Standby Generator (Genset) Sizing for Commercial Buildings

Generators fail acceptance tests when the estimator sizes them on running kW alone. Motor inrush and block loading routinely need a set 40–70% larger than the steady-state figure.

Define the load to be backed up

Standby generators rarely carry the whole building. List only the circuits on the emergency and legally-required standby systems: life-safety (fire pump, exit and emergency lighting, fire alarm, elevators for egress), plus any optional standby the client wants (data room, cold storage, selected aircon).

Running kW and kVA

Motor starting

Direct-on-line motors draw 5–7 × full-load current at a poor power factor during start. The generator must supply that surge without the voltage dipping below about 85%.

Start the largest motor last, with the rest of the load already online, and check the transient dip against the genset's published voltage-dip curve.

Step (block) loading

A diesel set accepts load in steps, not all at once. Typical acceptance is 60–80% of rated kW on the first step for a modern turbocharged engine. Sequence large loads through the transfer scheme so no single step exceeds the engine's block-load capability.

Site derating

• Altitude: roughly −3% per 300 m above 1000 m.
• Ambient temperature: roughly −2% per 5 °C above 40 °C.
• Ventilation and exhaust back-pressure reduce output further in tight plant rooms.

Select the rating

Take the governing value among running kVA, starting kVA requirement, and step-load requirement; divide by the combined derating factor; select the next standard standby-rated genset above it (standby rating, not prime, for emergency use). Add the ATS, day tank, exhaust, and acoustic enclosure to the BOQ.