Cold Room Backup Generator & UPS Sizing Calculator

Calculation method

1. running electrical load = refrigeration nameplate × 0.85 + auxiliaries
2. starting peak = running load + refrigeration nameplate × 0.40 × (start multiplier − 1)
3. generator kVA = starting peak ÷ 0.80 power factor × 1.20 margin, rounded up to 25 kVA
4. diesel allowance = running kW × 0.27 L/kWh × autonomy hours × 1.10

Power inputs can be entered in kW or mechanical hp; fuel output is shown in litres and US gallons. Generator output remains kVA because it is the standard electrical rating.

Worked example

A 120 kW refrigeration plant with 25 kW auxiliaries needs 24-hour standby power and soft starting.

Inputs

• • 120 kW (161 hp) refrigeration nameplate
• • 25 kW (34 hp) auxiliaries
• • Soft-start multiplier 1.4
• • 24 h autonomy

Calculation

• • Running load = 120 × 0.85 + 25 = 127 kW
• • Peak = 127 + 120 × 0.40 × 0.40 ≈ 146 kW
• • Generator = 146 ÷ 0.80 × 1.20, rounded up = 225 kVA
• • Fuel allowance ≈ 906 L (239 US gal)

Planning output: one 225 kVA generator, about 906 L (239 US gal) for 24 hours, plus the selected UPS scope.

Limitations

• • Actual motor starting current must come from compressor and drive manufacturers.
• • Altitude, ambient temperature, harmonics, load steps, fuel quality and local generator derating are not modelled.
• • Fuel consumption is a planning allowance; the generator supplier must confirm the selected set's curve and tank arrangement.

Illustrative planning result only. Confirm the final design, duty, equipment selection and performance with a qualified refrigeration engineer and supplier quotation.