In short. A UPS has both a kW and a kVA rating, and the load must fit within both after growth and a sensible maximum loading. When the load’s power factor is lower than the UPS output power factor, kVA usually decides the size. When it is higher, kW decides. Then check the generator behind the UPS, which should be well above the UPS input because rectifier load is non-linear.
Who this page is for: For IT managers, facility managers and electricians sizing a UPS for a known critical load.
Key facts
- Required kW = load kW × (1 + growth) ÷ maximum loading.
- Required kVA = load kVA × (1 + growth) ÷ maximum loading.
- The UPS must meet both; the larger decides.
- Check the UPS output power factor on its rating plate.
- The generator should be about 1.5 to 2 times the UPS input kVA.
- Battery size is a separate calculation.
Worked example: the calculator’s defaults
| Item | Working | Result |
|---|---|---|
| Load today | 24 kW at 0.9 power factor | 26.7 kVA |
| kW required | 24 × 1.2 growth ÷ 0.8 loading | 36 kW |
| kVA required | 26.7 × 1.2 ÷ 0.8 | 40 kVA |
| UPS chosen, output power factor 1.0 | Must give 40 kVA and 36 kW | 40 kVA (40 kW) |
| Generator behind it | UPS input about 40 kVA at full load (more with losses and battery recharge) × 1.5 to 2 | at least 60–80 kVA for the UPS alone |
Here kVA decides, because the load power factor (0.9) is below the UPS output power factor (1.0). See UPS architecture and battery autonomy.
Do not size a UPS from the nameplate kVA of the connected equipment. Measure the real load in kW and kVA, then add growth.
After sizing
| Check | Why |
|---|---|
| Redundancy: N+1 modules or a second UPS | Survive a UPS failure. See modular UPS |
| Overload and fault-clearing ability | Downstream breakers must trip on UPS supply |
| Input current and harmonics | Cable, breaker and generator sizing |
| Bypass source | What feeds the load if the UPS fails |
UPS sizing sheet
| Item | Value |
|---|---|
| Load kW and power factor | |
| Growth and maximum loading | |
| kW and kVA required | |
| UPS output power factor |
Common mistakes
- Only kVA checked.
- No growth allowance.
- Generator too small for the UPS.
- Downstream protection not checked on UPS supply.
What to send us
Send measured load and backup time needed. WhatsApp 98852 01608 or use the application engineering enquiry.
Evidence note. The worked example uses the calculator’s defaults.
Questions engineers ask
Is a UPS rated in kVA or kW?
Both. Check each, since the kW rating depends on the output power factor.
How much spare capacity should I leave?
Commonly 20 to 30 percent for growth and tolerance, depending on policy and redundancy.
Why must the generator be bigger than the UPS?
Because of UPS losses, battery charging and input power factor and harmonics.
Can a UPS clear faults downstream?
Inverters supply limited current, so protection may rely on bypass or fast-acting devices.
Figures are illustrative. Use manufacturer ratings for design.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 8 November 2026.