Generator Sizing for Commercial Buildings: kW, Runtime, and Transfer Switch Selection

Connected load is not the answer

Generator sizing pitches that lead with connected load almost always end up over- or under-sized. Connected load assumes everything turns on at once, which never happens in real buildings; demand load assumes everything operates at calculated diversity, which doesn't account for motor inrush. The right number lives between them, and it depends on what's actually critical. We start every sizing conversation by asking 'what stays on during a 4-hour outage?' — not 'what's the total connected load?' The answer narrows the gen down by 30–50% in most office and retail buildings.

Three things blow the calculation

Three loads cause more sizing errors than everything else combined: HVAC compressors (LRA can be 4–7× FLA — and they all want to start at once when power restores), elevator hydraulic pumps (intermittent but with 2–3× FLA peaks lasting several seconds), and large UPS systems with battery-charging inrush during recovery. Any one of these can trip a generator that looked perfectly sized on demand-load math. We always run a step-load analysis — sequencing which loads come on at what time intervals — and size for the worst step, not the steady state.

Runtime is a fuel-system question

8-hour runtime is the default for commercial standby because that's typically how long a JEA-restored outage lasts on the worst hurricane day. But runtime is a fuel-system question, not a gen-set question. A 200 kW generator on a 1,000-gallon diesel tank gets ~36 hours at 75% load. The same gen on a 100-gallon belly tank gets 3.5 hours. For hurricane-zone commercial in Northeast Florida, we typically spec 48–72 hours of fuel — meaning 1,500–3,000 gallons for mid-sized buildings, with tank monitoring and a fuel-vendor contract for refills during extended events.

ATS selection: open vs closed transition

Automatic transfer switch selection is where the right gen ends up paired with the wrong switchgear. Open-transition ATS is cheaper and simpler — a momentary outage during transfer, fine for most buildings. Closed-transition (parallels gen and utility for ~100ms during transfer) is required when you have process equipment that can't tolerate any blip — typically medical, data center, and some industrial. Closed-transition ATS costs 2–3× more and requires utility approval. Spec it for the load that actually needs it; don't blanket-spec it because the brochure said 'better.'

Commissioning that proves the spec

Every commercial generator install gets two things at commissioning: load-bank test at 100% rated load for at least 2 hours, and a real outage simulation with the actual building load. The load bank proves the gen makes nameplate power; the building test proves the ATS sequences correctly under your actual step loads. We've caught HVAC starters that needed phase-rotation correction, elevator controllers that required a brownout-detection setting tweak, and a UPS that needed its battery-charge mode disabled during gen runs. None of those would show up on a load-bank test alone.

Commercial generator sizing is part math, part interview. The math sets the steady-state size; the interview about what's critical, what's startup-heavy, and how long you need to run determines whether the size is right. Add an honest fuel-system design and a real commissioning that includes building load — and the gen will work the next time the grid blinks.