How to calculate kW needs, validate with load banking, and maintain for reliability
Sizing a commercial generator isn't guesswork — it's math. Undersize it and your critical loads drop during an outage. Oversize it and you're burning fuel, wasting money, and creating wet-stacking problems in the engine. Here's how we do it right.
We start with a full load survey — every piece of equipment, its running amps, and its starting amps. Motors are the killer here: a 50 HP motor might draw 65A running but 390A locked-rotor on startup. Your generator has to handle that inrush without voltage collapse.
We separate loads into tiers: Life Safety (egress lighting, fire pumps, alarms), Critical (servers, medical equipment, refrigeration), Essential (HVAC, general lighting), and Optional (general outlets, non-essential equipment).
Total generator kW = (Sum of all critical loads × 1.25) + (Largest motor starting kVA × 0.8)
We don't trust factory specs. Every generator we install gets a full load bank test before sign-off. We apply resistive load in 25% increments up to 100% rated capacity, measuring voltage regulation, frequency stability, and engine performance.
Annual load banking is required by NFPA 110 for Level 1 systems (hospitals, data centers) and recommended for all commercial installations. A generator that only exercises under no-load will develop wet-stacking, carbon buildup, and injector fouling. When the real outage hits, it won't start — or it won't carry the load.
A single large motor can require 5-7x its running amps to start. The generator must handle this without voltage dip exceeding 15%.
Diesel engines run best at 70-80% load. Chronic underloading causes wet-stacking, glazed cylinders, and premature engine failure.
Inductive loads have poor power factor. A 100kW generator might only deliver 80kW of real power at 0.8 PF.
We always spec for 25% minimum growth. Adding a new HVAC unit after installation shouldn't require complete re-engineering.
Visual inspection, check fuel level, verify no alarms
Exercise under load (30 min min), check coolant/oil
Load bank test, inspect belts/hoses
Full service: oil change, filters, coolant test, battery load test, full load bank
Coolant replacement, fuel system cleaning, valve adjustment
Generator sizing is engineering, not estimation. We size, install, load bank test, and maintain.
Get a Generator Sizing Evaluation →