standard practice. It covers inrush current from motors and switch-mode equipment at start-up, future load additions, and the gradual capacity loss that comes with battery ageing. Rounding up to the next standard UPS size usually lands you in this range automatically.
Yes. Since kVA = kW ÷ PF, a lower power factor increases the calculated kVA for the same real-power load. Server and IT loads with poor power factors often need a noticeably larger UPS than their kW rating alone would suggest.
Base the calculation on steady-state running load, then rely on the standard-size rounding and headroom margin to absorb short inrush spikes. For loads with very high inrush (large motors, compressors), check the UPS manufacturer's surge rating separately, since sustained oversizing just for inrush wastes capacity.