UPS Capacity and Load Guide

In short. A UPS has both a kVA and a kW rating, and the load must fit within both. Add redundancy and growth allowance, then check the battery, input and generator impact of the result. Modern UPS often have a rated power factor of 0.9 to 1.0, so kW is the more useful figure for many loads.

Step 1: Know the load

• List each load’s kW and kVA, not just the nameplate. Measure where possible.
• Check power factor and crest factor, since non-linear loads draw peaky current.
• Identify inrush on start-up, such as transformers or motors.
• Split into loads that need UPS and those that do not. See no-break, short-break and long-break loads.

Step 2: Size the UPS

Example (illustrative): load of 96 kW and 100 kVA, 20 per cent growth and target loading of 80 per cent. kVA needed = 100 × 1.2 ÷ 0.8 = 150 kVA. kW needed = 96 × 1.2 ÷ 0.8 = 144 kW. A UPS rated 150 kVA at 0.9 gives 135 kW, so it fails the kW check. A 160 kVA unit at unity power factor (160 kW) passes both. The lesson is to check kW and kVA separately.

Step 3: Redundancy

For N+1 with modules, add a module. For 2N, each system carries the whole load. See modular UPS redundancy.

Step 4: Overload and short-circuit ability

UPS units typically accept short overloads, such as 125 per cent for several minutes or 150 per cent for seconds, but this varies. Check the inverter’s ability to clear downstream faults, since inverters supply limited fault current and may transfer to bypass to clear a fault.

Step 5: Efficiency

Efficiency falls at low load. Redundant or oversized UPS may run at 20 to 40 per cent, where efficiency is lower. Check the efficiency curve at your expected load, and consider eco modes only where the load tolerates them.

Step 6: Upstream impact

• Input power factor and harmonic current. See harmonic screening.
• Input current includes battery charging, so the input is higher than the load.
• On generator, walk-in and soft-start features reduce the step.
• Upstream breaker and cable sizing.

Step 7: Battery

Battery size follows the load and the autonomy. See UPS autonomy.

Worked check on the generator (illustrative)

The 160 kVA UPS at 90 per cent load and 95 per cent efficiency draws about 144 ÷ 0.95 ≈ 152 kW, plus battery charging of perhaps 10 kW, so around 162 kW input. With a rectifier power factor of 0.95, that is about 170 kVA of input apparent power, and the generator must be sized for it, plus harmonic effects. The generator rating is therefore greater than the UPS output rating, not equal.

Sizing a UPS

Worked example (illustrative)

IT load of 160 kW with 20 percent growth and a target maximum loading of 80 percent: 160 × 1.2 ÷ 0.8 = 240 kW. Choose a 250 kW UPS (or 5 × 50 kW modules for N, with a sixth for N+1). If the UPS is rated 0.9 power factor in kVA, 250 kW corresponds to 278 kVA. Check the manufacturer’s kW rating, since many modern units are rated at unity.

Generator sizing with a UPS

The generator must carry the UPS input power: UPS output ÷ efficiency plus battery recharge power. A 250 kW UPS at 95 percent efficiency draws 263 kW plus recharge, perhaps 20 percent more. For a six-pulse rectifier, add alternator margin for harmonics. See UPS and nonlinear loads.

See battery autonomy guide.

Common mistakes

• Sizing on kVA alone.
• Forgetting battery charging power in the input.
• Ignoring crest factor of IT loads.
• Running a redundant UPS at very low efficiency without knowing.

What to send us

Send the load list, redundancy target and autonomy. Send your application details. See also UPS architecture.

Questions engineers ask

Figures are illustrative. Use manufacturer ratings for design.