How Do You Size a Standby Generator for an Office Building?

Generator sizing fails most often at the load list — either everything gets added at nameplate (and the generator is absurdly oversized) or the real loads get missed (and it trips on day one). The engineer walks the building's systems with the owner: which elevators, which floors of lighting, the IT loads, the fire pump, the security systems, the HVAC the business continuity plan requires. Each load gets its running current and its starting inrush, because motors lie about their appetite at the moment of starting. Demand factors turn the connected load into the design load. Not every load runs simultaneously, and the code and good practice both recognize it — but the factors get applied honestly, with the owner understanding what's assumed. The generator then gets sized with headroom: for the largest motor start, for future load growth, and for the reality that load lists are always slightly optimistic. An honestly built load list is the difference between backup power and a backup problem.

The transfer scheme is the generator's reason for being: automatic transfer switches that sense the outage, start the generator, and move the loads — emergency first, standby after — within the times the code demands. The engineer designs the one-line so normal and emergency distribution stay cleanly separated, with the selectivity that keeps a fault on one branch from darkening the others. Every transfer switch gets a bypass or maintenance provision, because transfer switches themselves need service. Fuel system design decides how long the building runs. Diesel means on-site storage — sized to the runtime target, with fuel polishing for long storage, leak containment, and fill logistics the fire marshal approves. Natural gas means utility coordination and an honest assessment of gas reliability during regional outages. Exhaust routing, combustion air, and noise control complete the installation: the generator has to breathe, vent, and not deafen the neighbors, all within code.

A generator that isn't tested is a rumor. The engineer designs the system for testability: load bank connections, transfer switches that exercise under load, and monitoring that reports test results to the facilities team automatically. The commissioning process proves the whole sequence — outage simulation, generator start, load transfer, retransfer, cooldown — before the owner accepts the system. That first full-load test is when sizing errors reveal themselves, while there's still time to fix them. Maintenance is the unglamorous contract that keeps the promise. Diesel fuel gets polished, batteries get replaced on schedule, and the monthly tests get logged and reviewed — not just run. The facilities team gets trained on the system: what transfers automatically, what needs a human decision, and who to call when the generator does something unexpected at 2 a.m. Backup power is only as reliable as its last test and its next maintenance date.

• Build the load list from actual equipment with honest demand factors and starting inrush
• Size fuel storage to the business continuity runtime target, not just code minimums
• Engineer emergency and standby as fully separate branches with code-compliant transfer switching
• Design every transfer switch and generator for genuine loaded monthly testing routines
• Train facilities staff thoroughly on transfer sequences and maintenance before final acceptance